Simple PKM image decoder.

third_party/externals/etc1/etc1.cpp

+// Copyright 2009 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+

[robertphillips, 2014/05/22 14:41:35]

Add fork comment?

[krajcevski, 2014/05/22 15:32:17]

Done.

+#include <etc1.h>
+
+#include <string.h>
+
+/* From http://www.khronos.org/registry/gles/extensions/OES/OES_compressed_ETC1_RGB8_texture.txt
+
+ The number of bits that represent a 4x4 texel block is 64 bits if
+ <internalformat> is given by ETC1_RGB8_OES.
+
+ The data for a block is a number of bytes,
+
+ {q0, q1, q2, q3, q4, q5, q6, q7}
+
+ where byte q0 is located at the lowest memory address and q7 at
+ the highest. The 64 bits specifying the block is then represented
+ by the following 64 bit integer:
+
+ int64bit = 256*(256*(256*(256*(256*(256*(256*q0+q1)+q2)+q3)+q4)+q5)+q6)+q7;
+
+ ETC1_RGB8_OES:
+
+ a) bit layout in bits 63 through 32 if diffbit = 0
+
+ 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48
+ -----------------------------------------------
+ | base col1 | base col2 | base col1 | base col2 |
+ | R1 (4bits)| R2 (4bits)| G1 (4bits)| G2 (4bits)|
+ -----------------------------------------------
+
+ 47 46 45 44 43 42 41 40 39 38 37 36 35 34  33  32
+ ---------------------------------------------------
+ | base col1 | base col2 | table  | table  |diff|flip|
+ | B1 (4bits)| B2 (4bits)| cw 1   | cw 2   |bit |bit |
+ ---------------------------------------------------
+
+
+ b) bit layout in bits 63 through 32 if diffbit = 1
+
+ 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48
+ -----------------------------------------------
+ | base col1    | dcol 2 | base col1    | dcol 2 |
+ | R1' (5 bits) | dR2    | G1' (5 bits) | dG2    |
+ -----------------------------------------------
+
+ 47 46 45 44 43 42 41 40 39 38 37 36 35 34  33  32
+ ---------------------------------------------------
+ | base col 1   | dcol 2 | table  | table  |diff|flip|
+ | B1' (5 bits) | dB2    | cw 1   | cw 2   |bit |bit |
+ ---------------------------------------------------
+
+
+ c) bit layout in bits 31 through 0 (in both cases)
+
+ 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16
+ -----------------------------------------------
+ |       most significant pixel index bits       |
+ | p| o| n| m| l| k| j| i| h| g| f| e| d| c| b| a|
+ -----------------------------------------------
+
+ 15 14 13 12 11 10  9  8  7  6  5  4  3   2   1  0
+ --------------------------------------------------
+ |         least significant pixel index bits       |
+ | p| o| n| m| l| k| j| i| h| g| f| e| d| c | b | a |
+ --------------------------------------------------
+
+
+ Add table 3.17.2: Intensity modifier sets for ETC1 compressed textures:
+
+ table codeword                modifier table
+ ------------------        ----------------------
+ 0                     -8  -2  2   8
+ 1                    -17  -5  5  17
+ 2                    -29  -9  9  29
+ 3                    -42 -13 13  42
+ 4                    -60 -18 18  60
+ 5                    -80 -24 24  80
+ 6                   -106 -33 33 106
+ 7                   -183 -47 47 183
+
+
+ Add table 3.17.3 Mapping from pixel index values to modifier values for
+ ETC1 compressed textures:
+
+ pixel index value
+ ---------------
+ msb     lsb           resulting modifier value
+ -----   -----          -------------------------
+ 1       1            -b (large negative value)
+ 1       0            -a (small negative value)
+ 0       0             a (small positive value)
+ 0       1             b (large positive value)
+
+
+ */
+
+static const int kModifierTable[] = {
+/* 0 */2, 8, -2, -8,
+/* 1 */5, 17, -5, -17,
+/* 2 */9, 29, -9, -29,
+/* 3 */13, 42, -13, -42,
+/* 4 */18, 60, -18, -60,
+/* 5 */24, 80, -24, -80,
+/* 6 */33, 106, -33, -106,
+/* 7 */47, 183, -47, -183 };
+
+static const int kLookup[8] = { 0, 1, 2, 3, -4, -3, -2, -1 };
+
+static inline etc1_byte clamp(int x) {
+    return (etc1_byte) (x >= 0 ? (x < 255 ? x : 255) : 0);
+}
+
+static
+inline int convert4To8(int b) {
+    int c = b & 0xf;
+    return (c << 4) | c;
+}
+
+static
+inline int convert5To8(int b) {
+    int c = b & 0x1f;
+    return (c << 3) | (c >> 2);
+}
+
+static
+inline int convert6To8(int b) {
+    int c = b & 0x3f;
+    return (c << 2) | (c >> 4);
+}
+
+static
+inline int divideBy255(int d) {
+    return (d + 128 + (d >> 8)) >> 8;
+}
+
+static
+inline int convert8To4(int b) {
+    int c = b & 0xff;
+    return divideBy255(c * 15);
+}
+
+static
+inline int convert8To5(int b) {
+    int c = b & 0xff;
+    return divideBy255(c * 31);
+}
+
+static
+inline int convertDiff(int base, int diff) {
+    return convert5To8((0x1f & base) + kLookup[0x7 & diff]);
+}
+
+static
+void decode_subblock(etc1_byte* pOut, int r, int g, int b, const int* table,
+        etc1_uint32 low, bool second, bool flipped) {
+    int baseX = 0;
+    int baseY = 0;
+    if (second) {
+        if (flipped) {
+            baseY = 2;
+        } else {
+            baseX = 2;
+        }
+    }
+    for (int i = 0; i < 8; i++) {
+        int x, y;
+        if (flipped) {
+            x = baseX + (i >> 1);
+            y = baseY + (i & 1);
+        } else {
+            x = baseX + (i >> 2);
+            y = baseY + (i & 3);
+        }
+        int k = y + (x * 4);
+        int offset = ((low >> k) & 1) | ((low >> (k + 15)) & 2);
+        int delta = table[offset];
+        etc1_byte* q = pOut + 3 * (x + 4 * y);
+        *q++ = clamp(r + delta);
+        *q++ = clamp(g + delta);
+        *q++ = clamp(b + delta);
+    }
+}
+
+// Input is an ETC1 compressed version of the data.
+// Output is a 4 x 4 square of 3-byte pixels in form R, G, B
+
+void etc1_decode_block(const etc1_byte* pIn, etc1_byte* pOut) {
+    etc1_uint32 high = (pIn[0] << 24) | (pIn[1] << 16) | (pIn[2] << 8) | pIn[3];
+    etc1_uint32 low = (pIn[4] << 24) | (pIn[5] << 16) | (pIn[6] << 8) | pIn[7];
+    int r1, r2, g1, g2, b1, b2;
+    if (high & 2) {
+        // differential
+        int rBase = high >> 27;
+        int gBase = high >> 19;
+        int bBase = high >> 11;
+        r1 = convert5To8(rBase);
+        r2 = convertDiff(rBase, high >> 24);
+        g1 = convert5To8(gBase);
+        g2 = convertDiff(gBase, high >> 16);
+        b1 = convert5To8(bBase);
+        b2 = convertDiff(bBase, high >> 8);
+    } else {
+        // not differential
+        r1 = convert4To8(high >> 28);
+        r2 = convert4To8(high >> 24);
+        g1 = convert4To8(high >> 20);
+        g2 = convert4To8(high >> 16);
+        b1 = convert4To8(high >> 12);
+        b2 = convert4To8(high >> 8);
+    }
+    int tableIndexA = 7 & (high >> 5);
+    int tableIndexB = 7 & (high >> 2);
+    const int* tableA = kModifierTable + tableIndexA * 4;
+    const int* tableB = kModifierTable + tableIndexB * 4;
+    bool flipped = (high & 1) != 0;
+    decode_subblock(pOut, r1, g1, b1, tableA, low, false, flipped);
+    decode_subblock(pOut, r2, g2, b2, tableB, low, true, flipped);
+}
+
+typedef struct {
+    etc1_uint32 high;
+    etc1_uint32 low;
+    etc1_uint32 score; // Lower is more accurate
+} etc_compressed;
+
+static
+inline void take_best(etc_compressed* a, const etc_compressed* b) {
+    if (a->score > b->score) {
+        *a = *b;
+    }
+}
+
+static
+void etc_average_colors_subblock(const etc1_byte* pIn, etc1_uint32 inMask,
+        etc1_byte* pColors, bool flipped, bool second) {
+    int r = 0;
+    int g = 0;
+    int b = 0;
+
+    if (flipped) {
+        int by = 0;
+        if (second) {
+            by = 2;
+        }
+        for (int y = 0; y < 2; y++) {
+            int yy = by + y;
+            for (int x = 0; x < 4; x++) {
+                int i = x + 4 * yy;
+                if (inMask & (1 << i)) {
+                    const etc1_byte* p = pIn + i * 3;
+                    r += *(p++);
+                    g += *(p++);
+                    b += *(p++);
+                }
+            }
+        }
+    } else {
+        int bx = 0;
+        if (second) {
+            bx = 2;
+        }
+        for (int y = 0; y < 4; y++) {
+            for (int x = 0; x < 2; x++) {
+                int xx = bx + x;
+                int i = xx + 4 * y;
+                if (inMask & (1 << i)) {
+                    const etc1_byte* p = pIn + i * 3;
+                    r += *(p++);
+                    g += *(p++);
+                    b += *(p++);
+                }
+            }
+        }
+    }
+    pColors[0] = (etc1_byte)((r + 4) >> 3);
+    pColors[1] = (etc1_byte)((g + 4) >> 3);
+    pColors[2] = (etc1_byte)((b + 4) >> 3);
+}
+
+static
+inline int square(int x) {
+    return x * x;
+}
+
+static etc1_uint32 chooseModifier(const etc1_byte* pBaseColors,
+        const etc1_byte* pIn, etc1_uint32 *pLow, int bitIndex,
+        const int* pModifierTable) {
+    etc1_uint32 bestScore = ~0;
+    int bestIndex = 0;
+    int pixelR = pIn[0];
+    int pixelG = pIn[1];
+    int pixelB = pIn[2];
+    int r = pBaseColors[0];
+    int g = pBaseColors[1];
+    int b = pBaseColors[2];
+    for (int i = 0; i < 4; i++) {
+        int modifier = pModifierTable[i];
+        int decodedG = clamp(g + modifier);
+        etc1_uint32 score = (etc1_uint32) (6 * square(decodedG - pixelG));
+        if (score >= bestScore) {
+            continue;
+        }
+        int decodedR = clamp(r + modifier);
+        score += (etc1_uint32) (3 * square(decodedR - pixelR));
+        if (score >= bestScore) {
+            continue;
+        }
+        int decodedB = clamp(b + modifier);
+        score += (etc1_uint32) square(decodedB - pixelB);
+        if (score < bestScore) {
+            bestScore = score;
+            bestIndex = i;
+        }
+    }
+    etc1_uint32 lowMask = (((bestIndex >> 1) << 16) | (bestIndex & 1))
+            << bitIndex;
+    *pLow |= lowMask;
+    return bestScore;
+}
+
+static
+void etc_encode_subblock_helper(const etc1_byte* pIn, etc1_uint32 inMask,
+        etc_compressed* pCompressed, bool flipped, bool second,
+        const etc1_byte* pBaseColors, const int* pModifierTable) {
+    int score = pCompressed->score;
+    if (flipped) {
+        int by = 0;
+        if (second) {
+            by = 2;
+        }
+        for (int y = 0; y < 2; y++) {
+            int yy = by + y;
+            for (int x = 0; x < 4; x++) {
+                int i = x + 4 * yy;
+                if (inMask & (1 << i)) {
+                    score += chooseModifier(pBaseColors, pIn + i * 3,
+                            &pCompressed->low, yy + x * 4, pModifierTable);
+                }
+            }
+        }
+    } else {
+        int bx = 0;
+        if (second) {
+            bx = 2;
+        }
+        for (int y = 0; y < 4; y++) {
+            for (int x = 0; x < 2; x++) {
+                int xx = bx + x;
+                int i = xx + 4 * y;
+                if (inMask & (1 << i)) {
+                    score += chooseModifier(pBaseColors, pIn + i * 3,
+                            &pCompressed->low, y + xx * 4, pModifierTable);
+                }
+            }
+        }
+    }
+    pCompressed->score = score;
+}
+
+static bool inRange4bitSigned(int color) {
+    return color >= -4 && color <= 3;
+}
+
+static void etc_encodeBaseColors(etc1_byte* pBaseColors,
+        const etc1_byte* pColors, etc_compressed* pCompressed) {
+    int r1, g1, b1, r2, g2, b2; // 8 bit base colors for sub-blocks
+    bool differential;
+    {
+        int r51 = convert8To5(pColors[0]);
+        int g51 = convert8To5(pColors[1]);
+        int b51 = convert8To5(pColors[2]);
+        int r52 = convert8To5(pColors[3]);
+        int g52 = convert8To5(pColors[4]);
+        int b52 = convert8To5(pColors[5]);
+
+        r1 = convert5To8(r51);
+        g1 = convert5To8(g51);
+        b1 = convert5To8(b51);
+
+        int dr = r52 - r51;
+        int dg = g52 - g51;
+        int db = b52 - b51;
+
+        differential = inRange4bitSigned(dr) && inRange4bitSigned(dg)
+                && inRange4bitSigned(db);
+        if (differential) {
+            r2 = convert5To8(r51 + dr);
+            g2 = convert5To8(g51 + dg);
+            b2 = convert5To8(b51 + db);
+            pCompressed->high |= (r51 << 27) | ((7 & dr) << 24) | (g51 << 19)
+                    | ((7 & dg) << 16) | (b51 << 11) | ((7 & db) << 8) | 2;
+        }
+    }
+
+    if (!differential) {
+        int r41 = convert8To4(pColors[0]);
+        int g41 = convert8To4(pColors[1]);
+        int b41 = convert8To4(pColors[2]);
+        int r42 = convert8To4(pColors[3]);
+        int g42 = convert8To4(pColors[4]);
+        int b42 = convert8To4(pColors[5]);
+        r1 = convert4To8(r41);
+        g1 = convert4To8(g41);
+        b1 = convert4To8(b41);
+        r2 = convert4To8(r42);
+        g2 = convert4To8(g42);
+        b2 = convert4To8(b42);
+        pCompressed->high |= (r41 << 28) | (r42 << 24) | (g41 << 20) | (g42
+                << 16) | (b41 << 12) | (b42 << 8);
+    }
+    pBaseColors[0] = r1;
+    pBaseColors[1] = g1;
+    pBaseColors[2] = b1;
+    pBaseColors[3] = r2;
+    pBaseColors[4] = g2;
+    pBaseColors[5] = b2;
+}
+
+static
+void etc_encode_block_helper(const etc1_byte* pIn, etc1_uint32 inMask,
+        const etc1_byte* pColors, etc_compressed* pCompressed, bool flipped) {
+    pCompressed->score = ~0;
+    pCompressed->high = (flipped ? 1 : 0);
+    pCompressed->low = 0;
+
+    etc1_byte pBaseColors[6];
+
+    etc_encodeBaseColors(pBaseColors, pColors, pCompressed);
+
+    int originalHigh = pCompressed->high;
+
+    const int* pModifierTable = kModifierTable;
+    for (int i = 0; i < 8; i++, pModifierTable += 4) {
+        etc_compressed temp;
+        temp.score = 0;
+        temp.high = originalHigh | (i << 5);
+        temp.low = 0;
+        etc_encode_subblock_helper(pIn, inMask, &temp, flipped, false,
+                pBaseColors, pModifierTable);
+        take_best(pCompressed, &temp);
+    }
+    pModifierTable = kModifierTable;
+    etc_compressed firstHalf = *pCompressed;
+    for (int i = 0; i < 8; i++, pModifierTable += 4) {
+        etc_compressed temp;
+        temp.score = firstHalf.score;
+        temp.high = firstHalf.high | (i << 2);
+        temp.low = firstHalf.low;
+        etc_encode_subblock_helper(pIn, inMask, &temp, flipped, true,
+                pBaseColors + 3, pModifierTable);
+        if (i == 0) {
+            *pCompressed = temp;
+        } else {
+            take_best(pCompressed, &temp);
+        }
+    }
+}
+
+static void writeBigEndian(etc1_byte* pOut, etc1_uint32 d) {
+    pOut[0] = (etc1_byte)(d >> 24);
+    pOut[1] = (etc1_byte)(d >> 16);
+    pOut[2] = (etc1_byte)(d >> 8);
+    pOut[3] = (etc1_byte) d;
+}
+
+// Input is a 4 x 4 square of 3-byte pixels in form R, G, B
+// inmask is a 16-bit mask where bit (1 << (x + y * 4)) tells whether the corresponding (x,y)
+// pixel is valid or not. Invalid pixel color values are ignored when compressing.
+// Output is an ETC1 compressed version of the data.
+
+void etc1_encode_block(const etc1_byte* pIn, etc1_uint32 inMask,
+        etc1_byte* pOut) {
+    etc1_byte colors[6];
+    etc1_byte flippedColors[6];
+    etc_average_colors_subblock(pIn, inMask, colors, false, false);
+    etc_average_colors_subblock(pIn, inMask, colors + 3, false, true);
+    etc_average_colors_subblock(pIn, inMask, flippedColors, true, false);
+    etc_average_colors_subblock(pIn, inMask, flippedColors + 3, true, true);
+
+    etc_compressed a, b;
+    etc_encode_block_helper(pIn, inMask, colors, &a, false);
+    etc_encode_block_helper(pIn, inMask, flippedColors, &b, true);
+    take_best(&a, &b);
+    writeBigEndian(pOut, a.high);
+    writeBigEndian(pOut + 4, a.low);
+}
+
+// Return the size of the encoded image data (does not include size of PKM header).
+
+etc1_uint32 etc1_get_encoded_data_size(etc1_uint32 width, etc1_uint32 height) {
+    return (((width + 3) & ~3) * ((height + 3) & ~3)) >> 1;
+}
+
+// Encode an entire image.
+// pIn - pointer to the image data. Formatted such that the Red component of
+//       pixel (x,y) is at pIn + pixelSize * x + stride * y + redOffset;
+// pOut - pointer to encoded data. Must be large enough to store entire encoded image.
+
+int etc1_encode_image(const etc1_byte* pIn, etc1_uint32 width, etc1_uint32 height,
+        etc1_uint32 pixelSize, etc1_uint32 stride, etc1_byte* pOut) {
+    if (pixelSize < 2 || pixelSize > 3) {
+        return -1;
+    }
+    static const unsigned short kYMask[] = { 0x0, 0xf, 0xff, 0xfff, 0xffff };
+    static const unsigned short kXMask[] = { 0x0, 0x1111, 0x3333, 0x7777,
+            0xffff };
+    etc1_byte block[ETC1_DECODED_BLOCK_SIZE];
+    etc1_byte encoded[ETC1_ENCODED_BLOCK_SIZE];
+
+    etc1_uint32 encodedWidth = (width + 3) & ~3;
+    etc1_uint32 encodedHeight = (height + 3) & ~3;
+
+    for (etc1_uint32 y = 0; y < encodedHeight; y += 4) {
+        etc1_uint32 yEnd = height - y;
+        if (yEnd > 4) {
+            yEnd = 4;
+        }
+        int ymask = kYMask[yEnd];
+        for (etc1_uint32 x = 0; x < encodedWidth; x += 4) {
+            etc1_uint32 xEnd = width - x;
+            if (xEnd > 4) {
+                xEnd = 4;
+            }
+            int mask = ymask & kXMask[xEnd];
+            for (etc1_uint32 cy = 0; cy < yEnd; cy++) {
+                etc1_byte* q = block + (cy * 4) * 3;
+                const etc1_byte* p = pIn + pixelSize * x + stride * (y + cy);
+                if (pixelSize == 3) {
+                    memcpy(q, p, xEnd * 3);
+                } else {
+                    for (etc1_uint32 cx = 0; cx < xEnd; cx++) {
+                        int pixel = (p[1] << 8) | p[0];
+                        *q++ = convert5To8(pixel >> 11);
+                        *q++ = convert6To8(pixel >> 5);
+                        *q++ = convert5To8(pixel);
+                        p += pixelSize;
+                    }
+                }
+            }
+            etc1_encode_block(block, mask, encoded);
+            memcpy(pOut, encoded, sizeof(encoded));
+            pOut += sizeof(encoded);
+        }
+    }
+    return 0;
+}
+
+// Decode an entire image.
+// pIn - pointer to encoded data.
+// pOut - pointer to the image data. Will be written such that the Red component of
+//       pixel (x,y) is at pIn + pixelSize * x + stride * y + redOffset. Must be
+//        large enough to store entire image.
+
+
+int etc1_decode_image(const etc1_byte* pIn, etc1_byte* pOut,
+        etc1_uint32 width, etc1_uint32 height,
+        etc1_uint32 pixelSize, etc1_uint32 stride) {
+    if (pixelSize < 2 || pixelSize > 3) {
+        return -1;
+    }
+    etc1_byte block[ETC1_DECODED_BLOCK_SIZE];
+
+    etc1_uint32 encodedWidth = (width + 3) & ~3;
+    etc1_uint32 encodedHeight = (height + 3) & ~3;
+
+    for (etc1_uint32 y = 0; y < encodedHeight; y += 4) {
+        etc1_uint32 yEnd = height - y;
+        if (yEnd > 4) {
+            yEnd = 4;
+        }
+        for (etc1_uint32 x = 0; x < encodedWidth; x += 4) {
+            etc1_uint32 xEnd = width - x;
+            if (xEnd > 4) {
+                xEnd = 4;
+            }
+            etc1_decode_block(pIn, block);
+            pIn += ETC1_ENCODED_BLOCK_SIZE;
+            for (etc1_uint32 cy = 0; cy < yEnd; cy++) {
+                const etc1_byte* q = block + (cy * 4) * 3;
+                etc1_byte* p = pOut + pixelSize * x + stride * (y + cy);
+                if (pixelSize == 3) {
+                    memcpy(p, q, xEnd * 3);
+                } else {
+                    for (etc1_uint32 cx = 0; cx < xEnd; cx++) {
+                        etc1_byte r = *q++;
+                        etc1_byte g = *q++;
+                        etc1_byte b = *q++;
+                        etc1_uint32 pixel = ((r >> 3) << 11) | ((g >> 2) << 5) | (b >> 3);
+                        *p++ = (etc1_byte) pixel;
+                        *p++ = (etc1_byte) (pixel >> 8);
+                    }
+                }
+            }
+        }
+    }
+    return 0;
+}
+
+static const char kMagic[] = { 'P', 'K', 'M', ' ', '1', '0' };
+
+static const etc1_uint32 ETC1_PKM_FORMAT_OFFSET = 6;
+static const etc1_uint32 ETC1_PKM_ENCODED_WIDTH_OFFSET = 8;
+static const etc1_uint32 ETC1_PKM_ENCODED_HEIGHT_OFFSET = 10;
+static const etc1_uint32 ETC1_PKM_WIDTH_OFFSET = 12;
+static const etc1_uint32 ETC1_PKM_HEIGHT_OFFSET = 14;
+
+static const etc1_uint32 ETC1_RGB_NO_MIPMAPS = 0;
+
+static void writeBEUint16(etc1_byte* pOut, etc1_uint32 data) {
+    pOut[0] = (etc1_byte) (data >> 8);
+    pOut[1] = (etc1_byte) data;
+}
+
+static etc1_uint32 readBEUint16(const etc1_byte* pIn) {
+    return (pIn[0] << 8) | pIn[1];
+}
+
+// Format a PKM header
+
+void etc1_pkm_format_header(etc1_byte* pHeader, etc1_uint32 width, etc1_uint32 height) {
+    memcpy(pHeader, kMagic, sizeof(kMagic));
+    etc1_uint32 encodedWidth = (width + 3) & ~3;
+    etc1_uint32 encodedHeight = (height + 3) & ~3;
+    writeBEUint16(pHeader + ETC1_PKM_FORMAT_OFFSET, ETC1_RGB_NO_MIPMAPS);
+    writeBEUint16(pHeader + ETC1_PKM_ENCODED_WIDTH_OFFSET, encodedWidth);
+    writeBEUint16(pHeader + ETC1_PKM_ENCODED_HEIGHT_OFFSET, encodedHeight);
+    writeBEUint16(pHeader + ETC1_PKM_WIDTH_OFFSET, width);
+    writeBEUint16(pHeader + ETC1_PKM_HEIGHT_OFFSET, height);
+}
+
+// Check if a PKM header is correctly formatted.
+
+etc1_bool etc1_pkm_is_valid(const etc1_byte* pHeader) {
+    if (memcmp(pHeader, kMagic, sizeof(kMagic))) {
+        return false;
+    }
+    etc1_uint32 format = readBEUint16(pHeader + ETC1_PKM_FORMAT_OFFSET);
+    etc1_uint32 encodedWidth = readBEUint16(pHeader + ETC1_PKM_ENCODED_WIDTH_OFFSET);
+    etc1_uint32 encodedHeight = readBEUint16(pHeader + ETC1_PKM_ENCODED_HEIGHT_OFFSET);
+    etc1_uint32 width = readBEUint16(pHeader + ETC1_PKM_WIDTH_OFFSET);
+    etc1_uint32 height = readBEUint16(pHeader + ETC1_PKM_HEIGHT_OFFSET);
+    return format == ETC1_RGB_NO_MIPMAPS &&
+            encodedWidth >= width && encodedWidth - width < 4 &&
+            encodedHeight >= height && encodedHeight - height < 4;
+}
+
+// Read the image width from a PKM header
+
+etc1_uint32 etc1_pkm_get_width(const etc1_byte* pHeader) {
+    return readBEUint16(pHeader + ETC1_PKM_WIDTH_OFFSET);
+}
+
+// Read the image height from a PKM header
+
+etc1_uint32 etc1_pkm_get_height(const etc1_byte* pHeader){
+    return readBEUint16(pHeader + ETC1_PKM_HEIGHT_OFFSET);
+}