cc/raster/texture_compressor_etc1.cc

   1 // Copyright 2015 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 // See the following specification for details on the ETC1 format:
   6 // https://www.khronos.org/registry/gles/extensions/OES/OES_compressed_ETC1_RGB8_texture.txt
   7
   8 #include "cc/raster/texture_compressor_etc1.h"
   9
  10 #include <string.h>
  11 #include <limits>
  12
  13 #include "base/logging.h"
  14
  15 // Defining the following macro will cause the error metric function to weigh
  16 // each color channel differently depending on how the human eye can perceive
  17 // them. This can give a slight improvement in image quality at the cost of a
  18 // performance hit.
  19 // #define USE_PERCEIVED_ERROR_METRIC
  20
  21 namespace cc {
  22
  23 namespace {
  24
  25 // Constructs a color from a given base color and luminance value.
  26 inline Color MakeColor(const Color& base, int16_t lum) {
  27   int b = static_cast<int>(base.channels.b) + lum;
  28   int g = static_cast<int>(base.channels.g) + lum;
  29   int r = static_cast<int>(base.channels.r) + lum;
  30   Color color;
  31   color.channels.b = static_cast<uint8_t>(clamp(b, 0, 255));
  32   color.channels.g = static_cast<uint8_t>(clamp(g, 0, 255));
  33   color.channels.r = static_cast<uint8_t>(clamp(r, 0, 255));
  34   return color;
  35 }
  36
  37 // Calculates the error metric for two colors. A small error signals that the
  38 // colors are similar to each other, a large error the signals the opposite.
  39 inline uint32_t GetColorError(const Color& u, const Color& v) {
  40 #ifdef USE_PERCEIVED_ERROR_METRIC
  41   float delta_b = static_cast<float>(u.channels.b) - v.channels.b;
  42   float delta_g = static_cast<float>(u.channels.g) - v.channels.g;
  43   float delta_r = static_cast<float>(u.channels.r) - v.channels.r;
  44   return static_cast<uint32_t>(0.299f * delta_b * delta_b +
  45                                0.587f * delta_g * delta_g +
  46                                0.114f * delta_r * delta_r);
  47 #else
  48   int delta_b = static_cast<int>(u.channels.b) - v.channels.b;
  49   int delta_g = static_cast<int>(u.channels.g) - v.channels.g;
  50   int delta_r = static_cast<int>(u.channels.r) - v.channels.r;
  51   return delta_b * delta_b + delta_g * delta_g + delta_r * delta_r;
  52 #endif
  53 }
  54
  55 void GetAverageColor(const Color* src, float* avg_color) {
  56   uint32_t sum_b = 0, sum_g = 0, sum_r = 0;
  57
  58   for (unsigned int i = 0; i < 8; ++i) {
  59     sum_b += src[i].channels.b;
  60     sum_g += src[i].channels.g;
  61     sum_r += src[i].channels.r;
  62   }
  63
  64   const float kInv8 = 1.0f / 8.0f;
  65   avg_color[0] = static_cast<float>(sum_b) * kInv8;
  66   avg_color[1] = static_cast<float>(sum_g) * kInv8;
  67   avg_color[2] = static_cast<float>(sum_r) * kInv8;
  68 }
  69
  70 void ComputeLuminance(uint8_t* block,
  71                       const Color* src,
  72                       const Color& base,
  73                       int sub_block_id,
  74                       const uint8_t* idx_to_num_tab) {
  75   uint32_t best_tbl_err = std::numeric_limits<uint32_t>::max();
  76   uint8_t best_tbl_idx = 0;
  77   uint8_t best_mod_idx[8][8];  // [table][texel]
  78
  79   // Try all codeword tables to find the one giving the best results for this
  80   // block.
  81   for (unsigned int tbl_idx = 0; tbl_idx < 8; ++tbl_idx) {
  82     // Pre-compute all the candidate colors; combinations of the base color and
  83     // all available luminance values.
  84     Color candidate_color[4];  // [modifier]
  85     for (unsigned int mod_idx = 0; mod_idx < 4; ++mod_idx) {
  86       int16_t lum = g_codeword_tables[tbl_idx][mod_idx];
  87       candidate_color[mod_idx] = MakeColor(base, lum);
  88     }
  89
  90     uint32_t tbl_err = 0;
  91
  92     for (unsigned int i = 0; i < 8; ++i) {
  93       // Try all modifiers in the current table to find which one gives the
  94       // smallest error.
  95       uint32_t best_mod_err = std::numeric_limits<uint32_t>::max();
  96       for (unsigned int mod_idx = 0; mod_idx < 4; ++mod_idx) {
  97         const Color& color = candidate_color[mod_idx];
  98
  99         uint32_t mod_err = GetColorError(src[i], color);
 100         if (mod_err < best_mod_err) {
 101           best_mod_idx[tbl_idx][i] = mod_idx;
 102           best_mod_err = mod_err;
 103
 104           if (mod_err == 0)
 105             break;  // We cannot do any better than this.
 106         }
 107       }
 108
 109       tbl_err += best_mod_err;
 110       if (tbl_err > best_tbl_err)
 111         break;  // We're already doing worse than the best table so skip.
 112     }
 113
 114     if (tbl_err < best_tbl_err) {
 115       best_tbl_err = tbl_err;
 116       best_tbl_idx = tbl_idx;
 117
 118       if (tbl_err == 0)
 119         break;  // We cannot do any better than this.
 120     }
 121   }
 122
 123   WriteCodewordTable(block, sub_block_id, best_tbl_idx);
 124
 125   uint32_t pix_data = 0;
 126
 127   for (unsigned int i = 0; i < 8; ++i) {
 128     uint8_t mod_idx = best_mod_idx[best_tbl_idx][i];
 129     uint8_t pix_idx = g_mod_to_pix[mod_idx];
 130
 131     uint32_t lsb = pix_idx & 0x1;
 132     uint32_t msb = pix_idx >> 1;
 133
 134     // Obtain the texel number as specified in the standard.
 135     int texel_num = idx_to_num_tab[i];
 136     pix_data |= msb << (texel_num + 16);
 137     pix_data |= lsb << (texel_num);
 138   }
 139
 140   WritePixelData(block, pix_data);
 141 }
 142
 143 /**
 144  * Tries to compress the block under the assumption that it's a single color
 145  * block. If it's not the function will bail out without writing anything to
 146  * the destination buffer.
 147  */
 148 bool TryCompressSolidBlock(uint8_t* dst, const Color* src) {
 149   for (unsigned int i = 1; i < 16; ++i) {
 150     if (src[i].bits != src[0].bits)
 151       return false;
 152   }
 153
 154   // Clear destination buffer so that we can "or" in the results.
 155   memset(dst, 0, 8);
 156
 157   float src_color_float[3] = {static_cast<float>(src->channels.b),
 158                               static_cast<float>(src->channels.g),
 159                               static_cast<float>(src->channels.r)};
 160   Color base = MakeColor555(src_color_float);
 161
 162   WriteDiff(dst, true);
 163   WriteFlip(dst, false);
 164   WriteColors555(dst, base, base);
 165
 166   uint8_t best_tbl_idx = 0;
 167   uint8_t best_mod_idx = 0;
 168   uint32_t best_mod_err = std::numeric_limits<uint32_t>::max();
 169
 170   // Try all codeword tables to find the one giving the best results for this
 171   // block.
 172   for (unsigned int tbl_idx = 0; tbl_idx < 8; ++tbl_idx) {
 173     // Try all modifiers in the current table to find which one gives the
 174     // smallest error.
 175     for (unsigned int mod_idx = 0; mod_idx < 4; ++mod_idx) {
 176       int16_t lum = g_codeword_tables[tbl_idx][mod_idx];
 177       const Color& color = MakeColor(base, lum);
 178
 179       uint32_t mod_err = GetColorError(*src, color);
 180       if (mod_err < best_mod_err) {
 181         best_tbl_idx = tbl_idx;
 182         best_mod_idx = mod_idx;
 183         best_mod_err = mod_err;
 184
 185         if (mod_err == 0)
 186           break;  // We cannot do any better than this.
 187       }
 188     }
 189
 190     if (best_mod_err == 0)
 191       break;
 192   }
 193
 194   WriteCodewordTable(dst, 0, best_tbl_idx);
 195   WriteCodewordTable(dst, 1, best_tbl_idx);
 196
 197   uint8_t pix_idx = g_mod_to_pix[best_mod_idx];
 198   uint32_t lsb = pix_idx & 0x1;
 199   uint32_t msb = pix_idx >> 1;
 200
 201   uint32_t pix_data = 0;
 202   for (unsigned int i = 0; i < 2; ++i) {
 203     for (unsigned int j = 0; j < 8; ++j) {
 204       // Obtain the texel number as specified in the standard.
 205       int texel_num = g_idx_to_num[i][j];
 206       pix_data |= msb << (texel_num + 16);
 207       pix_data |= lsb << (texel_num);
 208     }
 209   }
 210
 211   WritePixelData(dst, pix_data);
 212   return true;
 213 }
 214
 215 void CompressBlock(uint8_t* dst, const Color* ver_src, const Color* hor_src) {
 216   if (TryCompressSolidBlock(dst, ver_src))
 217     return;
 218
 219   const Color* sub_block_src[4] = {ver_src, ver_src + 8, hor_src, hor_src + 8};
 220
 221   Color sub_block_avg[4];
 222   bool use_differential[2] = {true, true};
 223
 224   // Compute the average color for each sub block and determine if differential
 225   // coding can be used.
 226   for (unsigned int i = 0, j = 1; i < 4; i += 2, j += 2) {
 227     float avg_color_0[3];
 228     GetAverageColor(sub_block_src[i], avg_color_0);
 229     Color avg_color_555_0 = MakeColor555(avg_color_0);
 230
 231     float avg_color_1[3];
 232     GetAverageColor(sub_block_src[j], avg_color_1);
 233     Color avg_color_555_1 = MakeColor555(avg_color_1);
 234
 235     for (unsigned int light_idx = 0; light_idx < 3; ++light_idx) {
 236       int u = avg_color_555_0.components[light_idx] >> 3;
 237       int v = avg_color_555_1.components[light_idx] >> 3;
 238
 239       int component_diff = v - u;
 240       if (component_diff < -4 || component_diff > 3) {
 241         use_differential[i / 2] = false;
 242         sub_block_avg[i] = MakeColor444(avg_color_0);
 243         sub_block_avg[j] = MakeColor444(avg_color_1);
 244       } else {
 245         sub_block_avg[i] = avg_color_555_0;
 246         sub_block_avg[j] = avg_color_555_1;
 247       }
 248     }
 249   }
 250
 251   // Compute the error of each sub block before adjusting for luminance. These
 252   // error values are later used for determining if we should flip the sub
 253   // block or not.
 254   uint32_t sub_block_err[4] = {0};
 255   for (unsigned int i = 0; i < 4; ++i) {
 256     for (unsigned int j = 0; j < 8; ++j) {
 257       sub_block_err[i] += GetColorError(sub_block_avg[i], sub_block_src[i][j]);
 258     }
 259   }
 260
 261   bool flip =
 262       sub_block_err[2] + sub_block_err[3] < sub_block_err[0] + sub_block_err[1];
 263
 264   // Clear destination buffer so that we can "or" in the results.
 265   memset(dst, 0, 8);
 266
 267   WriteDiff(dst, use_differential[!!flip]);
 268   WriteFlip(dst, flip);
 269
 270   uint8_t sub_block_off_0 = flip ? 2 : 0;
 271   uint8_t sub_block_off_1 = sub_block_off_0 + 1;
 272
 273   if (use_differential[!!flip]) {
 274     WriteColors555(dst, sub_block_avg[sub_block_off_0],
 275                    sub_block_avg[sub_block_off_1]);
 276   } else {
 277     WriteColors444(dst, sub_block_avg[sub_block_off_0],
 278                    sub_block_avg[sub_block_off_1]);
 279   }
 280
 281   // Compute luminance for the first sub block.
 282   ComputeLuminance(dst, sub_block_src[sub_block_off_0],
 283                    sub_block_avg[sub_block_off_0], 0,
 284                    g_idx_to_num[sub_block_off_0]);
 285   // Compute luminance for the second sub block.
 286   ComputeLuminance(dst, sub_block_src[sub_block_off_1],
 287                    sub_block_avg[sub_block_off_1], 1,
 288                    g_idx_to_num[sub_block_off_1]);
 289 }
 290
 291 }  // namespace
 292
 293 void TextureCompressorETC1::Compress(const uint8_t* src,
 294                                      uint8_t* dst,
 295                                      int width,
 296                                      int height,
 297                                      Quality quality) {
 298   DCHECK_GE(width, 4);
 299   DCHECK_EQ((width & 3), 0);
 300   DCHECK_GE(height, 4);
 301   DCHECK_EQ((height & 3), 0);
 302
 303   Color ver_blocks[16];
 304   Color hor_blocks[16];
 305
 306   for (int y = 0; y < height; y += 4, src += width * 4 * 4) {
 307     for (int x = 0; x < width; x += 4, dst += 8) {
 308       const Color* row0 = reinterpret_cast<const Color*>(src + x * 4);
 309       const Color* row1 = row0 + width;
 310       const Color* row2 = row1 + width;
 311       const Color* row3 = row2 + width;
 312
 313       memcpy(ver_blocks, row0, 8);
 314       memcpy(ver_blocks + 2, row1, 8);
 315       memcpy(ver_blocks + 4, row2, 8);
 316       memcpy(ver_blocks + 6, row3, 8);
 317       memcpy(ver_blocks + 8, row0 + 2, 8);
 318       memcpy(ver_blocks + 10, row1 + 2, 8);
 319       memcpy(ver_blocks + 12, row2 + 2, 8);
 320       memcpy(ver_blocks + 14, row3 + 2, 8);
 321
 322       memcpy(hor_blocks, row0, 16);
 323       memcpy(hor_blocks + 4, row1, 16);
 324       memcpy(hor_blocks + 8, row2, 16);
 325       memcpy(hor_blocks + 12, row3, 16);
 326
 327       CompressBlock(dst, ver_blocks, hor_blocks);
 328     }
 329   }
 330 }
 331
 332 }  // namespace cc