cc/resources/texture_compressor_etc1.cc - mojo - Git at Google

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 // See the following specification for details on the ETC1 format:
 // https://www.khronos.org/registry/gles/extensions/OES/OES_compressed_ETC1_RGB8_texture.txt

 #include "cc/resources/texture_compressor_etc1.h"

 #include <string.h>
 #include <limits>

 #include "base/logging.h"

 // Defining the following macro will cause the error metric function to weigh
 // each color channel differently depending on how the human eye can perceive
 // them. This can give a slight improvement in image quality at the cost of a
 // performance hit.
 // #define USE_PERCEIVED_ERROR_METRIC

 namespace {

 template <typename T>
 inline T clamp(T val, T min, T max) {
   return val < min ? min : (val > max ? max : val);
 }

 inline uint8_t round_to_5_bits(float val) {
   return clamp<uint8_t>(val * 31.0f / 255.0f + 0.5f, 0, 31);
 }

 inline uint8_t round_to_4_bits(float val) {
   return clamp<uint8_t>(val * 15.0f / 255.0f + 0.5f, 0, 15);
 }

 union Color {
   struct BgraColorType {
     uint8_t b;
     uint8_t g;
     uint8_t r;
     uint8_t a;
   } channels;
   uint8_t components[4];
   uint32_t bits;
 };

 /*
  * Codeword tables.
  * See: Table 3.17.2
  */
 static const int16_t g_codeword_tables[8][4] = {{-8, -2, 2, 8},
                                                 {-17, -5, 5, 17},
                                                 {-29, -9, 9, 29},
                                                 {-42, -13, 13, 42},
                                                 {-60, -18, 18, 60},
                                                 {-80, -24, 24, 80},
                                                 {-106, -33, 33, 106},
                                                 {-183, -47, 47, 183}};

 /*
  * Maps modifier indices to pixel index values.
  * See: Table 3.17.3
  */
 static const uint8_t g_mod_to_pix[4] = {3, 2, 0, 1};

 /*
  * The ETC1 specification index texels as follows:
  *
  * [a][e][i][m]     [ 0][ 4][ 8][12]
  * [b][f][j][n] <-> [ 1][ 5][ 9][13]
  * [c][g][k][o]     [ 2][ 6][10][14]
  * [d][h][l][p]     [ 3][ 7][11][15]
  *
  * However, when extracting sub blocks from BGRA data the natural array
  * indexing order ends up different:
  *
  * vertical0: [a][e][b][f]  horizontal0: [a][e][i][m]
  *            [c][g][d][h]               [b][f][j][n]
  * vertical1: [i][m][j][n]  horizontal1: [c][g][k][o]
  *            [k][o][l][p]               [d][h][l][p]
  *
  * In order to translate from the natural array indices in a sub block to the
  * indices (number) used by specification and hardware we use this table.
  */
 static const uint8_t g_idx_to_num[4][8] = {
     {0, 4, 1, 5, 2, 6, 3, 7},        // Vertical block 0.
     {8, 12, 9, 13, 10, 14, 11, 15},  // Vertical block 1.
     {0, 4, 8, 12, 1, 5, 9, 13},      // Horizontal block 0.
     {2, 6, 10, 14, 3, 7, 11, 15}     // Horizontal block 1.
 };

 inline void WriteColors444(uint8_t* block,
                            const Color& color0,
                            const Color& color1) {
   block[0] = (color0.channels.r & 0xf0) | (color1.channels.r >> 4);
   block[1] = (color0.channels.g & 0xf0) | (color1.channels.g >> 4);
   block[2] = (color0.channels.b & 0xf0) | (color1.channels.b >> 4);
 }

 inline void WriteColors555(uint8_t* block,
                            const Color& color0,
                            const Color& color1) {
   // Table for conversion to 3-bit two complement format.
   static const uint8_t two_compl_trans_table[8] = {
       4,  // -4 (100b)
       5,  // -3 (101b)
       6,  // -2 (110b)
       7,  // -1 (111b)
       0,  //  0 (000b)
       1,  //  1 (001b)
       2,  //  2 (010b)
       3,  //  3 (011b)
   };

   int16_t delta_r =
       static_cast<int16_t>(color1.channels.r >> 3) - (color0.channels.r >> 3);
   int16_t delta_g =
       static_cast<int16_t>(color1.channels.g >> 3) - (color0.channels.g >> 3);
   int16_t delta_b =
       static_cast<int16_t>(color1.channels.b >> 3) - (color0.channels.b >> 3);
   DCHECK(delta_r >= -4 && delta_r <= 3);
   DCHECK(delta_g >= -4 && delta_g <= 3);
   DCHECK(delta_b >= -4 && delta_b <= 3);

   block[0] = (color0.channels.r & 0xf8) | two_compl_trans_table[delta_r + 4];
   block[1] = (color0.channels.g & 0xf8) | two_compl_trans_table[delta_g + 4];
   block[2] = (color0.channels.b & 0xf8) | two_compl_trans_table[delta_b + 4];
 }

 inline void WriteCodewordTable(uint8_t* block,
                                uint8_t sub_block_id,
                                uint8_t table) {
   DCHECK_LT(sub_block_id, 2);
   DCHECK_LT(table, 8);

   uint8_t shift = (2 + (3 - sub_block_id * 3));
   block[3] &= ~(0x07 << shift);
   block[3] |= table << shift;
 }

 inline void WritePixelData(uint8_t* block, uint32_t pixel_data) {
   block[4] |= pixel_data >> 24;
   block[5] |= (pixel_data >> 16) & 0xff;
   block[6] |= (pixel_data >> 8) & 0xff;
   block[7] |= pixel_data & 0xff;
 }

 inline void WriteFlip(uint8_t* block, bool flip) {
   block[3] &= ~0x01;
   block[3] |= static_cast<uint8_t>(flip);
 }

 inline void WriteDiff(uint8_t* block, bool diff) {
   block[3] &= ~0x02;
   block[3] |= static_cast<uint8_t>(diff) << 1;
 }

 /**
  * Compress and rounds BGR888 into BGR444. The resulting BGR444 color is
  * expanded to BGR888 as it would be in hardware after decompression. The
  * actual 444-bit data is available in the four most significant bits of each
  * channel.
  */
 inline Color MakeColor444(const float* bgr) {
   uint8_t b4 = round_to_4_bits(bgr[0]);
   uint8_t g4 = round_to_4_bits(bgr[1]);
   uint8_t r4 = round_to_4_bits(bgr[2]);
   Color bgr444;
   bgr444.channels.b = (b4 << 4) | b4;
   bgr444.channels.g = (g4 << 4) | g4;
   bgr444.channels.r = (r4 << 4) | r4;
   return bgr444;
 }

 /**
  * Compress and rounds BGR888 into BGR555. The resulting BGR555 color is
  * expanded to BGR888 as it would be in hardware after decompression. The
  * actual 555-bit data is available in the five most significant bits of each
  * channel.
  */
 inline Color MakeColor555(const float* bgr) {
   uint8_t b5 = round_to_5_bits(bgr[0]);
   uint8_t g5 = round_to_5_bits(bgr[1]);
   uint8_t r5 = round_to_5_bits(bgr[2]);
   Color bgr555;
   bgr555.channels.b = (b5 << 3) | (b5 >> 2);
   bgr555.channels.g = (g5 << 3) | (g5 >> 2);
   bgr555.channels.r = (r5 << 3) | (r5 >> 2);
   return bgr555;
 }

 /**
  * Constructs a color from a given base color and luminance value.
  */
 inline Color MakeColor(const Color& base, int16_t lum) {
   int b = static_cast<int>(base.channels.b) + lum;
   int g = static_cast<int>(base.channels.g) + lum;
   int r = static_cast<int>(base.channels.r) + lum;
   Color color;
   color.channels.b = static_cast<uint8_t>(clamp(b, 0, 255));
   color.channels.g = static_cast<uint8_t>(clamp(g, 0, 255));
   color.channels.r = static_cast<uint8_t>(clamp(r, 0, 255));
   return color;
 }

 /**
  * Calculates the error metric for two colors. A small error signals that the
  * colors are similar to each other, a large error the signals the opposite.
  */
 inline uint32_t GetColorError(const Color& u, const Color& v) {
 #ifdef USE_PERCEIVED_ERROR_METRIC
   float delta_b = static_cast<float>(u.channels.b) - v.channels.b;
   float delta_g = static_cast<float>(u.channels.g) - v.channels.g;
   float delta_r = static_cast<float>(u.channels.r) - v.channels.r;
   return static_cast<uint32_t>(0.299f * delta_b * delta_b +
                                0.587f * delta_g * delta_g +
                                0.114f * delta_r * delta_r);
 #else
   int delta_b = static_cast<int>(u.channels.b) - v.channels.b;
   int delta_g = static_cast<int>(u.channels.g) - v.channels.g;
   int delta_r = static_cast<int>(u.channels.r) - v.channels.r;
   return delta_b * delta_b + delta_g * delta_g + delta_r * delta_r;
 #endif
 }

 void GetAverageColor(const Color* src, float* avg_color) {
   uint32_t sum_b = 0, sum_g = 0, sum_r = 0;

   for (unsigned int i = 0; i < 8; ++i) {
     sum_b += src[i].channels.b;
     sum_g += src[i].channels.g;
     sum_r += src[i].channels.r;
   }

   const float kInv8 = 1.0f / 8.0f;
   avg_color[0] = static_cast<float>(sum_b) * kInv8;
   avg_color[1] = static_cast<float>(sum_g) * kInv8;
   avg_color[2] = static_cast<float>(sum_r) * kInv8;
 }

 void ComputeLuminance(uint8_t* block,
                       const Color* src,
                       const Color& base,
                       int sub_block_id,
                       const uint8_t* idx_to_num_tab) {
   uint32_t best_tbl_err = std::numeric_limits<uint32_t>::max();
   uint8_t best_tbl_idx = 0;
   uint8_t best_mod_idx[8][8];  // [table][texel]

   // Try all codeword tables to find the one giving the best results for this
   // block.
   for (unsigned int tbl_idx = 0; tbl_idx < 8; ++tbl_idx) {
     // Pre-compute all the candidate colors; combinations of the base color and
     // all available luminance values.
     Color candidate_color[4];  // [modifier]
     for (unsigned int mod_idx = 0; mod_idx < 4; ++mod_idx) {
       int16_t lum = g_codeword_tables[tbl_idx][mod_idx];
       candidate_color[mod_idx] = MakeColor(base, lum);
     }

     uint32_t tbl_err = 0;

     for (unsigned int i = 0; i < 8; ++i) {
       // Try all modifiers in the current table to find which one gives the
       // smallest error.
       uint32_t best_mod_err = std::numeric_limits<uint32_t>::max();
       for (unsigned int mod_idx = 0; mod_idx < 4; ++mod_idx) {
         const Color& color = candidate_color[mod_idx];

         uint32_t mod_err = GetColorError(src[i], color);
         if (mod_err < best_mod_err) {
           best_mod_idx[tbl_idx][i] = mod_idx;
           best_mod_err = mod_err;

           if (mod_err == 0)
             break;  // We cannot do any better than this.
         }
       }

       tbl_err += best_mod_err;
       if (tbl_err > best_tbl_err)
         break;  // We're already doing worse than the best table so skip.
     }

     if (tbl_err < best_tbl_err) {
       best_tbl_err = tbl_err;
       best_tbl_idx = tbl_idx;

       if (tbl_err == 0)
         break;  // We cannot do any better than this.
     }
   }

   WriteCodewordTable(block, sub_block_id, best_tbl_idx);

   uint32_t pix_data = 0;

   for (unsigned int i = 0; i < 8; ++i) {
     uint8_t mod_idx = best_mod_idx[best_tbl_idx][i];
     uint8_t pix_idx = g_mod_to_pix[mod_idx];

     uint32_t lsb = pix_idx & 0x1;
     uint32_t msb = pix_idx >> 1;

     // Obtain the texel number as specified in the standard.
     int texel_num = idx_to_num_tab[i];
     pix_data |= msb << (texel_num + 16);
     pix_data |= lsb << (texel_num);
   }

   WritePixelData(block, pix_data);
 }

 /**
  * Tries to compress the block under the assumption that it's a single color
  * block. If it's not the function will bail out without writing anything to
  * the destination buffer.
  */
 bool TryCompressSolidBlock(uint8_t* dst, const Color* src) {
   for (unsigned int i = 1; i < 16; ++i) {
     if (src[i].bits != src[0].bits)
       return false;
   }

   // Clear destination buffer so that we can "or" in the results.
   memset(dst, 0, 8);

   float src_color_float[3] = {static_cast<float>(src->channels.b),
                               static_cast<float>(src->channels.g),
                               static_cast<float>(src->channels.r)};
   Color base = MakeColor555(src_color_float);

   WriteDiff(dst, true);
   WriteFlip(dst, false);
   WriteColors555(dst, base, base);

   uint8_t best_tbl_idx = 0;
   uint8_t best_mod_idx = 0;
   uint32_t best_mod_err = std::numeric_limits<uint32_t>::max();

   // Try all codeword tables to find the one giving the best results for this
   // block.
   for (unsigned int tbl_idx = 0; tbl_idx < 8; ++tbl_idx) {
     // Try all modifiers in the current table to find which one gives the
     // smallest error.
     for (unsigned int mod_idx = 0; mod_idx < 4; ++mod_idx) {
       int16_t lum = g_codeword_tables[tbl_idx][mod_idx];
       const Color& color = MakeColor(base, lum);

       uint32_t mod_err = GetColorError(*src, color);
       if (mod_err < best_mod_err) {
         best_tbl_idx = tbl_idx;
         best_mod_idx = mod_idx;
         best_mod_err = mod_err;

         if (mod_err == 0)
           break;  // We cannot do any better than this.
       }
     }

     if (best_mod_err == 0)
       break;
   }

   WriteCodewordTable(dst, 0, best_tbl_idx);
   WriteCodewordTable(dst, 1, best_tbl_idx);

   uint8_t pix_idx = g_mod_to_pix[best_mod_idx];
   uint32_t lsb = pix_idx & 0x1;
   uint32_t msb = pix_idx >> 1;

   uint32_t pix_data = 0;
   for (unsigned int i = 0; i < 2; ++i) {
     for (unsigned int j = 0; j < 8; ++j) {
       // Obtain the texel number as specified in the standard.
       int texel_num = g_idx_to_num[i][j];
       pix_data |= msb << (texel_num + 16);
       pix_data |= lsb << (texel_num);
     }
   }

   WritePixelData(dst, pix_data);
   return true;
 }

 void CompressBlock(uint8_t* dst, const Color* ver_src, const Color* hor_src) {
   if (TryCompressSolidBlock(dst, ver_src))
     return;

   const Color* sub_block_src[4] = {ver_src, ver_src + 8, hor_src, hor_src + 8};

   Color sub_block_avg[4];
   bool use_differential[2] = {true, true};

   // Compute the average color for each sub block and determine if differential
   // coding can be used.
   for (unsigned int i = 0, j = 1; i < 4; i += 2, j += 2) {
     float avg_color_0[3];
     GetAverageColor(sub_block_src[i], avg_color_0);
     Color avg_color_555_0 = MakeColor555(avg_color_0);

     float avg_color_1[3];
     GetAverageColor(sub_block_src[j], avg_color_1);
     Color avg_color_555_1 = MakeColor555(avg_color_1);

     for (unsigned int light_idx = 0; light_idx < 3; ++light_idx) {
       int u = avg_color_555_0.components[light_idx] >> 3;
       int v = avg_color_555_1.components[light_idx] >> 3;

       int component_diff = v - u;
       if (component_diff < -4 || component_diff > 3) {
         use_differential[i / 2] = false;
         sub_block_avg[i] = MakeColor444(avg_color_0);
         sub_block_avg[j] = MakeColor444(avg_color_1);
       } else {
         sub_block_avg[i] = avg_color_555_0;
         sub_block_avg[j] = avg_color_555_1;
       }
     }
   }

   // Compute the error of each sub block before adjusting for luminance. These
   // error values are later used for determining if we should flip the sub
   // block or not.
   uint32_t sub_block_err[4] = {0};
   for (unsigned int i = 0; i < 4; ++i) {
     for (unsigned int j = 0; j < 8; ++j) {
       sub_block_err[i] += GetColorError(sub_block_avg[i], sub_block_src[i][j]);
     }
   }

   bool flip =
       sub_block_err[2] + sub_block_err[3] < sub_block_err[0] + sub_block_err[1];

   // Clear destination buffer so that we can "or" in the results.
   memset(dst, 0, 8);

   WriteDiff(dst, use_differential[!!flip]);
   WriteFlip(dst, flip);

   uint8_t sub_block_off_0 = flip ? 2 : 0;
   uint8_t sub_block_off_1 = sub_block_off_0 + 1;

   if (use_differential[!!flip]) {
     WriteColors555(dst, sub_block_avg[sub_block_off_0],
                    sub_block_avg[sub_block_off_1]);
   } else {
     WriteColors444(dst, sub_block_avg[sub_block_off_0],
                    sub_block_avg[sub_block_off_1]);
   }

   // Compute luminance for the first sub block.
   ComputeLuminance(dst, sub_block_src[sub_block_off_0],
                    sub_block_avg[sub_block_off_0], 0,
                    g_idx_to_num[sub_block_off_0]);
   // Compute luminance for the second sub block.
   ComputeLuminance(dst, sub_block_src[sub_block_off_1],
                    sub_block_avg[sub_block_off_1], 1,
                    g_idx_to_num[sub_block_off_1]);
 }

 }  // namespace

 namespace cc {

 void TextureCompressorETC1::Compress(const uint8_t* src,
                                      uint8_t* dst,
                                      int width,
                                      int height,
                                      Quality quality) {
   DCHECK(width >= 4 && (width & 3) == 0);
   DCHECK(height >= 4 && (height & 3) == 0);

   Color ver_blocks[16];
   Color hor_blocks[16];

   for (int y = 0; y < height; y += 4, src += width * 4 * 4) {
     for (int x = 0; x < width; x += 4, dst += 8) {
       const Color* row0 = reinterpret_cast<const Color*>(src + x * 4);
       const Color* row1 = row0 + width;
       const Color* row2 = row1 + width;
       const Color* row3 = row2 + width;

       memcpy(ver_blocks, row0, 8);
       memcpy(ver_blocks + 2, row1, 8);
       memcpy(ver_blocks + 4, row2, 8);
       memcpy(ver_blocks + 6, row3, 8);
       memcpy(ver_blocks + 8, row0 + 2, 8);
       memcpy(ver_blocks + 10, row1 + 2, 8);
       memcpy(ver_blocks + 12, row2 + 2, 8);
       memcpy(ver_blocks + 14, row3 + 2, 8);

       memcpy(hor_blocks, row0, 16);
       memcpy(hor_blocks + 4, row1, 16);
       memcpy(hor_blocks + 8, row2, 16);
       memcpy(hor_blocks + 12, row3, 16);

       CompressBlock(dst, ver_blocks, hor_blocks);
     }
   }
 }

 }  // namespace cc
	// Copyright 2015 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	// See the following specification for details on the ETC1 format:
	// https://www.khronos.org/registry/gles/extensions/OES/OES_compressed_ETC1_RGB8_texture.txt

	#include "cc/resources/texture_compressor_etc1.h"

	#include <string.h>
	#include <limits>

	#include "base/logging.h"

	// Defining the following macro will cause the error metric function to weigh
	// each color channel differently depending on how the human eye can perceive
	// them. This can give a slight improvement in image quality at the cost of a
	// performance hit.
	// #define USE_PERCEIVED_ERROR_METRIC

	namespace {

	template <typename T>
	inline T clamp(T val, T min, T max) {
	return val < min ? min : (val > max ? max : val);
	}

	inline uint8_t round_to_5_bits(float val) {
	return clamp<uint8_t>(val * 31.0f / 255.0f + 0.5f, 0, 31);
	}

	inline uint8_t round_to_4_bits(float val) {
	return clamp<uint8_t>(val * 15.0f / 255.0f + 0.5f, 0, 15);
	}

	union Color {
	struct BgraColorType {
	uint8_t b;
	uint8_t g;
	uint8_t r;
	uint8_t a;
	} channels;
	uint8_t components[4];
	uint32_t bits;
	};

	/*
	* Codeword tables.
	* See: Table 3.17.2
	*/
	static const int16_t g_codeword_tables[8][4] = {{-8, -2, 2, 8},
	{-17, -5, 5, 17},
	{-29, -9, 9, 29},
	{-42, -13, 13, 42},
	{-60, -18, 18, 60},
	{-80, -24, 24, 80},
	{-106, -33, 33, 106},
	{-183, -47, 47, 183}};

	/*
	* Maps modifier indices to pixel index values.
	* See: Table 3.17.3
	*/
	static const uint8_t g_mod_to_pix[4] = {3, 2, 0, 1};

	/*
	* The ETC1 specification index texels as follows:
	*
	* [a][e][i][m] [ 0][ 4][ 8][12]
	* [b][f][j][n] <-> [ 1][ 5][ 9][13]
	* [c][g][k][o] [ 2][ 6][10][14]
	* [d][h][l][p] [ 3][ 7][11][15]
	*
	* However, when extracting sub blocks from BGRA data the natural array
	* indexing order ends up different:
	*
	* vertical0: [a][e][b][f] horizontal0: [a][e][i][m]
	* [c][g][d][h] [b][f][j][n]
	* vertical1: [i][m][j][n] horizontal1: [c][g][k][o]
	* [k][o][l][p] [d][h][l][p]
	*
	* In order to translate from the natural array indices in a sub block to the
	* indices (number) used by specification and hardware we use this table.
	*/
	static const uint8_t g_idx_to_num[4][8] = {
	{0, 4, 1, 5, 2, 6, 3, 7}, // Vertical block 0.
	{8, 12, 9, 13, 10, 14, 11, 15}, // Vertical block 1.
	{0, 4, 8, 12, 1, 5, 9, 13}, // Horizontal block 0.
	{2, 6, 10, 14, 3, 7, 11, 15} // Horizontal block 1.
	};

	inline void WriteColors444(uint8_t* block,
	const Color& color0,
	const Color& color1) {
	block[0] = (color0.channels.r & 0xf0) \| (color1.channels.r >> 4);
	block[1] = (color0.channels.g & 0xf0) \| (color1.channels.g >> 4);
	block[2] = (color0.channels.b & 0xf0) \| (color1.channels.b >> 4);
	}

	inline void WriteColors555(uint8_t* block,
	const Color& color0,
	const Color& color1) {
	// Table for conversion to 3-bit two complement format.
	static const uint8_t two_compl_trans_table[8] = {
	4, // -4 (100b)
	5, // -3 (101b)
	6, // -2 (110b)
	7, // -1 (111b)
	0, // 0 (000b)
	1, // 1 (001b)
	2, // 2 (010b)
	3, // 3 (011b)
	};

	int16_t delta_r =
	static_cast<int16_t>(color1.channels.r >> 3) - (color0.channels.r >> 3);
	int16_t delta_g =
	static_cast<int16_t>(color1.channels.g >> 3) - (color0.channels.g >> 3);
	int16_t delta_b =
	static_cast<int16_t>(color1.channels.b >> 3) - (color0.channels.b >> 3);
	DCHECK(delta_r >= -4 && delta_r <= 3);
	DCHECK(delta_g >= -4 && delta_g <= 3);
	DCHECK(delta_b >= -4 && delta_b <= 3);

	block[0] = (color0.channels.r & 0xf8) \| two_compl_trans_table[delta_r + 4];
	block[1] = (color0.channels.g & 0xf8) \| two_compl_trans_table[delta_g + 4];
	block[2] = (color0.channels.b & 0xf8) \| two_compl_trans_table[delta_b + 4];
	}

	inline void WriteCodewordTable(uint8_t* block,
	uint8_t sub_block_id,
	uint8_t table) {
	DCHECK_LT(sub_block_id, 2);
	DCHECK_LT(table, 8);

	uint8_t shift = (2 + (3 - sub_block_id * 3));
	block[3] &= ~(0x07 << shift);
	block[3] \|= table << shift;
	}

	inline void WritePixelData(uint8_t* block, uint32_t pixel_data) {
	block[4] \|= pixel_data >> 24;
	block[5] \|= (pixel_data >> 16) & 0xff;
	block[6] \|= (pixel_data >> 8) & 0xff;
	block[7] \|= pixel_data & 0xff;
	}

	inline void WriteFlip(uint8_t* block, bool flip) {
	block[3] &= ~0x01;
	block[3] \|= static_cast<uint8_t>(flip);
	}

	inline void WriteDiff(uint8_t* block, bool diff) {
	block[3] &= ~0x02;
	block[3] \|= static_cast<uint8_t>(diff) << 1;
	}

	/**
	* Compress and rounds BGR888 into BGR444. The resulting BGR444 color is
	* expanded to BGR888 as it would be in hardware after decompression. The
	* actual 444-bit data is available in the four most significant bits of each
	* channel.
	*/
	inline Color MakeColor444(const float* bgr) {
	uint8_t b4 = round_to_4_bits(bgr[0]);
	uint8_t g4 = round_to_4_bits(bgr[1]);
	uint8_t r4 = round_to_4_bits(bgr[2]);
	Color bgr444;
	bgr444.channels.b = (b4 << 4) \| b4;
	bgr444.channels.g = (g4 << 4) \| g4;
	bgr444.channels.r = (r4 << 4) \| r4;
	return bgr444;
	}

	/**
	* Compress and rounds BGR888 into BGR555. The resulting BGR555 color is
	* expanded to BGR888 as it would be in hardware after decompression. The
	* actual 555-bit data is available in the five most significant bits of each
	* channel.
	*/
	inline Color MakeColor555(const float* bgr) {
	uint8_t b5 = round_to_5_bits(bgr[0]);
	uint8_t g5 = round_to_5_bits(bgr[1]);
	uint8_t r5 = round_to_5_bits(bgr[2]);
	Color bgr555;
	bgr555.channels.b = (b5 << 3) \| (b5 >> 2);
	bgr555.channels.g = (g5 << 3) \| (g5 >> 2);
	bgr555.channels.r = (r5 << 3) \| (r5 >> 2);
	return bgr555;
	}

	/**
	* Constructs a color from a given base color and luminance value.
	*/
	inline Color MakeColor(const Color& base, int16_t lum) {
	int b = static_cast<int>(base.channels.b) + lum;
	int g = static_cast<int>(base.channels.g) + lum;
	int r = static_cast<int>(base.channels.r) + lum;
	Color color;
	color.channels.b = static_cast<uint8_t>(clamp(b, 0, 255));
	color.channels.g = static_cast<uint8_t>(clamp(g, 0, 255));
	color.channels.r = static_cast<uint8_t>(clamp(r, 0, 255));
	return color;
	}

	/**
	* Calculates the error metric for two colors. A small error signals that the
	* colors are similar to each other, a large error the signals the opposite.
	*/
	inline uint32_t GetColorError(const Color& u, const Color& v) {
	#ifdef USE_PERCEIVED_ERROR_METRIC
	float delta_b = static_cast<float>(u.channels.b) - v.channels.b;
	float delta_g = static_cast<float>(u.channels.g) - v.channels.g;
	float delta_r = static_cast<float>(u.channels.r) - v.channels.r;
	return static_cast<uint32_t>(0.299f * delta_b * delta_b +
	0.587f * delta_g * delta_g +
	0.114f * delta_r * delta_r);
	#else
	int delta_b = static_cast<int>(u.channels.b) - v.channels.b;
	int delta_g = static_cast<int>(u.channels.g) - v.channels.g;
	int delta_r = static_cast<int>(u.channels.r) - v.channels.r;
	return delta_b * delta_b + delta_g * delta_g + delta_r * delta_r;
	#endif
	}

	void GetAverageColor(const Color* src, float* avg_color) {
	uint32_t sum_b = 0, sum_g = 0, sum_r = 0;

	for (unsigned int i = 0; i < 8; ++i) {
	sum_b += src[i].channels.b;
	sum_g += src[i].channels.g;
	sum_r += src[i].channels.r;
	}

	const float kInv8 = 1.0f / 8.0f;
	avg_color[0] = static_cast<float>(sum_b) * kInv8;
	avg_color[1] = static_cast<float>(sum_g) * kInv8;
	avg_color[2] = static_cast<float>(sum_r) * kInv8;
	}

	void ComputeLuminance(uint8_t* block,
	const Color* src,
	const Color& base,
	int sub_block_id,
	const uint8_t* idx_to_num_tab) {
	uint32_t best_tbl_err = std::numeric_limits<uint32_t>::max();
	uint8_t best_tbl_idx = 0;
	uint8_t best_mod_idx[8][8]; // [table][texel]

	// Try all codeword tables to find the one giving the best results for this
	// block.
	for (unsigned int tbl_idx = 0; tbl_idx < 8; ++tbl_idx) {
	// Pre-compute all the candidate colors; combinations of the base color and
	// all available luminance values.
	Color candidate_color[4]; // [modifier]
	for (unsigned int mod_idx = 0; mod_idx < 4; ++mod_idx) {
	int16_t lum = g_codeword_tables[tbl_idx][mod_idx];
	candidate_color[mod_idx] = MakeColor(base, lum);
	}

	uint32_t tbl_err = 0;

	for (unsigned int i = 0; i < 8; ++i) {
	// Try all modifiers in the current table to find which one gives the
	// smallest error.
	uint32_t best_mod_err = std::numeric_limits<uint32_t>::max();
	for (unsigned int mod_idx = 0; mod_idx < 4; ++mod_idx) {
	const Color& color = candidate_color[mod_idx];

	uint32_t mod_err = GetColorError(src[i], color);
	if (mod_err < best_mod_err) {
	best_mod_idx[tbl_idx][i] = mod_idx;
	best_mod_err = mod_err;

	if (mod_err == 0)
	break; // We cannot do any better than this.
	}
	}

	tbl_err += best_mod_err;
	if (tbl_err > best_tbl_err)
	break; // We're already doing worse than the best table so skip.
	}

	if (tbl_err < best_tbl_err) {
	best_tbl_err = tbl_err;
	best_tbl_idx = tbl_idx;

	if (tbl_err == 0)
	break; // We cannot do any better than this.
	}
	}

	WriteCodewordTable(block, sub_block_id, best_tbl_idx);

	uint32_t pix_data = 0;

	for (unsigned int i = 0; i < 8; ++i) {
	uint8_t mod_idx = best_mod_idx[best_tbl_idx][i];
	uint8_t pix_idx = g_mod_to_pix[mod_idx];

	uint32_t lsb = pix_idx & 0x1;
	uint32_t msb = pix_idx >> 1;

	// Obtain the texel number as specified in the standard.
	int texel_num = idx_to_num_tab[i];
	pix_data \|= msb << (texel_num + 16);
	pix_data \|= lsb << (texel_num);
	}

	WritePixelData(block, pix_data);
	}

	/**
	* Tries to compress the block under the assumption that it's a single color
	* block. If it's not the function will bail out without writing anything to
	* the destination buffer.
	*/
	bool TryCompressSolidBlock(uint8_t* dst, const Color* src) {
	for (unsigned int i = 1; i < 16; ++i) {
	if (src[i].bits != src[0].bits)
	return false;
	}

	// Clear destination buffer so that we can "or" in the results.
	memset(dst, 0, 8);

	float src_color_float[3] = {static_cast<float>(src->channels.b),
	static_cast<float>(src->channels.g),
	static_cast<float>(src->channels.r)};
	Color base = MakeColor555(src_color_float);

	WriteDiff(dst, true);
	WriteFlip(dst, false);
	WriteColors555(dst, base, base);

	uint8_t best_tbl_idx = 0;
	uint8_t best_mod_idx = 0;
	uint32_t best_mod_err = std::numeric_limits<uint32_t>::max();

	// Try all codeword tables to find the one giving the best results for this
	// block.
	for (unsigned int tbl_idx = 0; tbl_idx < 8; ++tbl_idx) {
	// Try all modifiers in the current table to find which one gives the
	// smallest error.
	for (unsigned int mod_idx = 0; mod_idx < 4; ++mod_idx) {
	int16_t lum = g_codeword_tables[tbl_idx][mod_idx];
	const Color& color = MakeColor(base, lum);

	uint32_t mod_err = GetColorError(*src, color);
	if (mod_err < best_mod_err) {
	best_tbl_idx = tbl_idx;
	best_mod_idx = mod_idx;
	best_mod_err = mod_err;

	if (mod_err == 0)
	break; // We cannot do any better than this.
	}
	}

	if (best_mod_err == 0)
	break;
	}

	WriteCodewordTable(dst, 0, best_tbl_idx);
	WriteCodewordTable(dst, 1, best_tbl_idx);

	uint8_t pix_idx = g_mod_to_pix[best_mod_idx];
	uint32_t lsb = pix_idx & 0x1;
	uint32_t msb = pix_idx >> 1;

	uint32_t pix_data = 0;
	for (unsigned int i = 0; i < 2; ++i) {
	for (unsigned int j = 0; j < 8; ++j) {
	// Obtain the texel number as specified in the standard.
	int texel_num = g_idx_to_num[i][j];
	pix_data \|= msb << (texel_num + 16);
	pix_data \|= lsb << (texel_num);
	}
	}

	WritePixelData(dst, pix_data);
	return true;
	}

	void CompressBlock(uint8_t* dst, const Color* ver_src, const Color* hor_src) {
	if (TryCompressSolidBlock(dst, ver_src))
	return;

	const Color* sub_block_src[4] = {ver_src, ver_src + 8, hor_src, hor_src + 8};

	Color sub_block_avg[4];
	bool use_differential[2] = {true, true};

	// Compute the average color for each sub block and determine if differential
	// coding can be used.
	for (unsigned int i = 0, j = 1; i < 4; i += 2, j += 2) {
	float avg_color_0[3];
	GetAverageColor(sub_block_src[i], avg_color_0);
	Color avg_color_555_0 = MakeColor555(avg_color_0);

	float avg_color_1[3];
	GetAverageColor(sub_block_src[j], avg_color_1);
	Color avg_color_555_1 = MakeColor555(avg_color_1);

	for (unsigned int light_idx = 0; light_idx < 3; ++light_idx) {
	int u = avg_color_555_0.components[light_idx] >> 3;
	int v = avg_color_555_1.components[light_idx] >> 3;

	int component_diff = v - u;
	if (component_diff < -4 \|\| component_diff > 3) {
	use_differential[i / 2] = false;
	sub_block_avg[i] = MakeColor444(avg_color_0);
	sub_block_avg[j] = MakeColor444(avg_color_1);
	} else {
	sub_block_avg[i] = avg_color_555_0;
	sub_block_avg[j] = avg_color_555_1;
	}
	}
	}

	// Compute the error of each sub block before adjusting for luminance. These
	// error values are later used for determining if we should flip the sub
	// block or not.
	uint32_t sub_block_err[4] = {0};
	for (unsigned int i = 0; i < 4; ++i) {
	for (unsigned int j = 0; j < 8; ++j) {
	sub_block_err[i] += GetColorError(sub_block_avg[i], sub_block_src[i][j]);
	}
	}

	bool flip =
	sub_block_err[2] + sub_block_err[3] < sub_block_err[0] + sub_block_err[1];

	// Clear destination buffer so that we can "or" in the results.
	memset(dst, 0, 8);

	WriteDiff(dst, use_differential[!!flip]);
	WriteFlip(dst, flip);

	uint8_t sub_block_off_0 = flip ? 2 : 0;
	uint8_t sub_block_off_1 = sub_block_off_0 + 1;

	if (use_differential[!!flip]) {
	WriteColors555(dst, sub_block_avg[sub_block_off_0],
	sub_block_avg[sub_block_off_1]);
	} else {
	WriteColors444(dst, sub_block_avg[sub_block_off_0],
	sub_block_avg[sub_block_off_1]);
	}

	// Compute luminance for the first sub block.
	ComputeLuminance(dst, sub_block_src[sub_block_off_0],
	sub_block_avg[sub_block_off_0], 0,
	g_idx_to_num[sub_block_off_0]);
	// Compute luminance for the second sub block.
	ComputeLuminance(dst, sub_block_src[sub_block_off_1],
	sub_block_avg[sub_block_off_1], 1,
	g_idx_to_num[sub_block_off_1]);
	}

	} // namespace

	namespace cc {

	void TextureCompressorETC1::Compress(const uint8_t* src,
	uint8_t* dst,
	int width,
	int height,
	Quality quality) {
	DCHECK(width >= 4 && (width & 3) == 0);
	DCHECK(height >= 4 && (height & 3) == 0);

	Color ver_blocks[16];
	Color hor_blocks[16];

	for (int y = 0; y < height; y += 4, src += width * 4 * 4) {
	for (int x = 0; x < width; x += 4, dst += 8) {
	const Color* row0 = reinterpret_cast<const Color>(src + x 4);
	const Color* row1 = row0 + width;
	const Color* row2 = row1 + width;
	const Color* row3 = row2 + width;

	memcpy(ver_blocks, row0, 8);
	memcpy(ver_blocks + 2, row1, 8);
	memcpy(ver_blocks + 4, row2, 8);
	memcpy(ver_blocks + 6, row3, 8);
	memcpy(ver_blocks + 8, row0 + 2, 8);
	memcpy(ver_blocks + 10, row1 + 2, 8);
	memcpy(ver_blocks + 12, row2 + 2, 8);
	memcpy(ver_blocks + 14, row3 + 2, 8);

	memcpy(hor_blocks, row0, 16);
	memcpy(hor_blocks + 4, row1, 16);
	memcpy(hor_blocks + 8, row2, 16);
	memcpy(hor_blocks + 12, row3, 16);

	CompressBlock(dst, ver_blocks, hor_blocks);
	}
	}
	}

	} // namespace cc