Initial ASTC decoder -- currently only supports 2D LDR decomrpession modes.

src/utils/SkTextureCompressor_ASTC.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkTextureCompressor_ASTC.h"
 #include "SkTextureCompressor_Blitter.h"
 
 #include "SkBlitter.h"
 #include "SkEndian.h"
+#include "SkMath.h"
 
 // This table contains the weight values for each texel. This is used in determining
 // how to convert a 12x12 grid of alpha values into a 6x5 grid of index values. Since
 // we have a 6x5 grid, that gives 30 values that we have to compute. For each index,
 // we store up to 20 different triplets of values. In order the triplets are:
 // weight, texel-x, texel-y
 // The weight value corresponds to the amount that this index contributes to the final
 // index value of the given texel. Hence, we need to reconstruct the 6x5 index grid
 // from their relative contribution to the 12x12 texel grid.
 //
@@ skipping 231 matching lines @@
     top = top ^ t ^ (t << 2);
 
     send_packing(dst, SkEndian_SwapLE64(top), SkEndian_SwapLE64(bottom));
 }
 
 inline void CompressA8ASTCBlockVertical(uint8_t* dst, const uint8_t* src) {
     compress_a8_astc_block<GetAlphaTranspose>(&dst, src, 12);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
+//
+// ASTC Decoder
+//
+// Full details available in the spec:
+// http://www.khronos.org/registry/gles/extensions/OES/OES_texture_compression_astc.txt
+//
+////////////////////////////////////////////////////////////////////////////////
+

[robertphillips, 2014/08/06 17:24:21]

// Enable this to debug decoding errors. If not set, errors will result in
magenta images.

[krajcevski, 2014/08/06 18:14:30]

Done.

+#define ASSERT_ASTC_DECODE_ERROR 0
+
+// Reverse 64-bit integer taken from TAOCP 4a, although it's better
+// documented at this site:
+// http://matthewarcus.wordpress.com/2012/11/18/reversing-a-64-bit-word/
+
+template <typename T, T m, int k>
+static inline T swap_bits(T p) {
+    T q = ((p>>k)^p) & m;
+    return p^q^(q<<k);
+}
+
+static inline uint64_t reverse64(uint64_t n) {
+    static const uint64_t m0 = 0x5555555555555555LLU;
+    static const uint64_t m1 = 0x0300c0303030c303LLU;
+    static const uint64_t m2 = 0x00c0300c03f0003fLLU;
+    static const uint64_t m3 = 0x00000ffc00003fffLLU;
+    n = ((n>>1)&m0) | (n&m0)<<1;
+    n = swap_bits<uint64_t, m1, 4>(n);
+    n = swap_bits<uint64_t, m2, 8>(n);
+    n = swap_bits<uint64_t, m3, 20>(n);
+    n = (n >> 34) | (n << 30);
+    return n;
+}
+
+// An ASTC block is 128 bits. We represent it as two 64-bit integers in order
+// to efficiently operate on the block using bitwise operations.
+struct ASTCBlock {
+    uint64_t fLow;
+    uint64_t fHigh;
+
+    // Reverses the bits of an ASTC block, making the LSB of the
+    // 128 bit block the MSB.

[robertphillips, 2014/08/06 17:24:21]

lower case - since not static ?

[krajcevski, 2014/08/06 18:14:30]

Done.

+    inline void Reverse() {
+        const uint64_t newLow = reverse64(this->fHigh);

[robertphillips, 2014/08/06 17:24:20]

Why have the newHigh temp? Why not plop it into fHigh?

[krajcevski, 2014/08/06 18:14:30]

Done.

+        const uint64_t newHigh = reverse64(this->fLow);
+        this->fHigh = newHigh;
+        this->fLow = newLow;
+    }
+};
+
+// Writes the given color to every pixel in the block. This is used by void-extent
+// blocks (a special constant-color encoding of a block) and by the error function.
+static inline void write_constant_color(uint8_t* dst, int blockDimX, int blockDimY,
+                                        int dstRowBytes, SkColor color) {
+    for (int y = 0; y < blockDimY; ++y) {
+        SkColor *dstColors = reinterpret_cast<SkColor*>(dst);
+        for (int x = 0; x < blockDimX; ++x) {
+            dstColors[x] = color;
+        }
+        dst += dstRowBytes;
+    }
+}
+
+// Sets the entire block to the ASTC "error" color, a disgusting magenta
+// that's not supposed to appear in natural images.
+static inline void write_error_color(uint8_t* dst, int blockDimX, int blockDimY,
+                                     int dstRowBytes) {
+    static const SkColor kASTCErrorColor = SkColorSetRGB(0xFF, 0, 0xFF);
+
+#if ASSERT_ASTC_DECODE_ERROR
+    SkDEBUGFAIL("ASTC decoding error!\n");
+#endif
+
+    write_constant_color(dst, blockDimX, blockDimY, dstRowBytes, kASTCErrorColor);
+}
+
+// Reads up to 64 bits of the ASTC block starting from bit
+// 'from' and going up to but not including bit 'to'. 'from' starts
+// counting from the LSB, counting up to the MSB. Returns -1 on
+// error.
+static uint64_t read_astc_bits(const ASTCBlock &block, int from, int to) {
+    SkASSERT(0 <= from && from <= 128);
+    SkASSERT(0 <= to && to <= 128);
+
+    const int nBits = to - from;
+    if (0 == nBits) {
+        return 0;
+    }
+
+    if (nBits < 0 || 64 <= nBits) {
+        SkDEBUGFAIL("ASTC -- shouldn't read more than 64 bits");
+        return -1;
+    }
+
+    // Remember, the 'to' bit isn't read.
+    uint64_t result = 0;
+    if (to <= 64) {
+        // All desired bits are in the low 64-bits.
+        result = (block.fLow >> from) & ((1ULL << nBits) - 1);
+    } else if (from >= 64) {
+        // All desired bits are in the high 64-bits.
+        result = (block.fHigh >> (from - 64)) & ((1ULL << nBits) - 1);
+    } else {
+        // from < 64 && to > 64
+        SkASSERT(nBits > (64 - from));
+        const int nLow = 64 - from;
+        const int nHigh = nBits - nLow;
+        result = 
+            ((block.fLow >> from) & ((1ULL << nLow) - 1)) |
+            ((block.fHigh & ((1ULL << nHigh) - 1)) << nLow);
+    }
+
+    return result;
+}
+
+// Returns the number of bits needed to represent a number

[robertphillips, 2014/08/06 17:24:21]

add "(excluding the power-of-two itself)" ?

[krajcevski, 2014/08/06 18:14:29]

Done.

+// in the given power-of-two range.
+static inline int bits_for_range(int x) {
+    SkASSERT(SkIsPow2(x));
+    SkASSERT(0 != x);
+    // Since we know it's a power of two, there should only be one bit set,
+    // meaning the number of trailing zeros is 31 minus the number of leading
+    // zeros.
+    return 31 - SkCLZ(x);
+}
+
+// Clamps an integer to the range [0, 255]
+static inline int clamp_byte(int x) {
+    return SkClampMax(x, 255);
+}
+
+// Helper function defined in the ASTC spec, section C.2.14
+// It transfers a few bits of precision from one value to another.
+static inline void bit_transfer_signed(int *a, int *b) {
+    *b >>= 1;
+    *b |= *a & 0x80;
+    *a >>= 1;
+    *a &= 0x3F;
+    if ( (*a & 0x20) != 0 ) {
+        *a -= 0x40;
+    }
+}
+
+// Helper function defined in the ASTC spec, section C.2.14

[robertphillips, 2014/08/06 17:24:21]

modulate -> tint ?

[krajcevski, 2014/08/06 18:14:30]

Done.

+// It uses the value in the blue channel to modulate the red and green
+static inline SkColor blue_contract(int a, int r, int g, int b) {
+    return SkColorSetARGB(a, (r + b) >> 1, (g + b) >> 1, b);
+}
+

[robertphillips, 2014/08/06 17:24:21]

six or eight ?

[krajcevski, 2014/08/06 18:14:29]

Done.

+// Helper function that decodes two colors from eight values stored in
+// the pointer 'v'. This corresponds to the decode procedure for the following
+// endpoint modes:
+//   kLDR_RGB_Direct_ColorEndpointMode
+//   kLDR_RGBA_Direct_ColorEndpointMode
+static inline void decode_rgba_direct(const int *v, SkColor *endpoints, bool isRGB) {
+
+    int v6 = 0xFF;
+    int v7 = 0xFF;
+    if (!isRGB) {
+        v6 = v[6];
+        v7 = v[7];
+    }
+
+    const int s0 = v[0] + v[2] + v[4];
+    const int s1 = v[1] + v[3] + v[5];
+
+    if (s1 >= s0) {
+        endpoints[0] = SkColorSetARGB(v6, v[0], v[2], v[4]);
+        endpoints[1] = SkColorSetARGB(v7, v[1], v[3], v[5]);
+    } else {
+        endpoints[0] = blue_contract(v7, v[1], v[3], v[5]);
+        endpoints[1] = blue_contract(v6, v[0], v[2], v[4]);
+    }
+}
+

[robertphillips, 2014/08/06 17:24:20]

six or eight ?

[krajcevski, 2014/08/06 18:14:30]

Done.

+// Helper function that decodes two colors from eight values stored in
+// the pointer 'v'. This corresponds to the decode procedure for the following
+// endpoint modes:
+//   kLDR_RGB_BaseOffset_ColorEndpointMode
+//   kLDR_RGBA_BaseOffset_ColorEndpointMode
+//
+// If isRGB is true, then treat this as if v6 and v7 are meant to encode full alpha values.
+static inline void decode_rgba_baseoffset(const int *v, SkColor *endpoints, bool isRGB) {
+    int v0 = v[0];
+    int v1 = v[1];
+    int v2 = v[2];
+    int v3 = v[3];
+    int v4 = v[4];
+    int v5 = v[5];
+    int v6 = isRGB ? 0xFF : v[6];

[robertphillips, 2014/08/06 17:24:20]

0 -> 0xFF ?

[krajcevski, 2014/08/06 18:14:30]

It's supposed to be 0 because it's added to v6. The resulting values shouldn't
go over 0xFF.

+    int v7 = isRGB ? 0 : v[7];
+
+    bit_transfer_signed(&v1, &v0);
+    bit_transfer_signed(&v3, &v2);
+    bit_transfer_signed(&v5, &v4);
+    if (!isRGB) {
+        bit_transfer_signed(&v7, &v6);
+    }
+
+    int c[2][4];
+    if ((v1 + v3 + v5) >= 0) {
+        c[0][0] = v6;
+        c[0][1] = v0;
+        c[0][2] = v2;
+        c[0][3] = v4;
+
+        c[1][0] = v6 + v7;
+        c[1][1] = v0 + v1;
+        c[1][2] = v2 + v3;
+        c[1][3] = v4 + v5;
+    } else {
+        c[0][0] = v6 + v7;
+        c[0][1] = (v0 + v1 + v4 + v5) >> 1;
+        c[0][2] = (v2 + v3 + v4 + v5) >> 1;
+        c[0][3] = v4 + v5;
+
+        c[1][0] = v6;
+        c[1][1] = (v0 + v4) >> 1;
+        c[1][2] = (v2 + v4) >> 1;
+        c[1][3] = v4;
+    }
+
+    endpoints[0] = SkColorSetARGB(

[robertphillips, 2014/08/06 17:24:20]

tab these guys over? Maybe have them on two lines?

[krajcevski, 2014/08/06 18:14:29]

Done.

+        clamp_byte(c[0][0]),
+        clamp_byte(c[0][1]),
+        clamp_byte(c[0][2]),
+        clamp_byte(c[0][3]));
+
+    endpoints[1] = SkColorSetARGB(
+        clamp_byte(c[1][0]),
+        clamp_byte(c[1][1]),
+        clamp_byte(c[1][2]),
+        clamp_byte(c[1][3]));
+}
+
+
+// A helper class used to decode bit values from standard integer values.
+// We can't use this class with ASTCBlock because then it would need to
+// handle multi-value ranges, and it's non-trivial to lookup a range of bits
+// that splits across two different ints.
+template <typename T>
+class SkTBits {
+public:
+    SkTBits(const T val) : fVal(val) { }
+
+    // Returns the bit at the given position
+    T operator [](const int idx) const {
+        return (fVal >> idx) & 1;
+    }
+
+    // Returns the bits in the given range, inclusive
+    T operator ()(const int end, const int start) const {
+        SkASSERT(end >= start);
+        return (fVal >> start) & ((1ULL << ((end - start) + 1)) - 1);
+    }
+
+private:
+    const T fVal;
+};
+
+// This algorithm matches the trit block decoding in the spec (Table C.2.14)
+static void decode_trit_block(int* dst, int nBits, const uint64_t &block) {
+
+    SkTBits<uint64_t> blockBits(block);
+
+    // According to the spec, a trit block, which contains five values,
+    // has the following layout:
+    //
+    // 27  26  25  24  23  22  21  20  19  18  17  16
+    //  -----------------------------------------------
+    // |T7 |     m4        |T6  T5 |     m3        |T4 |
+    //  -----------------------------------------------
+    //
+    // 15  14  13  12  11  10  9   8   7   6   5   4   3   2   1   0
+    //  --------------------------------------------------------------
+    // |    m2        |T3  T2 |      m1       |T1  T0 |      m0       |
+    //  --------------------------------------------------------------
+    //
+    // Where the m's are variable width depending on the number of bits used
+    // to encode the values (anywhere from 0 to 6). Since 3^5 = 243, the extra
+    // byte labeled T (whose bits are interleaved where 0 is the LSB and 7 is
+    // the MSB), contains five trit values. To decode the trit values, the spec
+    // says that we need to follow the following algorithm:
+    //
+    // if T[4:2] = 111
+    //     C = { T[7:5], T[1:0] }; t4 = t3 = 2
+    // else
+    //     C = T[4:0]
+    //
+    // if T[6:5] = 11
+    //     t4 = 2; t3 = T[7]
+    // else
+    //     t4 = T[7]; t3 = T[6:5]
+    //
+    // if C[1:0] = 11
+    //     t2 = 2; t1 = C[4]; t0 = { C[3], C[2]&~C[3] }
+    // else if C[3:2] = 11
+    //     t2 = 2; t1 = 2; t0 = C[1:0]
+    // else
+    //     t2 = C[4]; t1 = C[3:2]; t0 = { C[1], C[0]&~C[1] }
+    //
+    // The following C++ code is meant to mirror this layout and algorithm as
+    // closely as possible.
+
+    int m[5];
+    if (0 == nBits) {
+        memset(m, 0, sizeof(m));
+    } else {
+        m[0] = blockBits(nBits - 1, 0);
+        m[1] = blockBits(2*nBits - 1 + 2, nBits + 2);
+        m[2] = blockBits(3*nBits - 1 + 4, 2*nBits + 4);
+        m[3] = blockBits(4*nBits - 1 + 5, 3*nBits + 5);
+        m[4] = blockBits(5*nBits - 1 + 7, 4*nBits + 7);
+    }
+
+    int T =
+        blockBits(nBits + 1, nBits) |
+        (blockBits(2*nBits + 2 + 1, 2*nBits + 2) << 2) |
+        (blockBits[3*nBits + 4] << 4) |
+        (blockBits(4*nBits + 5 + 1, 4*nBits + 5) << 5) |
+        (blockBits[5*nBits + 7] << 7);
+
+    int t[5];
+
+    int C;
+    SkTBits<int> Tbits(T);
+    if (0x7 == Tbits(4, 2)) {
+        C = (Tbits(7, 5) << 2) | Tbits(1, 0);
+        t[3] = t[4] = 2;
+    } else {
+        C = Tbits(4, 0);
+        if (Tbits(6, 5) == 0x3) {
+            t[4] = 2; t[3] = Tbits[7];
+        } else {
+            t[4] = Tbits[7]; t[3] = Tbits(6, 5);
+        }
+    }
+
+    SkTBits<int> Cbits(C);
+    if (Cbits(1, 0) == 0x3) {
+        t[2] = 2;
+        t[1] = Cbits[4];
+        t[0] = (Cbits[3] << 1) | (Cbits[2] & (0x1 & ~(Cbits[3])));
+    } else if (Cbits(3, 2) == 0x3) {
+        t[2] = 2;
+        t[1] = 2;
+        t[0] = Cbits(1, 0);
+    } else {
+        t[2] = Cbits[4];
+        t[1] = Cbits(3, 2);
+        t[0] = (Cbits[1] << 1) | (Cbits[0] & (0x1 & ~(Cbits[1])));
+    }
+
+#if SK_DEBUG
+    // Make sure all of the decoded values have a trit less than three
+    // and a bit value within the range of the allocated bits.
+    for (int i = 0; i < 5; ++i) {
+        SkASSERT(t[i] < 3);
+        SkASSERT(m[i] < (1 << nBits));
+    }
+#endif
+
+    for (int i = 0; i < 5; ++i) {
+        *dst = (t[i] << nBits) + m[i];
+        ++dst;
+    }
+}
+
+// This algorithm matches the quint block decoding in the spec (Table C.2.15)
+static void decode_quint_block(int* dst, int nBits, const uint64_t &block) {
+    SkTBits<uint64_t> blockBits(block);
+
+    // According to the spec, a quint block, which contains three values,
+    // has the following layout:
+    //
+    //
+    // 18  17  16  15  14  13  12  11  10  9   8   7   6   5   4   3   2   1   0
+    //  --------------------------------------------------------------------------
+    // |Q6  Q5 |     m2       |Q4  Q3 |     m1        |Q2  Q1  Q0 |      m0       |
+    //  --------------------------------------------------------------------------
+    //
+    // Where the m's are variable width depending on the number of bits used
+    // to encode the values (anywhere from 0 to 4). Since 5^3 = 125, the extra
+    // 7-bit value labeled Q (whose bits are interleaved where 0 is the LSB and 6 is
+    // the MSB), contains three quint values. To decode the quint values, the spec
+    // says that we need to follow the following algorithm:
+    //
+    // if Q[2:1] = 11 and Q[6:5] = 00
+    //     q2 = { Q[0], Q[4]&~Q[0], Q[3]&~Q[0] }; q1 = q0 = 4
+    // else
+    //     if Q[2:1] = 11
+    //         q2 = 4; C = { Q[4:3], ~Q[6:5], Q[0] }
+    //     else
+    //         q2 = T[6:5]; C = Q[4:0]
+    //
+    //     if C[2:0] = 101
+    //         q1 = 4; q0 = C[4:3]
+    //     else
+    //         q1 = C[4:3]; q0 = C[2:0]
+    //
+    // The following C++ code is meant to mirror this layout and algorithm as
+    // closely as possible.
+
+    int m[3];
+    if (0 == nBits) {
+        memset(m, 0, sizeof(m));
+    } else {
+        m[0] = blockBits(nBits - 1, 0);
+        m[1] = blockBits(2*nBits - 1 + 3, nBits + 3);
+        m[2] = blockBits(3*nBits - 1 + 5, 2*nBits + 5);
+    }
+
+    int Q =
+        blockBits(nBits + 2, nBits) |
+        (blockBits(2*nBits + 3 + 1, 2*nBits + 3) << 3) |
+        (blockBits(3*nBits + 5 + 1, 3*nBits + 5) << 5);
+
+    int q[3];
+    SkTBits<int> Qbits(Q); // quantum?
+
+    if (Qbits(2, 1) == 0x3 && Qbits(6, 5) == 0) {
+        const int notBitZero = (0x1 & ~(Qbits[0]));
+        q[2] = (Qbits[0] << 2) | ((Qbits[4] & notBitZero) << 1) | (Qbits[3] & notBitZero);
+        q[1] = 4;
+        q[0] = 4;
+    } else {
+        int C;
+        if (Qbits(2, 1) == 0x3) {
+            q[2] = 4;
+            C = (Qbits(4, 3) << 3) | ((0x3 & ~(Qbits(6, 5))) << 1) | Qbits[0];
+        } else {
+            q[2] = Qbits(6, 5);
+            C = Qbits(4, 0);
+        }
+
+        SkTBits<int> Cbits(C);
+        if (Cbits(2, 0) == 0x5) {
+            q[1] = 4;
+            q[0] = Cbits(4, 3);
+        } else {
+            q[1] = Cbits(4, 3);
+            q[0] = Cbits(2, 0);
+        }
+    }
+
+#if SK_DEBUG
+    for (int i = 0; i < 3; ++i) {
+        SkASSERT(q[i] < 5);
+        SkASSERT(m[i] < (1 << nBits));
+    }
+#endif
+
+    for (int i = 0; i < 3; ++i) {
+        *dst = (q[i] << nBits) + m[i];
+        ++dst;
+    }
+}
+
+// Function that decodes a sequence of integers stored as an ISE (Integer
+// Sequence Encoding) bit stream. The full details of this function are outlined
+// in section C.2.12 of the ASTC spec. A brief overview is as follows:
+//
+// - Each integer in the sequence is bounded by a specific range r.
+// - The range of each value determines the way the bit stream is interpreted,
+// - If the range is a power of two, then the sequence is a sequence of bits
+// - If the range is of the form 3*2^n, then the sequence is stored as a
+//   sequence of blocks, each block contains 5 trits and 5 bit sequences, which
+//   decodes into 5 values.
+// - Similarly, if the range is of the form 5*2^n, then the sequence is stored as a
+//   sequence of blocks, each block contains 3 quints and 3 bit sequences, which
+//   decodes into 3 values.
+static bool decode_integer_sequence(
+    int* dst,                 // The array holding the destination bits
+    int dstSize,              // The maximum size of the array
+    int nVals,                // The number of values that we'd like to decode
+    const ASTCBlock &block,   // The block that we're decoding from
+    int startBit,             // The bit from which we're going to do the reading
+    int endBit,               // The bit at which we stop reading (not inclusive)
+    bool bReadForward,        // If true, then read LSB -> MSB, else read MSB -> LSB
+    int nBits,                // The number of bits representing this encoding
+    int nTrits,               // The number of trits representing this encoding
+    int nQuints               // The number of quints representing this encoding
+) {
+    // If we want more values than we have, then fail.
+    if (nVals > dstSize) {
+        return false;
+    }
+
+    ASTCBlock src = block;
+
+    if (!bReadForward) {
+        src.Reverse();
+        startBit = 128 - startBit;
+        endBit = 128 - endBit;
+    }
+
+    while (nVals > 0) {
+
+        if (nTrits > 0) {
+            SkASSERT(0 == nQuints);
+
+            int endBlockBit = startBit + 8 + 5*nBits;
+            if (endBlockBit > endBit) {
+                endBlockBit = endBit;
+            }
+
+            decode_trit_block(dst, nBits, read_astc_bits(src, startBit, endBlockBit));
+            dst += 5;
+            nVals -= 5;
+            startBit = endBlockBit;
+
+        } else if (nQuints > 0) {
+            SkASSERT(0 == nTrits);
+
+            int endBlockBit = startBit + 7 + 3*nBits;
+            if (endBlockBit > endBit) {
+                endBlockBit = endBit;
+            }
+
+            decode_quint_block(dst, nBits, read_astc_bits(src, startBit, endBlockBit));
+            dst += 3;
+            nVals -= 3;
+            startBit = endBlockBit;
+
+        } else {
+            // Just read the bits, but don't read more than we have...
+            int endValBit = startBit + nBits;
+            if (endValBit > endBit) {
+                endValBit = endBit;
+            }
+
+            *dst = read_astc_bits(src, startBit, endValBit);
+            ++dst;
+            --nVals;
+            startBit = endValBit;
+        }
+    }
+
+    return true;
+}
+
+// Helper function that unquantizes some (seemingly random) generated
+// numbers... meant to match the ASTC hardware. This function is used
+// to unquantize both colors (Table C.2.16) and weights (Table C.2.26)
+static inline int unquantize_value(unsigned mask, int A, int B, int C, int D) {
+    int T = D * C + B;
+    T = T ^ A;
+    T = (A & mask) | (T >> 2);
+    SkASSERT(T < 256);
+    return T;
+}
+
+// Helper function to replicate the bits in x that represents an oldPrec
+// precision integer into a prec precision integer. For example:
+//   255 == replicate_bits(7, 3, 8);
+static inline int replicate_bits(int x, int oldPrec, int prec) {
+    while (oldPrec < prec) {
+        const int toShift = SkMin32(prec-oldPrec, oldPrec);
+        x = (x << toShift) | (x >> (oldPrec - toShift));
+        oldPrec += toShift;
+    }
+
+    // Make sure that no bits are set outside the desired precision.
+    SkASSERT((-(1 << prec) & x) == 0);
+    return x;
+}
+
+// Returns the unquantized value of a color that's represented only as
+// a set of bits.
+static inline int unquantize_bits_color(int val, int nBits) {
+    return replicate_bits(val, nBits, 8);
+}
+
+// Returns the unquantized value of a color that's represented as a
+// trit followed by nBits bits. This algorithm follows the sequence
+// defined in section C.2.13 of the ASTC spec.
+static inline int unquantize_trit_color(int val, int nBits) {
+    SkASSERT(nBits > 0);
+    SkASSERT(nBits < 7);
+
+    const int D = (val >> nBits) & 0x3;
+    SkASSERT(D < 3);
+
+    const int A = -(val & 0x1) & 0x1FF;
+
+    static const int Cvals[6] = { 204, 93, 44, 22, 11, 5 };
+    const int C = Cvals[nBits - 1];
+
+    int B = 0;
+    const SkTBits<int> valBits(val);
+    switch (nBits) {
+        case 1:
+            B = 0;
+            break;
+
+        case 2: {
+            const int b = valBits[1];
+            B = (b << 1) | (b << 2) | (b << 4) | (b << 8);
+        }
+        break;
+
+        case 3: {
+            const int cb = valBits(2, 1);
+            B = cb | (cb << 2) | (cb << 7);
+        }
+        break;
+
+        case 4: {
+            const int dcb = valBits(3, 1);
+            B = dcb | (dcb << 6);
+        }
+        break;
+
+        case 5: {
+            const int edcb = valBits(4, 1);
+            B = (edcb << 5) | (edcb >> 2);
+        }
+        break;
+
+        case 6: {
+            const int fedcb = valBits(5, 1);
+            B = (fedcb << 4) | (fedcb >> 4);
+        }
+        break;
+    }
+
+    return unquantize_value(0x80, A, B, C, D);
+}
+
+// Returns the unquantized value of a color that's represented as a
+// quint followed by nBits bits. This algorithm follows the sequence
+// defined in section C.2.13 of the ASTC spec.
+static inline int unquantize_quint_color(int val, int nBits) {
+    const int D = (val >> nBits) & 0x7;
+    SkASSERT(D < 5);
+
+    const int A = -(val & 0x1) & 0x1FF;
+
+    static const int Cvals[5] = { 113, 54, 26, 13, 6 };
+    SkASSERT(nBits > 0);
+    SkASSERT(nBits < 6);
+
+    const int C = Cvals[nBits - 1];
+
+    int B = 0;
+    const SkTBits<int> valBits(val);
+    switch (nBits) {
+        case 1:
+            B = 0;
+            break;
+
+        case 2: {
+            const int b = valBits[1];
+            B = (b << 2) | (b << 3) | (b << 8);
+        }
+        break;
+
+        case 3: {
+            const int cb = valBits(2, 1);
+            B = (cb >> 1) | (cb << 1) | (cb << 7);
+        }
+        break;
+
+        case 4: {
+            const int dcb = valBits(3, 1);
+            B = (dcb >> 1) | (dcb << 6);
+        }
+        break;
+
+        case 5: {
+            const int edcb = valBits(4, 1);
+            B = (edcb << 5) | (edcb >> 3);
+        }
+        break;
+    }
+
+    return unquantize_value(0x80, A, B, C, D);
+}
+
+// This algorithm takes a list of integers, stored in vals, and unquantizes them
+// in place. This follows the algorithm laid out in section C.2.13 of the ASTC spec.
+static void unquantize_colors(int *vals, int nVals, int nBits, int nTrits, int nQuints) {
+    for (int i = 0; i < nVals; ++i) {
+        if (nTrits > 0) {
+            SkASSERT(nQuints == 0);
+            vals[i] = unquantize_trit_color(vals[i], nBits);
+        } else if (nQuints > 0) {
+            SkASSERT(nTrits == 0);
+            vals[i] = unquantize_quint_color(vals[i], nBits);
+        } else {
+            SkASSERT(nQuints == 0 && nTrits == 0);
+            vals[i] = unquantize_bits_color(vals[i], nBits);
+        }
+    }
+}
+
+// Returns an interpolated value between c0 and c1 based on the weight. This
+// follows the algorithm laid out in section C.2.19 of the ASTC spec.
+static int interpolate_channel(int c0, int c1, int weight) {
+    SkASSERT(0 <= c0 && c0 < 256);
+    SkASSERT(0 <= c1 && c1 < 256);
+
+    c0 = (c0 << 8) | c0;
+    c1 = (c1 << 8) | c1;
+
+    const int result = ((c0*(64 - weight) + c1*weight + 32) / 64) >> 8;
+
+    if (result > 255) {
+        return 255;
+    }
+
+    SkASSERT(result >= 0);
+    return result;
+}
+
+// Returns an interpolated color between the two endpoints based on the weight.
+static SkColor interpolate_endpoints(const SkColor endpoints[2], int weight) {
+    return SkColorSetARGB(
+        interpolate_channel(SkColorGetA(endpoints[0]), SkColorGetA(endpoints[1]), weight),
+        interpolate_channel(SkColorGetR(endpoints[0]), SkColorGetR(endpoints[1]), weight),
+        interpolate_channel(SkColorGetG(endpoints[0]), SkColorGetG(endpoints[1]), weight),
+        interpolate_channel(SkColorGetB(endpoints[0]), SkColorGetB(endpoints[1]), weight));
+}
+
+// Returns an interpolated color between the two endpoints based on the weight.
+// It uses separate weights for the channel depending on the value of the 'plane'
+// variable. By default, all channels will use weight 0, and the value of plane
+// means that weight1 will be used for:
+// 0: red
+// 1: green
+// 2: blue
+// 3: alpha
+static SkColor interpolate_dual_endpoints(
+    const SkColor endpoints[2], int weight0, int weight1, int plane) {
+    int a = interpolate_channel(SkColorGetA(endpoints[0]), SkColorGetA(endpoints[1]), weight0);
+    int r = interpolate_channel(SkColorGetR(endpoints[0]), SkColorGetR(endpoints[1]), weight0);
+    int g = interpolate_channel(SkColorGetG(endpoints[0]), SkColorGetG(endpoints[1]), weight0);
+    int b = interpolate_channel(SkColorGetB(endpoints[0]), SkColorGetB(endpoints[1]), weight0);
+
+    switch (plane) {
+
+        case 0:
+            r = interpolate_channel(
+                SkColorGetR(endpoints[0]), SkColorGetR(endpoints[1]), weight1);
+            break;
+
+        case 1:
+            g = interpolate_channel(
+                SkColorGetG(endpoints[0]), SkColorGetG(endpoints[1]), weight1);
+            break;
+
+        case 2:
+            b = interpolate_channel(
+                SkColorGetB(endpoints[0]), SkColorGetB(endpoints[1]), weight1);
+            break;
+
+        case 3:
+            a = interpolate_channel(
+                SkColorGetA(endpoints[0]), SkColorGetA(endpoints[1]), weight1);
+            break;
+
+        default:
+            SkDEBUGFAIL("Plane should be 0-3");
+            break;
+    }
+
+    return SkColorSetARGB(a, r, g, b);
+}
+
+// A struct of decoded values that we use to carry around information
+// about the block. dimX and dimY are the dimension in texels of the block,
+// for which there is only a limited subset of valid values:
+//
+// 4x4, 5x4, 5x5, 6x5, 6x6, 8x5, 8x6, 8x8, 10x5, 10x6, 10x8, 10x10, 12x10, 12x12
+
+struct ASTCDecompressionData {
+    ASTCDecompressionData(int dimX, int dimY) : fDimX(dimX), fDimY(dimY) { }
+    const int   fDimX;      // the X dimension of the decompressed block
+    const int   fDimY;      // the Y dimension of the decompressed block
+    ASTCBlock   fBlock;     // the block data
+    int         fBlockMode; // the block header that contains the block mode.
+
+    bool fDualPlaneEnabled; // is this block compressing dual weight planes?
+    int  fDualPlane;        // the independent plane in dual plane mode.
+
+    bool fVoidExtent;       // is this block a single color?
+    bool fError;            // does this block have an error encoding?
+
+    int  fWeightDimX;       // the x dimension of the weight grid
+    int  fWeightDimY;       // the y dimension of the weight grid
+
+    int  fWeightBits;       // the number of bits used for each weight value
+    int  fWeightTrits;      // the number of trits used for each weight value
+    int  fWeightQuints;     // the number of quints used for each weight value
+
+    int  fPartCount;        // the number of partitions in this block
+    int  fPartIndex;        // the partition index: only relevant if fPartCount > 0
+
+    // CEM values can be anything in the range 0-15, and each corresponds to a different
+    // mode that represents the color data. We only support LDR modes.
+    enum ColorEndpointMode {
+        kLDR_Luminance_Direct_ColorEndpointMode          = 0,
+        kLDR_Luminance_BaseOffset_ColorEndpointMode      = 1,
+        kHDR_Luminance_LargeRange_ColorEndpointMode      = 2,
+        kHDR_Luminance_SmallRange_ColorEndpointMode      = 3,
+        kLDR_LuminanceAlpha_Direct_ColorEndpointMode     = 4,
+        kLDR_LuminanceAlpha_BaseOffset_ColorEndpointMode = 5,
+        kLDR_RGB_BaseScale_ColorEndpointMode             = 6,
+        kHDR_RGB_BaseScale_ColorEndpointMode             = 7,
+        kLDR_RGB_Direct_ColorEndpointMode                = 8,
+        kLDR_RGB_BaseOffset_ColorEndpointMode            = 9,
+        kLDR_RGB_BaseScaleWithAlpha_ColorEndpointMode    = 10,
+        kHDR_RGB_ColorEndpointMode                       = 11,
+        kLDR_RGBA_Direct_ColorEndpointMode               = 12,
+        kLDR_RGBA_BaseOffset_ColorEndpointMode           = 13,
+        kHDR_RGB_LDRAlpha_ColorEndpointMode              = 14,
+        kHDR_RGB_HDRAlpha_ColorEndpointMode              = 15
+    };
+    static const int kMaxColorEndpointModes = 16;
+
+    // the color endpoint modes for this block.
+    static const int kMaxPartitions = 4;
+    ColorEndpointMode fCEM[kMaxPartitions];
+
+    int  fColorStartBit;    // The bit position of the first bit of the color data
+    int  fColorEndBit;      // The bit position of the last *possible* bit of the color data
+
+    // Returns the number of partitions for this block.
+    int numPartitions() const {
+        return fPartCount;
+    }
+
+    // Returns the total number of weight values that are stored in this block
+    int numWeights() const {
+        return fWeightDimX * fWeightDimY * (fDualPlaneEnabled ? 2 : 1);
+    }
+
+#ifdef SK_DEBUG
+    // Returns the maximum value that any weight can take. We really only use
+    // this function for debugging.
+    int maxWeightValue() const {
+        int maxVal = (1 << fWeightBits);
+        if (fWeightTrits > 0) {
+            SkASSERT(0 == fWeightQuints);
+            maxVal *= 3;
+        } else if (fWeightQuints > 0) {
+            SkASSERT(0 == fWeightTrits);
+            maxVal *= 5;
+        }
+        return maxVal - 1;
+    }
+#endif
+
+    // The number of bits needed to represent the texel weight data. This
+    // comes from the 'data size determination' section of the ASTC spec (C.2.22)
+    int numWeightBits() const {
+        const int nWeights = this->numWeights();
+        return
+            ((nWeights*8*fWeightTrits + 4) / 5) +
+            ((nWeights*7*fWeightQuints + 2) / 3) +
+            (nWeights*fWeightBits);
+    }
+
+    // Returns the number of color values stored in this block. The number of
+    // values stored is directly a function of the color endpoint modes.
+    int numColorValues() const {
+        int numValues = 0;
+        for (int i = 0; i < this->numPartitions(); ++i) {
+            int cemInt = static_cast<int>(fCEM[i]);
+            numValues += ((cemInt >> 2) + 1) * 2;
+        }
+
+        return numValues;
+    }
+
+    // Figures out the number of bits available for color values, and fills
+    // in the maximum encoding that will fit the number of color values that
+    // we need. Returns false on error. (See section C.2.22 of the spec)
+    bool getColorValueEncoding(int *nBits, int *nTrits, int *nQuints) const {
+        if (NULL == nBits || NULL == nTrits || NULL == nQuints) {
+            return false;
+        }
+
+        const int nColorVals = this->numColorValues();
+        if (nColorVals <= 0) {
+            return false;
+        }
+
+        const int colorBits = fColorEndBit - fColorStartBit;
+        SkASSERT(colorBits > 0);
+
+        // This is the minimum amount of accuracy required by the spec.
+        if (colorBits < ((13 * nColorVals + 4) / 5)) {
+            return false;
+        }
+
+        // Values can be represented as at most 8-bit values.
+        // !SPEED! place this in a lookup table based on colorBits and nColorVals
+        for (int i = 255; i > 0; --i) {
+            int range = i + 1;
+            int bits = 0, trits = 0, quints = 0;
+            bool valid = false;
+            if (SkIsPow2(range)) {
+                bits = bits_for_range(range);
+                valid = true;
+            } else if ((range % 3) == 0 && SkIsPow2(range/3)) {
+                trits = 1;
+                bits = bits_for_range(range/3);
+                valid = true;
+            } else if ((range % 5) == 0 && SkIsPow2(range/5)) {
+                quints = 1;
+                bits = bits_for_range(range/5);
+                valid = true;
+            }
+
+            if (valid) {
+                const int actualColorBits =
+                    ((nColorVals*8*trits + 4) / 5) +
+                    ((nColorVals*7*quints + 2) / 3) +
+                    (nColorVals*bits);
+                if (actualColorBits <= colorBits) {
+                    *nTrits = trits;
+                    *nQuints = quints;
+                    *nBits = bits;
+                    return true;
+                }
+            }
+        }
+
+        return false;
+    }
+
+    // Converts the sequence of color values into endpoints. The algorithm here
+    // corresponds to the values determined by section C.2.14 of the ASTC spec
+    void colorEndpoints(SkColor endpoints[4][2], const int* colorValues) const {
+        for (int i = 0; i < this->numPartitions(); ++i) {
+            switch (fCEM[i]) {

[robertphillips, 2014/08/06 17:24:20]

Share subroutine with kLDR_LuminanceAlpha_Direct_ColorEndpointMode ?

[krajcevski, 2014/08/06 18:14:30]

This will probably create more upkeep than it saves.

+                case kLDR_Luminance_Direct_ColorEndpointMode: {
+                    const int* v = colorValues;
+                    endpoints[i][0] = SkColorSetARGB(0xFF, v[0], v[0], v[0]);
+                    endpoints[i][1] = SkColorSetARGB(0xFF, v[1], v[1], v[1]);
+
+                    colorValues += 2;
+                }
+                break;
+
+                case kLDR_Luminance_BaseOffset_ColorEndpointMode: {
+                    const int* v = colorValues;
+                    const int L0 = (v[0] >> 2) | (v[1] & 0xC0);
+                    const int L1 = clamp_byte(L0 + (v[1] & 0x3F));
+
+                    endpoints[i][0] = SkColorSetARGB(0xFF, L0, L0, L0);
+                    endpoints[i][1] = SkColorSetARGB(0xFF, L1, L1, L1);
+
+                    colorValues += 2;
+                }
+                break;
+
+                case kLDR_LuminanceAlpha_Direct_ColorEndpointMode: {
+                    const int* v = colorValues;
+                    
+                    endpoints[i][0] = SkColorSetARGB(v[2], v[0], v[0], v[0]);
+                    endpoints[i][1] = SkColorSetARGB(v[3], v[1], v[1], v[1]);
+
+                    colorValues += 4;
+                }
+                break;
+
+                case kLDR_LuminanceAlpha_BaseOffset_ColorEndpointMode: {
+                    int v0 = colorValues[0];
+                    int v1 = colorValues[1];
+                    int v2 = colorValues[2];
+                    int v3 = colorValues[3];
+
+                    bit_transfer_signed(&v1, &v0);
+                    bit_transfer_signed(&v3, &v2);
+                    
+                    endpoints[i][0] = SkColorSetARGB(v2, v0, v0, v0);
+                    endpoints[i][1] = SkColorSetARGB(
+                        clamp_byte(v3+v2),
+                        clamp_byte(v1+v0),
+                        clamp_byte(v1+v0),
+                        clamp_byte(v1+v0));
+
+                    colorValues += 4;
+                }
+                break;
+

[robertphillips, 2014/08/06 17:24:21]

Share subroutine with kLDR_RGB_BaseScaleWithAlpha_ColorEndpointMode?

[krajcevski, 2014/08/06 18:14:30]

Done.

+                case kLDR_RGB_BaseScale_ColorEndpointMode: {
+                    const int* v = colorValues;
+
+                    endpoints[i][0] = SkColorSetARGB(
+                        0xFF, v[0]*v[3] >> 8, v[1]*v[3] >> 8, v[2]*v[3] >> 8);
+                    endpoints[i][1] = SkColorSetARGB(0xFF, v[0], v[1], v[2]);
+                    
+                    colorValues += 4;
+                }
+                break;
+
+                case kLDR_RGB_Direct_ColorEndpointMode: {
+                    decode_rgba_direct(colorValues, endpoints[i], true);
+                    colorValues += 6;
+                }
+                break;
+
+                case kLDR_RGB_BaseOffset_ColorEndpointMode: {
+                    decode_rgba_baseoffset(colorValues, endpoints[i], true);
+                    colorValues += 6;
+                }
+                break;
+
+                case kLDR_RGB_BaseScaleWithAlpha_ColorEndpointMode: {
+                    const int* v = colorValues;
+
+                    endpoints[i][0] = SkColorSetARGB(v[4],
+                                                     (v[0]*v[3]) >> 8,
+                                                     (v[1]*v[3]) >> 8,
+                                                     (v[2]*v[3]) >> 8);
+                    endpoints[i][1] = SkColorSetARGB(v[5], v[0], v[1], v[2]);
+
+                    colorValues += 6;
+                }
+                break;
+
+                case kLDR_RGBA_Direct_ColorEndpointMode: {
+                    decode_rgba_direct(colorValues, endpoints[i], false);
+                    colorValues += 8;
+                }
+                break;
+
+                case kLDR_RGBA_BaseOffset_ColorEndpointMode: {
+                    decode_rgba_baseoffset(colorValues, endpoints[i], false);
+                    colorValues += 8;
+                }
+                break;
+
+                default:
+                    SkDEBUGFAIL("HDR mode unsupported! This should be caught sooner.");
+                    break;
+            }
+        }
+    }
+
+    // Follows the procedure from section C.2.17 of the ASTC specification
+    int unquantizeWeight(int x) const {
+        SkASSERT(x <= this->maxWeightValue());
+
+        const int D = (x >> fWeightBits) & 0x7;
+        const int A = -(x & 0x1) & 0x7F;
+
+        SkTBits<int> xbits(x);
+
+        int T = 0;
+        if (fWeightTrits > 0) {
+            SkASSERT(0 == fWeightQuints);
+            switch (fWeightBits) {
+                case 0: {
+                    // x is a single trit
+                    SkASSERT(x < 3);
+
+                    static const int kUnquantizationTable[3] = { 0, 32, 63 };
+                    T = kUnquantizationTable[x];
+                }
+                break;
+
+                case 1: {
+                    const int B = 0;
+                    const int C = 50;
+                    T = unquantize_value(0x20, A, B, C, D);
+                }
+                break;
+
+                case 2: {
+                    const int b = xbits[1];
+                    const int B = b | (b << 2) | (b << 6);
+                    const int C = 23;
+                    T = unquantize_value(0x20, A, B, C, D);
+                }
+                break;
+
+                case 3: {
+                    const int cb = xbits(2, 1);
+                    const int B = cb | (cb << 5);
+                    const int C = 11;
+                    T = unquantize_value(0x20, A, B, C, D);
+                }
+                break;
+
+                default:
+                    SkDEBUGFAIL("Too many bits for trit encoding");
+                    break;
+            }
+
+        } else if (fWeightQuints > 0) {
+            SkASSERT(0 == fWeightTrits);
+            switch (fWeightBits) {
+                case 0: {
+                    // x is a single quint
+                    SkASSERT(x < 5);
+
+                    static const int kUnquantizationTable[5] = { 0, 16, 32, 47, 63 };
+                    T = kUnquantizationTable[x];
+                }
+                break;
+
+                case 1: {
+                    const int B = 0;
+                    const int C = 28;
+                    T = unquantize_value(0x20, A, B, C, D);
+                }
+                break;
+
+                case 2: {
+                    const int b = xbits[1];
+                    const int B = (b << 1) | (b << 6);
+                    const int C = 13;
+                    T = unquantize_value(0x20, A, B, C, D);
+                }
+                break;
+
+                default:
+                    SkDEBUGFAIL("Too many bits for quint encoding");
+                    break;
+            }
+        } else {
+            SkASSERT(0 == fWeightTrits);
+            SkASSERT(0 == fWeightQuints);
+
+            T = replicate_bits(x, fWeightBits, 6);
+        }
+
+        // This should bring the value within [0, 63]..
+        SkASSERT(T <= 63);
+
+        if (T > 32) {
+            T += 1;
+        }
+
+        SkASSERT(T <= 64);
+
+        return T;
+    }
+
+    // Returns the weight at the associated index. If the index is out of bounds, it
+    // returns zero. It also chooses the weight appropriately based on the given dual
+    // plane.
+    int getWeight(const int* unquantizedWeights, int idx, bool dualPlane) const {

[robertphillips, 2014/08/06 17:24:21]

Space before '?' ?
Do we need the parens around fDualPlaneEnabled ?

[krajcevski, 2014/08/06 18:14:30]

Done.

+        const int maxIdx = ((fDualPlaneEnabled)? 2 : 1) * fWeightDimX * fWeightDimY - 1;
+        if (fDualPlaneEnabled) {
+            const int effectiveIdx = 2*idx + (dualPlane ? 1 : 0);
+            if (effectiveIdx > maxIdx) {
+                return 0;
+            }
+            return unquantizedWeights[effectiveIdx];
+        }
+
+        SkASSERT(!dualPlane);
+
+        if (idx > maxIdx) {
+            return 0;
+        } else {
+            return unquantizedWeights[idx];
+        }
+    }
+
+    // This computes the effective weight at location (s, t) of the block. This
+    // weight is computed by sampling the texel weight grid (it's usually not 1-1), and
+    // then applying a bilerp. The algorithm outlined here follows the algorithm
+    // defined in section C.2.18 of the ASTC spec.
+    int infillWeight(const int* unquantizedValues, int s, int t, bool dualPlane) const {
+        const int Ds = (1024 + fDimX/2) / (fDimX - 1);
+        const int Dt = (1024 + fDimY/2) / (fDimY - 1);
+
+        const int cs = Ds * s;
+        const int ct = Dt * t;
+
+        const int gs = (cs*(fWeightDimX - 1) + 32) >> 6;
+        const int gt = (ct*(fWeightDimY - 1) + 32) >> 6;
+
+        const int js = gs >> 4;
+        const int jt = gt >> 4;
+
+        const int fs = gs & 0xF;
+        const int ft = gt & 0xF;
+
+        const int idx = js + jt*fWeightDimX;
+        const int p00 = this->getWeight(unquantizedValues, idx, dualPlane);
+        const int p01 = this->getWeight(unquantizedValues, idx + 1, dualPlane);
+        const int p10 = this->getWeight(unquantizedValues, idx + fWeightDimX, dualPlane);
+        const int p11 = this->getWeight(unquantizedValues, idx + fWeightDimX + 1, dualPlane);
+
+        const int w11 = (fs*ft + 8) >> 4;
+        const int w10 = ft - w11;
+        const int w01 = fs - w11;
+        const int w00 = 16 - fs - ft + w11;
+
+        const int weight = (p00*w00 + p01*w01 + p10*w10 + p11*w11 + 8) >> 4;
+        SkASSERT(weight <= 64);
+        return weight;
+    }
+
+    // Unquantizes the decoded texel weights as described in section C.2.17 of
+    // the ASTC specification. Additionally, it populates texelWeights with
+    // the expanded weight grid, which is computed according to section C.2.18
+    void texelWeights(int texelWeights[2][12][12], const int* texelValues) const {
+        // Unquantized texel weights...
+        int unquantizedValues[144*2]; // 12x12 blocks with dual plane decoding...
+        SkASSERT(this->numWeights() <= 144*2);
+
+        // Unquantize the weights and cache them
+        for (int j = 0; j < this->numWeights(); ++j) {
+            unquantizedValues[j] = this->unquantizeWeight(texelValues[j]);
+        }
+
+        // Do weight infill...
+        for (int y = 0; y < fDimY; ++y) {
+            for (int x = 0; x < fDimX; ++x) {
+                texelWeights[0][x][y] = this->infillWeight(unquantizedValues, x, y, false);
+                if (fDualPlaneEnabled) {
+                    texelWeights[1][x][y] = this->infillWeight(unquantizedValues, x, y, true);
+                }
+            }
+        }
+    }
+
+    // Returns the partition for the texel located at position (x, y).
+    // Adapted from C.2.21 of the ASTC specification
+    int getPartition(int x, int y) const {
+        const int partitionCount = this->numPartitions();
+        int seed = fPartIndex;
+        if ((fDimX * fDimY) < 31) {
+            x <<= 1;
+            y <<= 1;
+        }
+
+        seed += (partitionCount - 1) * 1024;
+
+        uint32_t p = seed;
+        p ^= p >> 15;  p -= p << 17;  p += p << 7; p += p <<  4;
+        p ^= p >>  5;  p += p << 16;  p ^= p >> 7; p ^= p >> 3;
+        p ^= p <<  6;  p ^= p >> 17;
+
+        uint32_t rnum = p;
+        uint8_t seed1  =  rnum        & 0xF;
+        uint8_t seed2  = (rnum >>  4) & 0xF;
+        uint8_t seed3  = (rnum >>  8) & 0xF;
+        uint8_t seed4  = (rnum >> 12) & 0xF;
+        uint8_t seed5  = (rnum >> 16) & 0xF;
+        uint8_t seed6  = (rnum >> 20) & 0xF;
+        uint8_t seed7  = (rnum >> 24) & 0xF;
+        uint8_t seed8  = (rnum >> 28) & 0xF;
+        uint8_t seed9  = (rnum >> 18) & 0xF;
+        uint8_t seed10 = (rnum >> 22) & 0xF;
+        uint8_t seed11 = (rnum >> 26) & 0xF;
+        uint8_t seed12 = ((rnum >> 30) | (rnum << 2)) & 0xF;
+
+        seed1 *= seed1;     seed2 *= seed2;
+        seed3 *= seed3;     seed4 *= seed4;
+        seed5 *= seed5;     seed6 *= seed6;
+        seed7 *= seed7;     seed8 *= seed8;
+        seed9 *= seed9;     seed10 *= seed10;
+        seed11 *= seed11;   seed12 *= seed12;
+
+        int sh1, sh2, sh3;
+        if (0 != (seed & 1)) {
+            sh1 = (0 != (seed & 2))? 4 : 5;
+            sh2 = (partitionCount == 3)? 6 : 5;
+        } else {
+            sh1 = (partitionCount==3)? 6 : 5;
+            sh2 = (0 != (seed & 2))? 4 : 5;
+        }
+        sh3 = (0 != (seed & 0x10))? sh1 : sh2;
+
+        seed1 >>= sh1; seed2  >>= sh2; seed3  >>= sh1; seed4  >>= sh2;
+        seed5 >>= sh1; seed6  >>= sh2; seed7  >>= sh1; seed8  >>= sh2;
+        seed9 >>= sh3; seed10 >>= sh3; seed11 >>= sh3; seed12 >>= sh3;
+
+        const int z = 0;
+        int a = seed1*x + seed2*y + seed11*z + (rnum >> 14);
+        int b = seed3*x + seed4*y + seed12*z + (rnum >> 10);
+        int c = seed5*x + seed6*y + seed9 *z + (rnum >>  6);
+        int d = seed7*x + seed8*y + seed10*z + (rnum >>  2);
+
+        a &= 0x3F;
+        b &= 0x3F;
+        c &= 0x3F;
+        d &= 0x3F;
+
+        if (partitionCount < 4) {
+            d = 0;
+        }
+
+        if (partitionCount < 3) {
+            c = 0;
+        }
+
+        if (a >= b && a >= c && a >= d) {
+            return 0;
+        } else if (b >= c && b >= d) {
+            return 1;
+        } else if (c >= d) {
+            return 2;
+        } else {
+            return 3;
+        }
+    }
+
+    // Performs the proper interpolation of the texel based on the
+    // endpoints and weights.
+    SkColor getTexel(const SkColor endpoints[4][2],
+                     const int weights[2][12][12],
+                     int x, int y) const {
+        int part = 0;
+        if (this->numPartitions() > 1) {
+            part = this->getPartition(x, y);
+        }
+
+        SkColor result;
+        if (fDualPlaneEnabled) {
+            result = interpolate_dual_endpoints(
+                endpoints[part], weights[0][x][y], weights[1][x][y], fDualPlane);
+        } else {
+            result = interpolate_endpoints(endpoints[part], weights[0][x][y]);
+        }
+
+#if 1
+        // !FIXME! if we're writing directly to a bitmap, then we don't need
+        // to swap the red and blue channels, but since we're usually being used
+        // by the SkImageDecoder_astc module, the results are expected to be in RGBA.
+        result = SkColorSetARGB(
+            SkColorGetA(result), SkColorGetB(result), SkColorGetG(result), SkColorGetR(result));
+#endif
+
+        return result;
+    }
+
+    void decode() {
+        // First decode the block mode.
+        this->decodeBlockMode();
+
+        // Now we can decode the partition information.
+        fPartIndex = read_astc_bits(fBlock, 11, 23);
+        fPartCount = (fPartIndex & 0x3) + 1;
+        fPartIndex >>= 2;
+
+        // This is illegal
+        if (fDualPlaneEnabled && this->numPartitions() == 4) {
+            fError = true;
+            return;
+        }
+
+        // Based on the partition info, we can decode the color information.
+        this->decodeColorData();
+    }
+
+    // Decodes the dual plane based on the given bit location. The final
+    // location, if the dual plane is enabled, is also the end of our color data.
+    // This function is only meant to be used from this->decodeColorData()
+    void decodeDualPlane(int bitLoc) {
+        if (fDualPlaneEnabled) {
+            fDualPlane = read_astc_bits(fBlock, bitLoc - 2, bitLoc);
+            fColorEndBit = bitLoc - 2;
+        } else {
+            fColorEndBit = bitLoc;
+        }
+    }
+
+    // Decodes the color information based on the ASTC spec.
+    void decodeColorData() {
+
+        // By default, the last color bit is at the end of the texel weights
+        const int lastWeight = 128 - this->numWeightBits();
+
+        // If we have a dual plane then it will be at this location, too.
+        int dualPlaneBitLoc = lastWeight;
+
+        // If there's only one partition, then our job is (relatively) easy.
+        if (this->numPartitions() == 1) {
+            fCEM[0] = static_cast<ColorEndpointMode>(read_astc_bits(fBlock, 13, 17));
+            fColorStartBit = 17;
+
+            // Handle dual plane mode...
+            this->decodeDualPlane(dualPlaneBitLoc);
+
+            return;
+        } 
+
+        // If we have more than one partition, then we need to make
+        // room for the partition index.
+        fColorStartBit = 29;
+
+        // Read the base CEM. If it's zero, then we have no additional
+        // CEM data and the endpoints for each partition share the same CEM.
+        const int baseCEM = read_astc_bits(fBlock, 23, 25);
+        if (0 == baseCEM) {
+
+            const ColorEndpointMode sameCEM =
+                static_cast<ColorEndpointMode>(read_astc_bits(fBlock, 25, 29));
+
+            for (int i = 0; i < kMaxPartitions; ++i) {
+                fCEM[i] = sameCEM;
+            }
+
+            // Handle dual plane mode...
+            this->decodeDualPlane(dualPlaneBitLoc);
+
+            return;
+        } 
+
+        // Move the dual plane selector bits down based on how many
+        // partitions the block contains.
+        switch (this->numPartitions()) {
+            case 2:
+                dualPlaneBitLoc -= 2;
+                break;
+
+            case 3:
+                dualPlaneBitLoc -= 5;
+                break;
+
+            case 4:
+                dualPlaneBitLoc -= 8;
+                break;
+
+            default:
+                SkDEBUGFAIL("Internal ASTC decoding error.");
+                break;
+        }
+
+        // The rest of the CEM config will be between the dual plane bit selector
+        // and the texel weight grid.
+        const int lowCEM = read_astc_bits(fBlock, 23, 29);
+        int fullCEM = read_astc_bits(fBlock, dualPlaneBitLoc, lastWeight);
+
+        // Attach the config at the end of the weight grid to the CEM values
+        // in the beginning of the block.
+        fullCEM = (fullCEM << 6) | lowCEM;
+
+        // Ignore the two least significant bits, since those are our baseCEM above.
+        fullCEM = fullCEM >> 2;
+
+        int C[kMaxPartitions]; // Next, decode C and M from the spec (Table C.2.12)
+        for (int i = 0; i < this->numPartitions(); ++i) {
+            C[i] = fullCEM & 1;
+            fullCEM = fullCEM >> 1;
+        }
+
+        int M[kMaxPartitions];
+        for (int i = 0; i < this->numPartitions(); ++i) {
+            M[i] = fullCEM & 0x3;
+            fullCEM = fullCEM >> 2;
+        }
+
+        // Construct our CEMs..
+        SkASSERT(baseCEM > 0);
+        for (int i = 0; i < this->numPartitions(); ++i) {
+            int cem = (baseCEM - 1) * 4;
+            cem += (0 == C[i])? 0 : 4;
+            cem += M[i];
+
+            SkASSERT(cem < 16);
+            fCEM[i] = static_cast<ColorEndpointMode>(cem);
+        }
+
+        // Finally, if we have dual plane mode, then read the plane selector.
+        this->decodeDualPlane(dualPlaneBitLoc);
+    }
+
+    // Decodes the block mode. This function determines whether or not we use
+    // dual plane encoding, the size of the texel weight grid, and the number of
+    // bits, trits and quints that are used to encode it. For more information, 
+    // see section C.2.10 of the ASTC spec.
+    //
+    // For 2D blocks, the Block Mode field is laid out as follows:
+    //
+    // -------------------------------------------------------------------------
+    // 10  9   8   7   6   5   4   3   2   1   0   Width Height Notes
+    // -------------------------------------------------------------------------
+    // D   H     B       A     R0  0   0   R2  R1  B+4   A+2
+    // D   H     B       A     R0  0   1   R2  R1  B+8   A+2
+    // D   H     B       A     R0  1   0   R2  R1  A+2   B+8
+    // D   H   0   B     A     R0  1   1   R2  R1  A+2   B+6
+    // D   H   1   B     A     R0  1   1   R2  R1  B+2   A+2
+    // D   H   0   0     A     R0  R2  R1  0   0   12    A+2
+    // D   H   0   1     A     R0  R2  R1  0   0   A+2   12
+    // D   H   1   1   0   0   R0  R2  R1  0   0   6     10
+    // D   H   1   1   0   1   R0  R2  R1  0   0   10    6
+    //   B     1   0     A     R0  R2  R1  0   0   A+6   B+6   D=0, H=0
+    // x   x   1   1   1   1   1   1   1   0   0   -     -     Void-extent
+    // x   x   1   1   1   x   x   x   x   0   0   -     -     Reserved*
+    // x   x   x   x   x   x   x   0   0   0   0   -     -     Reserved
+    // -------------------------------------------------------------------------
+    //

[robertphillips, 2014/08/06 17:24:20]

A, B ?

[krajcevski, 2014/08/06 18:14:30]

Done.

+    // D - dual plane enabled
+    // H, R - used to determine the number of bits/trits/quints in texel weight encoding

[robertphillips, 2014/08/06 17:24:21]

R0, R1, R2 ?

[krajcevski, 2014/08/06 18:14:29]

Done.

+    // Width, Height - dimensions of the texel weight grid.
+
+    void decodeBlockMode() {
+        const int blockMode = read_astc_bits(fBlock, 0, 11);
+
+        // Check for special void extent encoding
+        fVoidExtent = (blockMode & 0x1FF) == 0x1FC;
+
+        // Check for reserved block modes
+        fError = ((blockMode & 0x1C3) == 0x1C0) || ((blockMode & 0xF) == 0);
+
+        // Neither reserved nor void-extent, decode as usual
+        // This code corresponds to table C.2.8 of the ASTC spec
+        bool highPrecision = false;
+        int R = 0;
+        if ((blockMode & 0x3) == 0) {
+            R = ((0xC & blockMode) >> 1) | ((0x10 & blockMode) >> 4);
+            const int bitsSevenAndEight = (blockMode & 0x180) >> 7;
+            SkASSERT(0 <= bitsSevenAndEight && bitsSevenAndEight < 4);
+
+            const int A = (blockMode >> 5) & 0x3;
+            const int B = (blockMode >> 9) & 0x3;
+
+            fDualPlaneEnabled = (blockMode >> 10) & 0x1;
+            highPrecision = (blockMode >> 9) & 0x1;
+
+            switch (bitsSevenAndEight) {
+                default:
+                case 0:
+                    fWeightDimX = 12;
+                    fWeightDimY = A + 2;
+                    break;
+
+                case 1:
+                    fWeightDimX = A + 2;
+                    fWeightDimY = 12;
+                    break;
+
+                case 2:
+                    fWeightDimX = A + 6;
+                    fWeightDimY = B + 6;
+                    fDualPlaneEnabled = false;
+                    highPrecision = false;
+                    break;
+
+                case 3:
+                    if (0 == A) {
+                        fWeightDimX = 6;
+                        fWeightDimY = 10;
+                    } else {
+                        fWeightDimX = 10;
+                        fWeightDimY = 6;
+                    }
+                    break;
+            }
+        } else { // (blockMode & 0x3) != 0
+            R = ((blockMode & 0x3) << 1) | ((blockMode & 0x10) >> 4);
+
+            const int bitsTwoAndThree = (blockMode >> 2) & 0x3;
+            SkASSERT(0 <= bitsTwoAndThree && bitsTwoAndThree < 4);
+
+            const int A = (blockMode >> 5) & 0x3;
+            const int B = (blockMode >> 7) & 0x3;
+
+            fDualPlaneEnabled = (blockMode >> 10) & 0x1;
+            highPrecision = (blockMode >> 9) & 0x1;
+
+            switch (bitsTwoAndThree) {
+                case 0:
+                    fWeightDimX = B + 4;
+                    fWeightDimY = A + 2;
+                    break;
+                case 1:
+                    fWeightDimX = B + 8;
+                    fWeightDimY = A + 2;
+                    break;
+                case 2:
+                    fWeightDimX = A + 2;
+                    fWeightDimY = B + 8;
+                    break;
+                case 3:
+                    if ((B & 0x2) == 0) {
+                        fWeightDimX = A + 2;
+                        fWeightDimY = (B & 1) + 6;
+                    } else {
+                        fWeightDimX = (B & 1) + 2;
+                        fWeightDimY = A + 2;
+                    }
+                    break;
+            }
+        }
+
+        // We should have set the values of R and highPrecision
+        // from decoding the block mode, these are used to determine
+        // the proper dimensions of our weight grid.
+        if ((R & 0x6) == 0) {
+            fError = true;
+        } else {
+            static const int kBitAllocationTable[2][6][3] = {
+                {
+                    {  1, 0, 0 },
+                    {  0, 1, 0 },
+                    {  2, 0, 0 },
+                    {  0, 0, 1 },
+                    {  1, 1, 0 },
+                    {  3, 0, 0 }
+                },
+                {
+                    {  1, 0, 1 },
+                    {  2, 1, 0 },
+                    {  4, 0, 0 },
+                    {  2, 0, 1 },
+                    {  3, 1, 0 },
+                    {  5, 0, 0 }
+                }
+            };
+
+            fWeightBits = kBitAllocationTable[highPrecision][R - 2][0];
+            fWeightTrits = kBitAllocationTable[highPrecision][R - 2][1];
+            fWeightQuints = kBitAllocationTable[highPrecision][R - 2][2];
+        }
+    }
+};
+
+// Reads an ASTC block from the given pointer.
+static inline void read_astc_block(ASTCDecompressionData *dst, const uint8_t* src) {
+    const uint64_t* qword = reinterpret_cast<const uint64_t*>(src);
+    dst->fBlock.fLow = SkEndian_SwapLE64(qword[0]);
+    dst->fBlock.fHigh = SkEndian_SwapLE64(qword[1]);
+    dst->decode();
+}
+
+// Take a known void-extent block, and write out the values as a constant color.
+static void decompress_void_extent(uint8_t* dst, int dstRowBytes,
+                                   const ASTCDecompressionData &data) {
+    // The top 64 bits contain 4 16-bit RGBA values.
+    int a = (read_astc_bits(data.fBlock, 112, 128) + 255) >> 8;
+    int b = (read_astc_bits(data.fBlock, 96, 112) + 255) >> 8;
+    int g = (read_astc_bits(data.fBlock, 80, 96) + 255) >> 8;
+    int r = (read_astc_bits(data.fBlock, 64, 80) + 255) >> 8;
+
+    write_constant_color(dst, data.fDimX, data.fDimY, dstRowBytes, SkColorSetARGB(a, r, g, b));
+}
+
+// Decompresses a single ASTC block. It's assumed that data.fDimX and data.fDimY are
+// set and that the block has already been decoded (i.e. data.decode() has been called)
+static void decompress_astc_block(uint8_t* dst, int dstRowBytes,
+                                  const ASTCDecompressionData &data) {
+    if (data.fError) {
+        write_error_color(dst, data.fDimX, data.fDimY, dstRowBytes);
+        return;
+    }
+

[robertphillips, 2014/08/06 17:24:21]

space after "if" ?

[krajcevski, 2014/08/06 18:14:30]

Done.

+    if(data.fVoidExtent) {
+        decompress_void_extent(dst, dstRowBytes, data);
+        return;
+    }
+
+    // According to the spec, any more than 64 values is illegal. (C.2.24)
+    static const int kMaxTexelValues = 64;
+
+    // Decode the texel weights.
+    int texelValues[kMaxTexelValues];
+    bool success = decode_integer_sequence(
+        texelValues, kMaxTexelValues, data.numWeights(),
+        // texel data goes to the end of the 128 bit block.
+        data.fBlock, 128, 128 - data.numWeightBits(), false,
+        data.fWeightBits, data.fWeightTrits, data.fWeightQuints);
+
+    if (!success) {
+        write_error_color(dst, data.fDimX, data.fDimY, dstRowBytes);
+        return;
+    }
+
+    // Decode the color endpoints
+    int colorBits, colorTrits, colorQuints;
+    if (!data.getColorValueEncoding(&colorBits, &colorTrits, &colorQuints)) {
+        write_error_color(dst, data.fDimX, data.fDimY, dstRowBytes);
+        return;
+    }
+
+    // According to the spec, any more than 18 color values is illegal. (C.2.24)
+    static const int kMaxColorValues = 18;
+
+    int colorValues[kMaxColorValues];
+    success = decode_integer_sequence(
+        colorValues, kMaxColorValues, data.numColorValues(),
+        data.fBlock, data.fColorStartBit, data.fColorEndBit, true,
+        colorBits, colorTrits, colorQuints);
+
+    if (!success) {
+        write_error_color(dst, data.fDimX, data.fDimY, dstRowBytes);
+        return;
+    }
+
+    // Unquantize the color values after they've been decoded.
+    unquantize_colors(colorValues, data.numColorValues(), colorBits, colorTrits, colorQuints);
+
+    // Decode the colors into the appropriate endpoints.
+    SkColor endpoints[4][2];
+    data.colorEndpoints(endpoints, colorValues);
+
+    // Do texel infill and decode the texel values.
+    int texelWeights[2][12][12];
+    data.texelWeights(texelWeights, texelValues);
+
+    // Write the texels by interpolating them based on the information
+    // stored in the block.
+    dst += data.fDimY * dstRowBytes;
+    for (int y = 0; y < data.fDimY; ++y) {
+        dst -= dstRowBytes;
+        SkColor* colorPtr = reinterpret_cast<SkColor*>(dst);
+        for (int x = 0; x < data.fDimX; ++x) {
+            colorPtr[x] = data.getTexel(endpoints, texelWeights, x, y);
+        }
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
 
 namespace SkTextureCompressor {
 
-bool CompressA8To12x12ASTC(uint8_t* dst, const uint8_t* src, int width, int height, int rowBytes) {
+bool CompressA8To12x12ASTC(uint8_t* dst, const uint8_t* src,
+                           int width, int height, int rowBytes) {
     if (width < 0 || ((width % 12) != 0) || height < 0 || ((height % 12) != 0)) {
         return false;
     }
 
     uint8_t** dstPtr = &dst;
     for (int y = 0; y < height; y += 12) {
         for (int x = 0; x < width; x += 12) {
             compress_a8_astc_block<GetAlpha>(dstPtr, src + y*rowBytes + x, rowBytes);
         }
     }
 
     return true;
 }
 
 SkBlitter* CreateASTCBlitter(int width, int height, void* outputBuffer) {
     return new
         SkTCompressedAlphaBlitter<12, 16, CompressA8ASTCBlockVertical>
         (width, height, outputBuffer);
 }
 
+void DecompressASTC(uint8_t* dst, int dstRowBytes, const uint8_t* src,
+                    int width, int height, int blockDimX, int blockDimY) {
+    // ASTC is encoded in what they call "raster order", so that the first
+    // block is the bottom-left block in the image, and the first pixel
+    // is the bottom-left pixel of the image
+    dst += height * dstRowBytes;
+
+    ASTCDecompressionData data(blockDimX, blockDimY);

[robertphillips, 2014/08/06 17:24:20]

i,j -> x,y ?

[krajcevski, 2014/08/06 18:14:29]

Done.

+    for (int j = 0; j < height; j += blockDimY) {
+        dst -= blockDimY * dstRowBytes;
+        SkColor *colorPtr = reinterpret_cast<SkColor*>(dst);
+        for (int i = 0; i < width; i += blockDimX) {
+            read_astc_block(&data, src);
+            decompress_astc_block(reinterpret_cast<uint8_t*>(colorPtr + i), dstRowBytes, data);
+
+            // ASTC encoded blocks are 16 bytes (128 bits) large.
+            src += 16;
+        }
+    }
+}
+
 }  // SkTextureCompressor