Refactor texture compressors into separate files

src/utils/SkTextureCompressor_LATC.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_LATC.h"
+
+#include "SkEndian.h"
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// Utility Functions
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// Absolute difference between two values. More correct than SkTAbs(a - b)
+// because it works on unsigned values.
+template <typename T> inline T abs_diff(const T &a, const T &b) {
+    return (a > b) ? (a - b) : (b - a);
+}
+
+static bool is_extremal(uint8_t pixel) {
+    return 0 == pixel || 255 == pixel;
+}
+
+typedef uint64_t (*A84x4To64BitProc)(const uint8_t block[]);
+
+// This function is used by both R11 EAC and LATC to compress 4x4 blocks
+// of 8-bit alpha into 64-bit values that comprise the compressed data.
+// For both formats, we need to make sure that the dimensions of the 
+// src pixels are divisible by 4, and copy 4x4 blocks one at a time
+// for compression.
+static bool compress_4x4_a8_to_64bit(uint8_t* dst, const uint8_t* src,
+                                     int width, int height, int rowBytes,
+                                     A84x4To64BitProc proc) {
+    // Make sure that our data is well-formed enough to be considered for compression
+    if (0 == width || 0 == height || (width % 4) != 0 || (height % 4) != 0) {
+        return false;
+    }
+
+    int blocksX = width >> 2;
+    int blocksY = height >> 2;
+
+    uint8_t block[16];
+    uint64_t* encPtr = reinterpret_cast<uint64_t*>(dst);
+    for (int y = 0; y < blocksY; ++y) {
+        for (int x = 0; x < blocksX; ++x) {
+            // Load block
+            for (int k = 0; k < 4; ++k) {
+                memcpy(block + k*4, src + k*rowBytes + 4*x, 4);
+            }
+
+            // Compress it
+            *encPtr = proc(block);
+            ++encPtr;
+        }
+        src += 4 * rowBytes;
+    }
+
+    return true;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+//
+// LATC compressor
+//
+////////////////////////////////////////////////////////////////////////////////
+
+// LATC compressed texels down into square 4x4 blocks
+static const int kLATCPaletteSize = 8;
+static const int kLATCBlockSize = 4;
+static const int kLATCPixelsPerBlock = kLATCBlockSize * kLATCBlockSize;
+
+// Generates an LATC palette. LATC constructs
+// a palette of eight colors from LUM0 and LUM1 using the algorithm:
+//
+// LUM0,              if lum0 > lum1 and code(x,y) == 0
+// LUM1,              if lum0 > lum1 and code(x,y) == 1
+// (6*LUM0+  LUM1)/7, if lum0 > lum1 and code(x,y) == 2
+// (5*LUM0+2*LUM1)/7, if lum0 > lum1 and code(x,y) == 3
+// (4*LUM0+3*LUM1)/7, if lum0 > lum1 and code(x,y) == 4
+// (3*LUM0+4*LUM1)/7, if lum0 > lum1 and code(x,y) == 5
+// (2*LUM0+5*LUM1)/7, if lum0 > lum1 and code(x,y) == 6
+// (  LUM0+6*LUM1)/7, if lum0 > lum1 and code(x,y) == 7
+//
+// LUM0,              if lum0 <= lum1 and code(x,y) == 0
+// LUM1,              if lum0 <= lum1 and code(x,y) == 1
+// (4*LUM0+  LUM1)/5, if lum0 <= lum1 and code(x,y) == 2
+// (3*LUM0+2*LUM1)/5, if lum0 <= lum1 and code(x,y) == 3
+// (2*LUM0+3*LUM1)/5, if lum0 <= lum1 and code(x,y) == 4
+// (  LUM0+4*LUM1)/5, if lum0 <= lum1 and code(x,y) == 5
+// 0,                 if lum0 <= lum1 and code(x,y) == 6
+// 255,               if lum0 <= lum1 and code(x,y) == 7
+
+static void generate_latc_palette(uint8_t palette[], uint8_t lum0, uint8_t lum1) {
+    palette[0] = lum0;
+    palette[1] = lum1;
+    if (lum0 > lum1) {
+        for (int i = 1; i < 7; i++) {
+            palette[i+1] = ((7-i)*lum0 + i*lum1) / 7;
+        }
+    } else {
+        for (int i = 1; i < 5; i++) {
+            palette[i+1] = ((5-i)*lum0 + i*lum1) / 5;
+        }
+        palette[6] = 0;
+        palette[7] = 255;
+    }
+}
+
+// Compress a block by using the bounding box of the pixels. It is assumed that
+// there are no extremal pixels in this block otherwise we would have used
+// compressBlockBBIgnoreExtremal.
+static uint64_t compress_latc_block_bb(const uint8_t pixels[]) {
+    uint8_t minVal = 255;
+    uint8_t maxVal = 0;
+    for (int i = 0; i < kLATCPixelsPerBlock; ++i) {
+        minVal = SkTMin(pixels[i], minVal);
+        maxVal = SkTMax(pixels[i], maxVal);
+    }
+
+    SkASSERT(!is_extremal(minVal));
+    SkASSERT(!is_extremal(maxVal));
+
+    uint8_t palette[kLATCPaletteSize];
+    generate_latc_palette(palette, maxVal, minVal);
+
+    uint64_t indices = 0;
+    for (int i = kLATCPixelsPerBlock - 1; i >= 0; --i) {
+
+        // Find the best palette index
+        uint8_t bestError = abs_diff(pixels[i], palette[0]);
+        uint8_t idx = 0;
+        for (int j = 1; j < kLATCPaletteSize; ++j) {
+            uint8_t error = abs_diff(pixels[i], palette[j]);
+            if (error < bestError) {
+                bestError = error;
+                idx = j;
+            }
+        }
+
+        indices <<= 3;
+        indices |= idx;
+    }
+
+    return
+        SkEndian_SwapLE64(
+            static_cast<uint64_t>(maxVal) |
+            (static_cast<uint64_t>(minVal) << 8) |
+            (indices << 16));
+}
+
+// Compress a block by using the bounding box of the pixels without taking into
+// account the extremal values. The generated palette will contain extremal values
+// and fewer points along the line segment to interpolate.
+static uint64_t compress_latc_block_bb_ignore_extremal(const uint8_t pixels[]) {
+    uint8_t minVal = 255;
+    uint8_t maxVal = 0;
+    for (int i = 0; i < kLATCPixelsPerBlock; ++i) {
+        if (is_extremal(pixels[i])) {
+            continue;
+        }
+
+        minVal = SkTMin(pixels[i], minVal);
+        maxVal = SkTMax(pixels[i], maxVal);
+    }
+
+    SkASSERT(!is_extremal(minVal));
+    SkASSERT(!is_extremal(maxVal));
+
+    uint8_t palette[kLATCPaletteSize];
+    generate_latc_palette(palette, minVal, maxVal);
+
+    uint64_t indices = 0;
+    for (int i = kLATCPixelsPerBlock - 1; i >= 0; --i) {
+
+        // Find the best palette index
+        uint8_t idx = 0;
+        if (is_extremal(pixels[i])) {
+            if (0xFF == pixels[i]) {
+                idx = 7;
+            } else if (0 == pixels[i]) {
+                idx = 6;
+            } else {
+                SkFAIL("Pixel is extremal but not really?!");
+            }
+        } else {
+            uint8_t bestError = abs_diff(pixels[i], palette[0]);
+            for (int j = 1; j < kLATCPaletteSize - 2; ++j) {
+                uint8_t error = abs_diff(pixels[i], palette[j]);
+                if (error < bestError) {
+                    bestError = error;
+                    idx = j;
+                }
+            }
+        }
+
+        indices <<= 3;
+        indices |= idx;
+    }
+
+    return
+        SkEndian_SwapLE64(
+            static_cast<uint64_t>(minVal) |
+            (static_cast<uint64_t>(maxVal) << 8) |
+            (indices << 16));
+}
+
+
+// Compress LATC block. Each 4x4 block of pixels is decompressed by LATC from two
+// values LUM0 and LUM1, and an index into the generated palette. Details of how
+// the palette is generated can be found in the comments of generatePalette above.
+//
+// We choose which palette type to use based on whether or not 'pixels' contains
+// any extremal values (0 or 255). If there are extremal values, then we use the
+// palette that has the extremal values built in. Otherwise, we use the full bounding
+// box.
+
+static uint64_t compress_latc_block(const uint8_t pixels[]) {
+    // Collect unique pixels
+    int nUniquePixels = 0;
+    uint8_t uniquePixels[kLATCPixelsPerBlock];
+    for (int i = 0; i < kLATCPixelsPerBlock; ++i) {
+        bool foundPixel = false;
+        for (int j = 0; j < nUniquePixels; ++j) {
+            foundPixel = foundPixel || uniquePixels[j] == pixels[i];
+        }
+
+        if (!foundPixel) {
+            uniquePixels[nUniquePixels] = pixels[i];
+            ++nUniquePixels;
+        }
+    }
+
+    // If there's only one unique pixel, then our compression is easy.
+    if (1 == nUniquePixels) {
+        return SkEndian_SwapLE64(pixels[0] | (pixels[0] << 8));
+
+    // Similarly, if there are only two unique pixels, then our compression is
+    // easy again: place the pixels in the block header, and assign the indices
+    // with one or zero depending on which pixel they belong to.
+    } else if (2 == nUniquePixels) {
+        uint64_t outBlock = 0;
+        for (int i = kLATCPixelsPerBlock - 1; i >= 0; --i) {
+            int idx = 0;
+            if (pixels[i] == uniquePixels[1]) {
+                idx = 1;
+            }
+
+            outBlock <<= 3;
+            outBlock |= idx;
+        }
+        outBlock <<= 16;
+        outBlock |= (uniquePixels[0] | (uniquePixels[1] << 8));
+        return SkEndian_SwapLE64(outBlock);
+    }
+
+    // Count non-maximal pixel values
+    int nonExtremalPixels = 0;
+    for (int i = 0; i < nUniquePixels; ++i) {
+        if (!is_extremal(uniquePixels[i])) {
+            ++nonExtremalPixels;
+        }
+    }
+
+    // If all the pixels are nonmaximal then compute the palette using
+    // the bounding box of all the pixels.
+    if (nonExtremalPixels == nUniquePixels) {
+        // This is really just for correctness, in all of my tests we
+        // never take this step. We don't lose too much perf here because
+        // most of the processing in this function is worth it for the 
+        // 1 == nUniquePixels optimization.
+        return compress_latc_block_bb(pixels);
+    } else {
+        return compress_latc_block_bb_ignore_extremal(pixels);
+    }
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+namespace SkTextureCompressor {
+
+bool CompressA8ToLATC(uint8_t* dst, const uint8_t* src, int width, int height, int rowBytes) {
+    return compress_4x4_a8_to_64bit(dst, src, width, height, rowBytes, compress_latc_block);
+}
+
+SkBlitter* CreateLATCBlitter(int width, int height, void* outputBuffer) {
+    // TODO (krajcevski)
+    return NULL;
+}
+
+}  // SkTextureCompressor