Tile Compression

cc/resources/texture_compress/arm/etc1_neon.cc

+// Copyright 2014 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
+
+#ifdef __ARM_NEON__
+
+#include "cc/resources/texture_compress/arm/etc1_neon.h"
+
+#include <arm_neon.h>
+#include <limits>
+
+#include "base/compiler_specific.h"
+#include "base/logging.h"
+
+// GCC 4.6 suffers from a bug, raising an internal error when mixing
+// interleaved load instructions with linear load instructions. By fiddling
+// with variable declaration order this problem can be avoided which is done
+// when the following macro is defined.
+#if (__GNUC__ == 4) && (__GNUC_MINOR__ == 6)
+#define GCC46_INTERNAL_ERROR_WORKAROUND
+#endif
+
+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);
+}
+
+}  // namespace
+
+namespace etc1_neon {
+
+union Color {
+  struct {
+    uint8_t b;
+    uint8_t g;
+    uint8_t r;
+    uint8_t a;
+  };
+  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}};
+
+/*
+ * NEON optimized codeword tables.
+ *
+ * It allows for a single table entry to be loaded into a 64-bit register
+ * without duplication and with the alpha channel already cleared.
+ *
+ * See: Table 3.17.2
+ */
+ALIGNAS(8) static const int16_t g_codeword_tables_neon_opt[8][16] = {
+    {-8, -8, -8, 0, -2, -2, -2, 0, 2, 2, 2, 0, 8, 8, 8, 0},
+    {-17, -17, -17, 0, -5, -5, -5, 0, 5, 5, 5, 0, 17, 17, 17, 0},
+    {-29, -29, -29, 0, -9, -9, -9, 0, 9, 9, 9, 0, 29, 29, 29, 0},
+    {-42, -42, -42, 0, -13, -13, -13, 0, 13, 13, 13, 0, 42, 42, 42, 0},
+    {-60, -60, -60, 0, -18, -18, -18, 0, 18, 18, 18, 0, 60, 60, 60, 0},
+    {-80, -80, -80, 0, -24, -24, -24, 0, 24, 24, 24, 0, 80, 80, 80, 0},
+    {-106, -106, -106, 0, -33, -33, -33, 0, 33, 33, 33, 0, 106, 106, 106, 0},
+    {-183, -183, -183, 0, -47, -47, -47, 0, 47, 47, 47, 0, 183, 183, 183, 0}};
+
+/*
+ * 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][b][c][d]  horizontal0: [a][e][i][m]
+ *            [e][f][g][h]               [b][f][j][n]
+ * vertical1: [i][j][k][l]  horizontal1: [c][g][k][o]
+ *            [m][n][o][p]               [d][h][l][p]
+ *
+ * In order to translate from the natural array indices in a sub block to the
+ * indices (numbers) used by specification and hardware we use this table.
+ *
+ * NOTE: Since we can efficiently transpose matrixes using NEON we end up with
+ *       near perfect indexing for vertical sub blocks.
+ */
+static const uint8_t g_idx_to_num[4][8] = {
+    {0, 1, 2, 3, 4, 5, 6, 7},        // Vertical block 0.
+    {8, 9, 10, 11, 12, 13, 14, 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.r & 0xf0) | (color1.r >> 4);
+  block[1] = (color0.g & 0xf0) | (color1.g >> 4);
+  block[2] = (color0.b & 0xf0) | (color1.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.r >> 3) - (color0.r >> 3);
+  int16_t delta_g = static_cast<int16_t>(color1.g >> 3) - (color0.g >> 3);
+  int16_t delta_b = static_cast<int16_t>(color1.b >> 3) - (color0.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.r & 0xf8) | two_compl_trans_table[delta_r + 4];
+  block[1] = (color0.g & 0xf8) | two_compl_trans_table[delta_g + 4];
+  block[2] = (color0.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.b = (b4 << 4) | b4;
+  bgr444.g = (g4 << 4) | g4;
+  bgr444.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.b = (b5 << 3) | (b5 >> 2);
+  bgr555.g = (g5 << 3) | (g5 >> 2);
+  bgr555.r = (r5 << 3) | (r5 >> 2);
+  return bgr555;
+}
+
+/**
+ * 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.
+ *
+ * IMPORTANT: This function call has been inlined and NEON optimized in the
+ *            ComputeLuminance() function. The inlined version should be kept
+ *            in sync with this function implementation.
+ */
+inline uint32_t GetColorError(const Color& u, const Color& v) {
+  int delta_b = static_cast<int>(u.b) - v.b;
+  int delta_g = static_cast<int>(u.g) - v.g;
+  int delta_r = static_cast<int>(u.r) - v.r;
+  return delta_b * delta_b + delta_g * delta_g + delta_r * delta_r;
+}
+
+void GetAverageColor(const Color* src, float* avg_color_bgr) {
+  const uint8_t* src_ptr = reinterpret_cast<const uint8_t*>(src);
+#ifdef GCC46_INTERNAL_ERROR_WORKAROUND
+  uint8x8x4_t src0;
+  src0 = vld4_u8(src_ptr);
+#else
+  uint8x8x4_t src0 = vld4_u8(src_ptr);
+#endif
+
+  uint64x1_t sum_b0 = vpaddl_u32(vpaddl_u16(vpaddl_u8(src0.val[0])));
+  uint64x1_t sum_g0 = vpaddl_u32(vpaddl_u16(vpaddl_u8(src0.val[1])));
+  uint64x1_t sum_r0 = vpaddl_u32(vpaddl_u16(vpaddl_u8(src0.val[2])));
+
+  ALIGNAS(8) uint64_t sum_b, sum_g, sum_r;
+  vst1_u64(&sum_b, sum_b0);
+  vst1_u64(&sum_g, sum_g0);
+  vst1_u64(&sum_r, sum_r0);
+
+  const float kInv8 = 1.0f / 8.0f;
+  avg_color_bgr[0] = static_cast<float>(sum_b) * kInv8;
+  avg_color_bgr[1] = static_cast<float>(sum_g) * kInv8;
+  avg_color_bgr[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_idxs[8][8];  // [table][texel]
+
+  // Load immutable data that is shared through iteration.
+  ALIGNAS(8) const int16_t base_color_ptr[4] = {base.b, base.g, base.r, 0x00};
+  int16x8_t base_color =
+      vcombine_s16(vld1_s16(base_color_ptr), vld1_s16(base_color_ptr));
+
+  ALIGNAS(8) const uint32_t idx_mask_ptr[4] = {0x00, 0x01, 0x02, 0x03};
+  uint32x4_t idx_mask = vld1q_u32(idx_mask_ptr);
+
+  // Preload source color registers.
+  uint8x16_t src_color[8];
+  for (unsigned int i = 0; i < 8; ++i) {
+    DCHECK_EQ(src[i].a, 0x00);
+    const uint32_t* src_ptr = reinterpret_cast<const uint32_t*>(&src[i]);
+    src_color[i] = vreinterpretq_u8_u32(vld1q_dup_u32(src_ptr));
+  }
+
+  // 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) {
+    uint32_t tbl_err = 0;
+
+    // For the current table, compute the candidate color: base + lum for all
+    // four luminance entries.
+    const int16_t* lum_ptr = g_codeword_tables_neon_opt[tbl_idx];
+    int16x8_t lum01 = vld1q_s16(lum_ptr);
+    int16x8_t lum23 = vld1q_s16(lum_ptr + 8);
+
+    int16x8_t color01 = vaddq_s16(base_color, lum01);
+    int16x8_t color23 = vaddq_s16(base_color, lum23);
+
+    // Clamp the candidate colors to [0, 255].
+    color01 = vminq_s16(color01, vdupq_n_s16(255));
+    color01 = vmaxq_s16(color01, vdupq_n_s16(0));
+    color23 = vminq_s16(color23, vdupq_n_s16(255));
+    color23 = vmaxq_s16(color23, vdupq_n_s16(0));
+
+    uint8x16_t candidate_color =
+        vcombine_u8(vmovn_u16(vreinterpretq_u16_s16(color01)),
+                    vmovn_u16(vreinterpretq_u16_s16(color23)));
+
+    for (unsigned int i = 0; i < 8; ++i) {
+      // Compute the squared distance between the source and candidate colors.
+      uint8x16_t diff = vabdq_u8(src_color[i], candidate_color);
+      uint8x8_t diff01 = vget_low_u8(diff);
+      uint8x8_t diff23 = vget_high_u8(diff);
+
+      uint16x8_t square01 = vmull_u8(diff01, diff01);
+      uint16x8_t square23 = vmull_u8(diff23, diff23);
+
+      uint32x4_t psum01 = vpaddlq_u16(square01);
+      uint32x4_t psum23 = vpaddlq_u16(square23);
+      uint32x2_t err01 = vpadd_u32(vget_low_u32(psum01), vget_high_u32(psum01));
+      uint32x2_t err23 = vpadd_u32(vget_low_u32(psum23), vget_high_u32(psum23));
+      uint32x4_t errs = vcombine_u32(err01, err23);
+
+      // Find the minimum error.
+      uint32x2_t min_err = vpmin_u32(err01, err23);
+      min_err = vpmin_u32(min_err, min_err);
+
+      // Find the modifier index which produced the minimum error. This is
+      // essentially the lane number of the lane containing the minimum error.
+      uint32x4_t min_mask = vceqq_u32(vcombine_u32(min_err, min_err), errs);
+      uint32x4_t idxs = vbslq_u32(min_mask, idx_mask, vdupq_n_u32(0xffffffff));
+
+      uint32x2_t min_idx = vpmin_u32(vget_low_u32(idxs), vget_high_u32(idxs));
+      min_idx = vpmin_u32(min_idx, min_idx);
+
+      uint32_t best_mod_err = vget_lane_u32(min_err, 0);
+      uint32_t best_mod_idx = vget_lane_u32(min_idx, 0);
+
+      best_mod_idxs[tbl_idx][i] = best_mod_idx;
+
+      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_idxs[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);
+}
+
+/**
+ * Compress a solid, single colored block.
+ */
+void CompressSolidBlock(uint8_t* dst, const Color& src) {
+  // Clear destination buffer so that we can "or" in the results.
+  memset(dst, 0, 8);
+
+  float src_color_float[3] = {static_cast<float>(src.b),
+                              static_cast<float>(src.g),
+                              static_cast<float>(src.r)};
+  Color base = MakeColor555(src_color_float);
+
+  WriteDiff(dst, true);
+  WriteFlip(dst, false);
+  WriteColors555(dst, base, base);
+
+  uint32_t best_tbl_err = std::numeric_limits<uint32_t>::max();
+  uint8_t best_tbl_idx = 0;
+  uint8_t best_mod_idx = 0;
+
+  // Load immutable data that is shared through iteration.
+  ALIGNAS(8) const int16_t base_color_ptr[4] = {base.b, base.g, base.r, 0x00};
+  int16x8_t base_color =
+      vcombine_s16(vld1_s16(base_color_ptr), vld1_s16(base_color_ptr));
+
+  ALIGNAS(8) const uint32_t idx_mask_ptr[4] = {0x00, 0x01, 0x02, 0x03};
+  uint32x4_t idx_mask = vld1q_u32(idx_mask_ptr);
+
+  // Preload source color registers.
+  DCHECK_EQ(src.a, 0x00);
+  uint8x16_t src_color = vreinterpretq_u8_u32(
+      vld1q_dup_u32(reinterpret_cast<const uint32_t*>(&src)));
+
+  // 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) {
+    // For the current table, compute the candidate color: base + lum for all
+    // four luminance entries.
+    const int16_t* lum_ptr = g_codeword_tables_neon_opt[tbl_idx];
+    int16x8_t lum01 = vld1q_s16(lum_ptr);
+    int16x8_t lum23 = vld1q_s16(lum_ptr + 8);
+
+    int16x8_t color01 = vaddq_s16(base_color, lum01);
+    int16x8_t color23 = vaddq_s16(base_color, lum23);
+
+    // Clamp the candidate colors to [0, 255].
+    color01 = vminq_s16(color01, vdupq_n_s16(255));
+    color01 = vmaxq_s16(color01, vdupq_n_s16(0));
+    color23 = vminq_s16(color23, vdupq_n_s16(255));
+    color23 = vmaxq_s16(color23, vdupq_n_s16(0));
+
+    uint8x16_t candidate_color =
+        vcombine_u8(vmovn_u16(vreinterpretq_u16_s16(color01)),
+                    vmovn_u16(vreinterpretq_u16_s16(color23)));
+
+    // Compute the squared distance between the source and candidate colors.
+    uint8x16_t diff = vabdq_u8(src_color, candidate_color);
+    uint8x8_t diff01 = vget_low_u8(diff);
+    uint8x8_t diff23 = vget_high_u8(diff);
+
+    uint16x8_t square01 = vmull_u8(diff01, diff01);
+    uint16x8_t square23 = vmull_u8(diff23, diff23);
+
+    uint32x4_t psum01 = vpaddlq_u16(square01);
+    uint32x4_t psum23 = vpaddlq_u16(square23);
+    uint32x2_t err01 = vpadd_u32(vget_low_u32(psum01), vget_high_u32(psum01));
+    uint32x2_t err23 = vpadd_u32(vget_low_u32(psum23), vget_high_u32(psum23));
+    uint32x4_t errs = vcombine_u32(err01, err23);
+
+    // Find the minimum error.
+    uint32x2_t min_err = vpmin_u32(err01, err23);
+    min_err = vpmin_u32(min_err, min_err);
+
+    // Find the modifier index which produced the minimum error. This is
+    // essentially the lane number of the lane containing the minimum error.
+    uint32x4_t min_mask = vceqq_u32(vcombine_u32(min_err, min_err), errs);
+    uint32x4_t idxs = vbslq_u32(min_mask, idx_mask, vdupq_n_u32(0xffffffff));
+
+    uint32x2_t min_idx = vpmin_u32(vget_low_u32(idxs), vget_high_u32(idxs));
+    min_idx = vpmin_u32(min_idx, min_idx);
+
+    uint32_t cur_best_mod_err = vget_lane_u32(min_err, 0);
+    uint32_t cur_best_mod_idx = vget_lane_u32(min_idx, 0);
+
+    uint32_t tbl_err = cur_best_mod_err;
+    if (tbl_err < best_tbl_err) {
+      best_tbl_err = tbl_err;
+      best_tbl_idx = tbl_idx;
+      best_mod_idx = cur_best_mod_idx;
+
+      if (tbl_err == 0)
+        break;  // We cannot do any better than this.
+    }
+  }
+
+  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);
+}
+
+void CompressBlock(uint8_t* dst, const Color* ver_src, const Color* hor_src) {
+  ALIGNAS(8) 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]);
+}
+
+void CompressTexture(const uint8_t* src, uint8_t* dst, int width, int height) {
+  DCHECK(width >= 4 && (width & 3) == 0);
+  DCHECK(height >= 4 && (height & 3) == 0);
+
+  ALIGNAS(8) uint32_t ver_blocks[16];
+  ALIGNAS(8) uint32_t hor_blocks[16];
+
+  // Mask for clearing the alpha channel.
+  ALIGNAS(8) const uint32_t clear_mask_ptr[4] = {
+      0xff000000, 0xff000000, 0xff000000, 0xff000000};
+  uint32x4_t clear_mask = vld1q_u32(clear_mask_ptr);
+
+  for (int y = 0; y < height; y += 4, src += width * 4 * 4) {
+    for (int x = 0; x < width; x += 4, dst += 8) {
+      const uint32_t* row0 = reinterpret_cast<const uint32_t*>(src + x * 4);
+      const uint32_t* row1 = row0 + width;
+      const uint32_t* row2 = row1 + width;
+      const uint32_t* row3 = row2 + width;
+
+#ifdef GCC46_INTERNAL_ERROR_WORKAROUND
+      uint32x4x4_t block_transposed;
+#endif
+      ALIGNAS(8) uint32x4_t block[4];
+      block[0] = vld1q_u32(row0);
+      block[1] = vld1q_u32(row1);
+      block[2] = vld1q_u32(row2);
+      block[3] = vld1q_u32(row3);
+
+      // Clear alpha channel.
+      for (unsigned int i = 0; i < 4; ++i) {
+        block[i] = vbicq_u32(block[i], clear_mask);
+      }
+
+      // Check if the block is solid.
+      uint32x4_t solid = vbicq_u32(vdupq_n_u32(*row0), clear_mask);
+
+      uint16x4_t eq0 = vmovn_u32(vceqq_u32(block[0], solid));
+      uint16x4_t eq1 = vmovn_u32(vceqq_u32(block[1], solid));
+      uint16x4_t eq2 = vmovn_u32(vceqq_u32(block[2], solid));
+      uint16x4_t eq3 = vmovn_u32(vceqq_u32(block[3], solid));
+      uint16x4_t tst = vand_u16(vand_u16(eq0, eq1), vand_u16(eq2, eq3));
+
+      ALIGNAS(8) uint64_t solid_block_tst_bits;
+      vst1_u64(&solid_block_tst_bits, vreinterpret_u64_u16(tst));
+
+      if (solid_block_tst_bits == 0xffffffffffffffff) {
+        CompressSolidBlock(dst, *reinterpret_cast<const Color*>(row0));
+        continue;
+      }
+
+      vst1q_u32(hor_blocks, block[0]);
+      vst1q_u32(hor_blocks + 4, block[1]);
+      vst1q_u32(hor_blocks + 8, block[2]);
+      vst1q_u32(hor_blocks + 12, block[3]);
+
+// Texel ordering according to specification:
+// [ 0][ 4][ 8][12]
+// [ 1][ 5][ 9][13]
+// [ 2][ 6][10][14]
+// [ 3][ 7][11][15]
+//
+// To access the vertical blocks using C-style indexing we
+// transpose the block:
+// [ 0][ 1][ 2][ 3]
+// [ 4][ 5][ 6][ 7]
+// [ 8][ 9][10][11]
+// [12][13][14][15]
+#ifdef GCC46_INTERNAL_ERROR_WORKAROUND
+      block_transposed = vld4q_u32(hor_blocks);
+#else
+      uint32x4x4_t block_transposed = vld4q_u32(hor_blocks);
+#endif
+
+      vst1q_u32(ver_blocks, block_transposed.val[0]);
+      vst1q_u32(ver_blocks + 4, block_transposed.val[1]);
+      vst1q_u32(ver_blocks + 8, block_transposed.val[2]);
+      vst1q_u32(ver_blocks + 12, block_transposed.val[3]);
+
+      CompressBlock(dst, reinterpret_cast<const Color*>(ver_blocks),
+                    reinterpret_cast<const Color*>(hor_blocks));
+    }
+  }
+}
+
+}  // namespace etc1_neon
+
+#endif  // __ARM_NEON__