Tile Compression

cc/resources/texture_compress/arm/atc_dxt_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 links below for detailed descriptions of the algorithms used.
+// http://cbloomrants.blogspot.se/2008/12/12-08-08-dxtc-summary.html
+// http://fgiesen.wordpress.com/2009/12/15/dxt5-alpha-block-index-determination
+
+#if defined(__ARM_NEON__)
+
+#include "cc/resources/texture_compress/arm/atc_dxt_neon.h"
+
+#include <arm_neon.h>
+
+#include "base/compiler_specific.h"
+#include "base/logging.h"
+#include "cc/resources/texture_compress/atc_dxt.h"
+
+namespace cc {
+namespace texture_compress {
+
+struct TYPE_ATC_NEON : public TYPE_ATC {
+  typedef TYPE_ATC BASE_TYPE;
+  static const uint8x8_t kRemap;
+  static const uint64_t kProds[3];
+};
+
+struct TYPE_DXT_NEON : public TYPE_DXT {
+  typedef TYPE_DXT BASE_TYPE;
+  static const uint8x8_t kRemap;
+  static const int8x8_t kW1Table;
+  static const uint64_t kProds[3];
+};
+
+const uint8x8_t TYPE_ATC_NEON::kRemap = {0, 1, 0, 1, 2, 2, 3, 3};
+const uint64_t TYPE_ATC_NEON::kProds[3] = {0x00010409, 0x09040100, 0x00020200};
+
+const uint8x8_t TYPE_DXT_NEON::kRemap = {0, 2, 0, 2, 3, 3, 1, 1};
+const int8x8_t TYPE_DXT_NEON::kW1Table = {3, 0, 2, 1, 0, 0, 0, 0};
+const uint64_t TYPE_DXT_NEON::kProds[3] = {0x01040009, 0x04010900, 0x02020000};
+
+// Number of passes over the block that's done to refine the base colors.
+// Only applies to high quality compression mode.
+const int kNumRefinements = 2;
+
+namespace {
+
+template <typename T>
+ALWAYS_INLINE int8x16_t DoW1TableLookup(uint8x16_t indices);
+
+template <>
+ALWAYS_INLINE int8x16_t DoW1TableLookup<TYPE_ATC_NEON>(uint8x16_t indices) {
+  // Take a shortcut for ATC which gives the same result as the table lookup.
+  // {0, 1, 2, 3} -> {3, 2, 1, 0}
+  return veorq_s8(vreinterpretq_s8_u8(indices), vdupq_n_s8(3));
+}
+
+template <>
+ALWAYS_INLINE int8x16_t DoW1TableLookup<TYPE_DXT_NEON>(uint8x16_t indices) {
+  // Do table lookup for each color index.
+  return vcombine_s8(vtbl1_s8(TYPE_DXT_NEON::kW1Table,
+                              vreinterpret_s8_u8(vget_low_u8(indices))),
+                     vtbl1_s8(TYPE_DXT_NEON::kW1Table,
+                              vreinterpret_s8_u8(vget_high_u8(indices))));
+}
+
+// Returns max and min base green colors matching the given single green color
+// when solved via linear interpolation. Output format differs for ATC and DXT.
+// See explicitly instantiated template functions below.
+template <typename T>
+ALWAYS_INLINE uint16_t MatchSingleGreenMax(int g);
+template <typename T>
+ALWAYS_INLINE uint16_t MatchSingleGreenMin(int g);
+
+template <>
+ALWAYS_INLINE uint16_t MatchSingleGreenMax<TYPE_ATC>(int g) {
+  return g_o_match56[g][0] << 1;
+}
+
+template <>
+ALWAYS_INLINE uint16_t MatchSingleGreenMin<TYPE_ATC>(int g) {
+  return g_o_match56[g][1];
+}
+
+template <>
+ALWAYS_INLINE uint16_t MatchSingleGreenMax<TYPE_DXT>(int g) {
+  return g_o_match66[g][0];
+}
+
+template <>
+ALWAYS_INLINE uint16_t MatchSingleGreenMin<TYPE_DXT>(int g) {
+  return g_o_match66[g][1];
+}
+
+// This converts the output data to either ATC or DXT format.
+// See explicitly instantiated template functions below.
+template <typename T>
+ALWAYS_INLINE void FormatFixup(uint16x4_t* base_colors, uint64x1_t* indices);
+
+template <>
+ALWAYS_INLINE void FormatFixup<TYPE_ATC_NEON>(uint16x4_t* base_colors,
+                                              uint64x1_t* indices) {
+  // First color in ATC format is 555.
+  *base_colors = vorr_u16(
+      vand_u16(*base_colors, vreinterpret_u16_u64(vdup_n_u64(0xffff001f))),
+      vshr_n_u16(
+          vand_u16(*base_colors, vreinterpret_u16_u64(vdup_n_u64(0x0000ffC0))),
+          1));
+}
+
+template <>
+ALWAYS_INLINE void FormatFixup<TYPE_DXT_NEON>(uint16x4_t* base_colors,
+                                              uint64x1_t* indices) {
+  // Swap min/max colors if necessary.
+  uint16x4_t max = vdup_lane_u16(*base_colors, 0);
+  uint16x4_t min = vdup_lane_u16(*base_colors, 1);
+  uint16x4_t cmp = vclt_u16(max, min);
+  *base_colors =
+      vorr_u16(vand_u16(vbsl_u16(cmp, min, max),
+                        vreinterpret_u16_u64(vdup_n_u64(0x0000ffff))),
+               vand_u16(vbsl_u16(cmp, max, min),
+                        vreinterpret_u16_u64(vdup_n_u64(0xffff0000))));
+  *indices = vbsl_u64(vreinterpret_u64_u16(cmp),
+                      veor_u64(*indices, vdup_n_u64(0x55555555)), *indices);
+}
+
+// Check if all the 8 bits elements in the given quad register are equal.
+ALWAYS_INLINE bool ElementsEqual(uint8x16_t elements) {
+  uint8x16_t first = vdupq_lane_u8(vget_low_u8(elements), 0);
+  uint8x16_t eq = vceqq_u8(elements, first);
+  uint8x8_t tst = vand_u8(vget_low_u8(eq), vget_high_u8(eq));
+  return vget_lane_u64(vreinterpret_u64_u8(tst), 0) == 0xffffffffffffffff;
+}
+
+ALWAYS_INLINE bool Equal(uint8x16_t e1, uint8x16_t e2) {
+  uint8x16_t eq = vceqq_u8(e1, e2);
+  uint8x8_t tst = vand_u8(vget_low_u8(eq), vget_high_u8(eq));
+  return vget_lane_u64(vreinterpret_u64_u8(tst), 0) == 0xffffffffffffffff;
+}
+
+ALWAYS_INLINE bool Equal(uint16x8_t e1, uint16x8_t e2) {
+  uint16x8_t eq = vceqq_u16(e1, e2);
+  uint16x4_t tst = vand_u16(vget_low_u16(eq), vget_high_u16(eq));
+  return vget_lane_u64(vreinterpret_u64_u16(tst), 0) == 0xffffffffffffffff;
+}
+
+ALWAYS_INLINE bool Equal(uint16x4_t e1, uint16x4_t e2) {
+  uint16x4_t eq = vceq_u16(e1, e2);
+  return vget_lane_u64(vreinterpret_u64_u16(eq), 0) == 0xffffffffffffffff;
+}
+
+ALWAYS_INLINE int16x8x2_t ExpandRGBATo16(const uint8x16_t& channel) {
+  int16x8x2_t result;
+  result.val[0] = vreinterpretq_s16_u16(vmovl_u8(vget_low_u8(channel)));
+  result.val[1] = vreinterpretq_s16_u16(vmovl_u8(vget_high_u8(channel)));
+  return result;
+}
+
+ALWAYS_INLINE int32x4x4_t ExpandRGBATo32(const uint8x16_t& channel) {
+  uint16x8_t lo = vmovl_u8(vget_low_u8(channel));
+  uint16x8_t hi = vmovl_u8(vget_high_u8(channel));
+  int32x4x4_t result;
+  result.val[0] = vreinterpretq_s32_u32(vmovl_u16(vget_low_u16(lo)));
+  result.val[1] = vreinterpretq_s32_u32(vmovl_u16(vget_high_u16(lo)));
+  result.val[2] = vreinterpretq_s32_u32(vmovl_u16(vget_low_u16(hi)));
+  result.val[3] = vreinterpretq_s32_u32(vmovl_u16(vget_high_u16(hi)));
+  return result;
+}
+
+// NEON doesn't have support for division.
+// Instead it's recommended to use Newton-Raphson refinement to get a close
+// approximation.
+template <int REFINEMENT_STEPS>
+ALWAYS_INLINE float32x4_t Divide(float32x4_t a, float32x4_t b) {
+#ifdef VERIFY_RESULTS
+  ALIGNAS(8) float a_[4];
+  ALIGNAS(8) float b_[4];
+  vst1q_f32(a_, a);
+  vst1q_f32(b_, b);
+  for (int i = 0; i < 4; ++i)
+    a_[i] /= b_[i];
+  return vld1q_f32(a_);
+#else
+  // Get an initial estimate of 1/b.
+  float32x4_t reciprocal = vrecpeq_f32(b);
+  // Use a number of Newton-Raphson steps to refine the estimate.
+  for (int i = 0; i < REFINEMENT_STEPS; ++i)
+    reciprocal = vmulq_f32(vrecpsq_f32(b, reciprocal), reciprocal);
+  // Calculate the final estimate.
+  return vmulq_f32(a, reciprocal);
+#endif
+}
+
+namespace vec_ops {
+
+struct Max {
+  ALWAYS_INLINE int32x4_t Calc(int32x4_t a, int32x4_t b) {
+    return vmaxq_s32(a, b);
+  }
+
+  ALWAYS_INLINE uint32x4_t Calc(uint32x4_t a, uint32x4_t b) {
+    return vmaxq_u32(a, b);
+  }
+
+  ALWAYS_INLINE uint8x8_t Fold(uint8x8_t a, uint8x8_t b) {
+    return vpmax_u8(a, b);
+  }
+
+  ALWAYS_INLINE int32x2_t Fold(int32x2_t a, int32x2_t b) {
+    return vpmax_s32(a, b);
+  }
+
+  ALWAYS_INLINE uint32x2_t Fold(uint32x2_t a, uint32x2_t b) {
+    return vpmax_u32(a, b);
+  }
+};
+
+struct Min {
+  ALWAYS_INLINE int32x4_t Calc(int32x4_t a, int32x4_t b) {
+    return vminq_s32(a, b);
+  }
+
+  ALWAYS_INLINE uint32x4_t Calc(uint32x4_t a, uint32x4_t b) {
+    return vminq_u32(a, b);
+  }
+
+  ALWAYS_INLINE uint8x8_t Fold(uint8x8_t a, uint8x8_t b) {
+    return vpmin_u8(a, b);
+  }
+
+  ALWAYS_INLINE int32x2_t Fold(int32x2_t a, int32x2_t b) {
+    return vpmin_s32(a, b);
+  }
+
+  ALWAYS_INLINE uint32x2_t Fold(uint32x2_t a, uint32x2_t b) {
+    return vpmin_u32(a, b);
+  }
+};
+
+}  // namespace vec_ops
+
+template <typename Operator>
+ALWAYS_INLINE uint8x8_t FoldRGBA(const uint8x16x4_t& src) {
+  Operator op;
+
+  // Fold each adjacent pair.
+  uint8x8_t r = op.Fold(vget_low_u8(src.val[0]), vget_high_u8(src.val[0]));
+  uint8x8_t g = op.Fold(vget_low_u8(src.val[1]), vget_high_u8(src.val[1]));
+  uint8x8_t b = op.Fold(vget_low_u8(src.val[2]), vget_high_u8(src.val[2]));
+  uint8x8_t a = op.Fold(vget_low_u8(src.val[3]), vget_high_u8(src.val[3]));
+
+  // Do both red and green channels at the same time.
+  uint8x8_t rg = op.Fold(r, g);
+
+  // Do both blue and alpha channels at the same time.
+  uint8x8_t ba = op.Fold(b, a);
+
+  // Do all the channels at the same time.
+  uint8x8_t rgba = op.Fold(rg, ba);
+
+  // Finally, we need to pad it to get the final reduction.
+  return op.Fold(rgba, rgba);
+}
+
+template <typename Operator>
+ALWAYS_INLINE int32x2_t Fold(const int32x4x4_t& src) {
+  Operator op;
+
+  int32x4_t fold0 = op.Calc(src.val[0], src.val[1]);
+  int32x4_t fold1 = op.Calc(src.val[2], src.val[3]);
+  int32x4_t fold01 = op.Calc(fold0, fold1);
+  int32x2_t fold0123 = op.Fold(vget_low_s32(fold01), vget_high_s32(fold01));
+  return op.Fold(fold0123, vdup_n_s32(0));
+}
+
+template <typename Operator>
+ALWAYS_INLINE uint32x2_t Fold(const uint32x4x4_t& src) {
+  Operator op;
+
+  uint32x4_t fold0 = op.Calc(src.val[0], src.val[1]);
+  uint32x4_t fold1 = op.Calc(src.val[2], src.val[3]);
+  uint32x4_t fold01 = op.Calc(fold0, fold1);
+  uint32x2_t fold0123 = op.Fold(vget_low_u32(fold01), vget_high_u32(fold01));
+  return op.Fold(fold0123, vdup_n_u32(0));
+}
+
+template <typename Operator>
+ALWAYS_INLINE int32x4_t FoldDup(const int32x4x4_t& src) {
+  return vdupq_lane_s32(Fold<Operator>(src), 0);
+}
+
+ALWAYS_INLINE uint16x4_t SumRGB(const uint8x16x4_t& src) {
+  // Add up all red values for 16 pixels.
+  uint16x8_t r = vpaddlq_u8(src.val[0]);
+  uint16x4_t r2 = vpadd_u16(vget_low_u16(r), vget_high_u16(r));
+
+  // Add up all green values for 16 pixels.
+  uint16x8_t g = vpaddlq_u8(src.val[1]);
+  uint16x4_t g2 = vpadd_u16(vget_low_u16(g), vget_high_u16(g));
+
+  uint16x4_t rg = vpadd_u16(r2, g2);
+
+  // Add up all blue values for 16 pixels.
+  uint16x8_t b = vpaddlq_u8(src.val[2]);
+  uint16x4_t b2 = vpadd_u16(vget_low_u16(b), vget_high_u16(b));
+
+  uint16x4_t ba = vpadd_u16(b2, vdup_n_u16(0));
+
+  return vpadd_u16(rg, ba);
+}
+
+ALWAYS_INLINE int32x4_t SumRGB(const int16x8x4_t& src) {
+  // Add up all red values for 8 pixels.
+  int32x4_t r = vpaddlq_s16(src.val[0]);
+  int32x2_t r2 = vpadd_s32(vget_low_s32(r), vget_high_s32(r));
+
+  // Add up all green values for 8 pixels.
+  int32x4_t g = vpaddlq_s16(src.val[1]);
+  int32x2_t g2 = vpadd_s32(vget_low_s32(g), vget_high_s32(g));
+
+  int32x2_t rg = vpadd_s32(r2, g2);
+
+  // Add up all blue values for 8 pixels.
+  int32x4_t b = vpaddlq_s16(src.val[2]);
+  int32x2_t b2 = vpadd_s32(vget_low_s32(b), vget_high_s32(b));
+
+  int32x2_t ba = vpadd_s32(b2, vdup_n_s32(0));
+
+  return vcombine_s32(rg, ba);
+}
+
+ALWAYS_INLINE int32x4_t SumRGB(const int32x4x4_t& src) {
+  // Add up all red values for 8 pixels.
+  int32x2_t r = vmovn_s64(vpaddlq_s32(src.val[0]));
+
+  // Add up all green values for 8 pixels.
+  int32x2_t g = vmovn_s64(vpaddlq_s32(src.val[1]));
+
+  int32x2_t rg = vpadd_s32(r, g);
+
+  // Add up all blue values for 8 pixels.
+  int32x2_t b = vmovn_s64(vpaddlq_s32(src.val[2]));
+
+  int32x2_t ba = vpadd_s32(b, vdup_n_s32(0));
+
+  return vcombine_s32(rg, ba);
+}
+
+ALWAYS_INLINE int32x4_t DotProduct(int32x4_t r,
+                                   int32x4_t g,
+                                   int32x4_t b,
+                                   int32x4_t dir_r,
+                                   int32x4_t dir_g,
+                                   int32x4_t dir_b) {
+  // Multiply and accumulate each 32 bits element.
+  int32x4_t dots = vmulq_s32(r, dir_r);
+  dots = vmlaq_s32(dots, g, dir_g);
+  dots = vmlaq_s32(dots, b, dir_b);
+  return dots;
+}
+
+ALWAYS_INLINE int32x4x4_t CalculateDots(const int32x4x4_t& r,
+                                        const int32x4x4_t& g,
+                                        const int32x4x4_t& b,
+                                        const int32x4_t& v_vec) {
+  // Duplicate the red, green and blue luminance values.
+  int32x4_t r_vec = vdupq_n_s32(vgetq_lane_s32(v_vec, 0));
+  int32x4_t g_vec = vdupq_n_s32(vgetq_lane_s32(v_vec, 1));
+  int32x4_t b_vec = vdupq_n_s32(vgetq_lane_s32(v_vec, 2));
+
+  int32x4x4_t result;
+  result.val[0] = DotProduct(r.val[0], g.val[0], b.val[0], r_vec, g_vec, b_vec);
+  result.val[1] = DotProduct(r.val[1], g.val[1], b.val[1], r_vec, g_vec, b_vec);
+  result.val[2] = DotProduct(r.val[2], g.val[2], b.val[2], r_vec, g_vec, b_vec);
+  result.val[3] = DotProduct(r.val[3], g.val[3], b.val[3], r_vec, g_vec, b_vec);
+  return result;
+}
+
+ALWAYS_INLINE uint16x8_t QuantizeTo565(uint8x8_t pixels) {
+  // in:  [min_r min_g min_b 0 max_r max_g max_b 0]
+  // out: [min_r5 min_g6 min_b5 0][max_r5 max_g6 max_b5 0]
+
+  // Expand the components to signed 16 bit.
+  uint16x8_t pixels16 = vmovl_u8(pixels);
+
+  // {31, 63, 31, 0, 31, 63, 31, 0};
+  const uint16x8_t kMultiply = vreinterpretq_u16_u64(vdupq_n_u64(0x1f003f001f));
+  uint16x8_t pixel0 = vmulq_u16(pixels16, kMultiply);
+
+  // {128, 128, 128, 0, 128, 128, 128, 0};
+  const uint16x8_t kAdd = vreinterpretq_u16_u64(vdupq_n_u64(0x8000800080));
+  uint16x8_t pixel1 = vaddq_u16(pixel0, kAdd);
+
+  // Create a shifted copy.
+  uint16x8_t pixel2 = vsraq_n_u16(pixel1, pixel1, 8);
+
+  // Shift and return.
+  return vshrq_n_u16(pixel2, 8);
+}
+
+// Combine the components of base colors in to 16 bits.
+ALWAYS_INLINE uint16x4_t PackBaseColors(uint16x8_t base_colors) {
+  // in:  [max_r5 max_g6 max_b5 0][min_r5 min_g6 min_b5 0]
+  // out: [max_rgb565 min_rgb565 0 0]
+
+  // Swapping r and b channels to match Skia.
+  base_colors = vrev64q_u16(base_colors);
+  base_colors = vcombine_u16(
+      vext_u16(vget_low_u16(base_colors), vget_low_u16(base_colors), 1),
+      vext_u16(vget_high_u16(base_colors), vget_high_u16(base_colors), 1));
+
+  // Shift to pack RGB565 in 16-bit.
+  uint64x2_t r =
+      vshlq_u64(vreinterpretq_u64_u16(base_colors), vdupq_n_s64(-32));
+  uint64x2_t g =
+      vshlq_u64(vreinterpretq_u64_u16(base_colors), vdupq_n_s64(-11));
+  uint64x2_t b = vshlq_u64(vreinterpretq_u64_u16(base_colors), vdupq_n_s64(11));
+  uint64x2_t base_colors_16 = vorrq_u64(r, vorrq_u64(g, b));
+
+  // Shift to pack 16-bit base colors in 32-bit and return.
+  return vreinterpret_u16_u64(
+      vorr_u64(vshl_n_u64(vget_high_u64(base_colors_16), 16),
+               vand_u64(vget_low_u64(base_colors_16), vdup_n_u64(0xffff))));
+}
+
+// Combine the given color indices.
+//
+// Params:
+//  S        Size of an index in bits.
+//  indices  Indices to be combined. Each of 8 bits element represents an index.
+template <int S>
+ALWAYS_INLINE uint64x1_t PackIndices(uint8x16_t indices) {
+  uint64x2_t ind = vshlq_n_u64(vreinterpretq_u64_u8(indices), 8 - S);
+  const uint64x2_t mask = vdupq_n_u64(0xff00000000000000);
+  uint64x2_t ind2 = vandq_u64(vshlq_n_u64(ind, 56), mask);
+  ind2 = vorrq_u64(vshrq_n_u64(ind2, S), vandq_u64(vshlq_n_u64(ind, 48), mask));
+  ind2 = vorrq_u64(vshrq_n_u64(ind2, S), vandq_u64(vshlq_n_u64(ind, 40), mask));
+  ind2 = vorrq_u64(vshrq_n_u64(ind2, S), vandq_u64(vshlq_n_u64(ind, 32), mask));
+  ind2 = vorrq_u64(vshrq_n_u64(ind2, S), vandq_u64(vshlq_n_u64(ind, 24), mask));
+  ind2 = vorrq_u64(vshrq_n_u64(ind2, S), vandq_u64(vshlq_n_u64(ind, 16), mask));
+  ind2 = vorrq_u64(vshrq_n_u64(ind2, S), vandq_u64(vshlq_n_u64(ind, 8), mask));
+  ind2 = vorrq_u64(vshrq_n_u64(ind2, S), vandq_u64(ind, mask));
+  return vshr_n_u64(
+      vorr_u64(vshr_n_u64(vget_low_u64(ind2), (8 * S)), vget_high_u64(ind2)),
+      64 - 16 * S);
+}
+
+ALWAYS_INLINE int32x4_t
+CovarianceChannels(const int16x8x2_t& ch1, const int16x8x2_t& ch2) {
+  // Multiply and accumulate.
+  int32x4_t cov;
+  cov = vmull_s16(vget_low_s16(ch1.val[0]), vget_low_s16(ch2.val[0]));
+  cov = vmlal_s16(cov, vget_high_s16(ch1.val[0]), vget_high_s16(ch2.val[0]));
+  cov = vmlal_s16(cov, vget_low_s16(ch1.val[1]), vget_low_s16(ch2.val[1]));
+  cov = vmlal_s16(cov, vget_high_s16(ch1.val[1]), vget_high_s16(ch2.val[1]));
+  return cov;
+}
+
+ALWAYS_INLINE int32x4x2_t
+Covariance(uint16x4_t average_rgb, const uint8x16x4_t& pixels_scattered) {
+  int16x8_t average_r = vreinterpretq_s16_u16(vdupq_lane_u16(average_rgb, 0));
+  int16x8_t average_g = vreinterpretq_s16_u16(vdupq_lane_u16(average_rgb, 1));
+  int16x8_t average_b = vreinterpretq_s16_u16(vdupq_lane_u16(average_rgb, 2));
+
+  // Subtract red values from the average red.
+  int16x8x2_t diff_r;
+  diff_r.val[0] = vsubq_s16(
+      vreinterpretq_s16_u16(vmovl_u8(vget_low_u8(pixels_scattered.val[0]))),
+      average_r);
+  diff_r.val[1] = vsubq_s16(
+      vreinterpretq_s16_u16(vmovl_u8(vget_high_u8(pixels_scattered.val[0]))),
+      average_r);
+
+  // Subtract green values from the average green.
+  int16x8x2_t diff_g;
+  diff_g.val[0] = vsubq_s16(
+      vreinterpretq_s16_u16(vmovl_u8(vget_low_u8(pixels_scattered.val[1]))),
+      average_g);
+  diff_g.val[1] = vsubq_s16(
+      vreinterpretq_s16_u16(vmovl_u8(vget_high_u8(pixels_scattered.val[1]))),
+      average_g);
+
+  // Subtract blue values from the average blue.
+  int16x8x2_t diff_b;
+  diff_b.val[0] = vsubq_s16(
+      vreinterpretq_s16_u16(vmovl_u8(vget_low_u8(pixels_scattered.val[2]))),
+      average_b);
+  diff_b.val[1] = vsubq_s16(
+      vreinterpretq_s16_u16(vmovl_u8(vget_high_u8(pixels_scattered.val[2]))),
+      average_b);
+
+  int32x4x4_t cov1;
+  cov1.val[0] = CovarianceChannels(diff_r, diff_r);
+  cov1.val[1] = CovarianceChannels(diff_r, diff_g);
+  cov1.val[2] = CovarianceChannels(diff_r, diff_b);
+  cov1.val[3] = vdupq_n_s32(0);
+
+  int32x4x4_t cov2;
+  cov2.val[0] = CovarianceChannels(diff_g, diff_g);
+  cov2.val[1] = CovarianceChannels(diff_g, diff_b);
+  cov2.val[2] = CovarianceChannels(diff_b, diff_b);
+  cov2.val[3] = vdupq_n_s32(0);
+
+  int32x4x2_t covariance;
+  covariance.val[0] = SumRGB(cov1);
+  covariance.val[1] = SumRGB(cov2);
+  return covariance;
+}
+
+ALWAYS_INLINE uint32x2_t MaskOutPixel(const uint8x16x4_t& pixels_linear,
+                                      const int32x4x4_t& dots,
+                                      int32x4_t max_dot_vec) {
+  // Mask out any of the 16 pixels where the dot product matches exactly.
+  uint32x4x4_t pixels;
+  pixels.val[0] = vandq_u32(vceqq_s32(dots.val[0], max_dot_vec),
+                            vreinterpretq_u32_u8(pixels_linear.val[0]));
+
+  pixels.val[1] = vandq_u32(vceqq_s32(dots.val[1], max_dot_vec),
+                            vreinterpretq_u32_u8(pixels_linear.val[1]));
+
+  pixels.val[2] = vandq_u32(vceqq_s32(dots.val[2], max_dot_vec),
+                            vreinterpretq_u32_u8(pixels_linear.val[2]));
+
+  pixels.val[3] = vandq_u32(vceqq_s32(dots.val[3], max_dot_vec),
+                            vreinterpretq_u32_u8(pixels_linear.val[3]));
+
+  // Fold it down.
+  return Fold<vec_ops::Max>(pixels);
+}
+
+ALWAYS_INLINE uint16x8_t GetBaseColors(const uint8x16x4_t& pixels_linear,
+                                       const uint8x16x4_t& pixels_scattered,
+                                       int32x4_t dir) {
+  // Expand all pixels to signed 32-bit integers.
+  int32x4x4_t r = ExpandRGBATo32(pixels_scattered.val[0]);
+  int32x4x4_t g = ExpandRGBATo32(pixels_scattered.val[1]);
+  int32x4x4_t b = ExpandRGBATo32(pixels_scattered.val[2]);
+
+  int32x4x4_t dots = CalculateDots(r, g, b, dir);
+
+  // Mask out the pixel(s) that matches the max dot.
+  uint32x2_t max_pixel =
+      MaskOutPixel(pixels_linear, dots, FoldDup<vec_ops::Max>(dots));
+
+  // Mask out the pixel(s) that matches the min dot.
+  uint32x2_t min_pixel =
+      MaskOutPixel(pixels_linear, dots, FoldDup<vec_ops::Min>(dots));
+
+  return QuantizeTo565(
+      vreinterpret_u8_u32(vzip_u32(max_pixel, min_pixel).val[0]));
+}
+
+// Figure out the two base colors to use from a block of 16 pixels
+// by Primary Component Analysis and map along principal axis.
+ALWAYS_INLINE uint16x8_t
+OptimizeColorsBlock(const uint8x16x4_t& pixels_linear,
+                    const uint8x16x4_t& pixels_scattered,
+                    uint16x4_t sum_rgb,
+                    uint8x8_t min_rgba,
+                    uint8x8_t max_rgba) {
+  // min_rgba: [min_r min_g min_b min_a x x x x]
+  // max_rgba: [max_r max_g max_b max_a x x x x]
+
+  // Determine color distribution. We already have the max and min, now we need
+  // the average of the 16 pixels. Divide sum_rgb with rounding.
+  uint16x4_t average_rgb = vrshr_n_u16(sum_rgb, 4);
+
+  // Determine covariance matrix.
+  int32x4x2_t covariance = Covariance(average_rgb, pixels_scattered);
+
+  // Convert covariance matrix to float, find principal axis via power
+  // iteration.
+  float32x4x2_t covariance_float;
+  const float32x4_t kInv255 = vdupq_n_f32(1.0f / 255.0f);
+  covariance_float.val[0] =
+      vmulq_f32(vcvtq_f32_s32(covariance.val[0]), kInv255);
+  covariance_float.val[1] =
+      vmulq_f32(vcvtq_f32_s32(covariance.val[1]), kInv255);
+
+  int16x4_t max_16 = vreinterpret_s16_u16(vget_low_u16(vmovl_u8(max_rgba)));
+  int16x4_t min_16 = vreinterpret_s16_u16(vget_low_u16(vmovl_u8(min_rgba)));
+  float32x4_t vf4 = vcvtq_f32_s32(vsubl_s16(max_16, min_16));
+
+  for (int i = 0; i < 4; ++i) {
+    float32x4_t vfr4 = vdupq_n_f32(vgetq_lane_f32(vf4, 0));
+    float32x4_t vfg4 = vdupq_n_f32(vgetq_lane_f32(vf4, 1));
+    float32x4_t vfb4 = vdupq_n_f32(vgetq_lane_f32(vf4, 2));
+
+    // from: [0 1 2 x] [3 4 5 x]
+    // to:   [1 3 4 x]
+    float32x4_t cov_134 =
+        vextq_f32(covariance_float.val[1], covariance_float.val[1], 3);
+    cov_134 =
+        vsetq_lane_f32(vgetq_lane_f32(covariance_float.val[0], 1), cov_134, 0);
+
+    // from: [0 1 2 x] [3 4 5 x]
+    // to:   [2 4 5 x]
+    float32x4_t cov_245 = vsetq_lane_f32(
+        vgetq_lane_f32(covariance_float.val[0], 2), covariance_float.val[1], 0);
+
+    vf4 = vmulq_f32(vfr4, covariance_float.val[0]);
+    vf4 = vmlaq_f32(vf4, vfg4, cov_134);
+    vf4 = vmlaq_f32(vf4, vfb4, cov_245);
+  }
+
+  float32x4_t magnitude = vabsq_f32(vf4);
+  magnitude = vsetq_lane_f32(0.0f, magnitude, 3);  // Null out alpha.
+  float32x4_t mag4 = vdupq_lane_f32(
+      vpmax_f32(vpmax_f32(vget_low_f32(magnitude), vget_high_f32(magnitude)),
+                vdup_n_f32(0.0f)),
+      0);
+
+  const int32x4_t kLuminance = {299, 587, 114, 0};
+
+  // Note that this quite often means dividing by zero. The math still works
+  // when comparing with Inf though.
+  float32x4_t inv_magnitude = Divide<2>(vdupq_n_f32(512.0f), mag4);
+
+  int32x4_t vf4_mag = vcvtq_s32_f32(vmulq_f32(vf4, inv_magnitude));
+  int32x4_t v =
+      vbslq_s32(vcltq_f32(mag4, vdupq_n_f32(4.0f)), kLuminance, vf4_mag);
+
+  return GetBaseColors(pixels_linear, pixels_scattered, v);
+}
+
+ALWAYS_INLINE uint16x8_t
+GetApproximateBaseColors(const uint8x16x4_t& pixels_linear,
+                         const uint8x16x4_t& pixels_scattered,
+                         uint8x8_t min_rgba,
+                         uint8x8_t max_rgba) {
+  // min_rgba: [min_r min_g min_b min_a x x x x]
+  // max_rgba: [max_r max_g max_b max_a x x x x]
+
+  // Get direction vector and expand to 32-bit.
+  int16x4_t max_16 = vreinterpret_s16_u16(vget_low_u16(vmovl_u8(max_rgba)));
+  int16x4_t min_16 = vreinterpret_s16_u16(vget_low_u16(vmovl_u8(min_rgba)));
+  int32x4_t v = vsubl_s16(max_16, min_16);
+
+  return GetBaseColors(pixels_linear, pixels_scattered, v);
+}
+
+// Take two base colors and generate 4 RGBX colors where:
+//  0 = baseColor0
+//  1 = baseColor1
+//  2 = (2 * baseColor0 + baseColor1) / 3
+//  3 = (2 * baseColor1 + baseColor0) / 3
+ALWAYS_INLINE uint16x4x4_t EvalColors(const uint16x8_t& base_colors) {
+  // The base colors are expanded by reusing the top bits at the end. That makes
+  // sure that white is still white after being quantized and converted back.
+  //
+  // [(r<<3 | r>>2) (g<<2 | g>>4) (b<<3 | b>>2) 0]
+
+  // The upper shift values for each component.
+  // {3, 2, 3, 0, 3, 2, 3, 0};
+  const int16x8_t kShiftUp = vreinterpretq_s16_u64(vdupq_n_u64(0x300020003));
+  uint16x8_t pixels_up = vshlq_u16(base_colors, kShiftUp);
+  // [r0<<3 g0<<2 b0<<3 0] [r1<<3 g1<<2 b1<<3 0]
+
+  // The lower shift values for each component.
+  // Note that we need to use negative values to shift right.
+  // {-2, -4, -2, 0, -2, -4, -2, 0};
+  const int16x8_t kShiftDown =
+      vreinterpretq_s16_u64(vdupq_n_u64(0xfffefffcfffe));
+  uint16x8_t pixels_down = vshlq_u16(base_colors, kShiftDown);
+  // [r0>>2 g0>>4 b0>>2 0] [r1>>2 g1>>4 b1>>2 0]
+
+  uint16x8_t pixels = vorrq_u16(pixels_up, pixels_down);
+  // [(r0<<3 | r0>>2) (g0<<2 | g0>>4) (b0<<3 | b0>>2) 0]
+  // [(r1<<3 | r1>>2) (g1<<2 | g1>>4) (b1<<3 | b1>>2) 0]
+
+  // Linear interpolate the two other colors:
+  // (2 * max + min) / 3
+  // (2 * min + max) / 3
+
+  uint16x8_t pixels_mul2 = vaddq_u16(pixels, pixels);
+
+  uint16x8_t swapped = vreinterpretq_u16_u64(vextq_u64(
+      vreinterpretq_u64_u16(pixels), vreinterpretq_u64_u16(pixels), 1));
+  int16x8_t output = vreinterpretq_s16_u16(vaddq_u16(pixels_mul2, swapped));
+
+  // There's no division in NEON, but we can use "x * ((1 << 16) / 3 + 1))"
+  // instead.
+  output = vqdmulhq_s16(output, vdupq_n_s16(((1 << 16) / 3 + 1) >> 1));
+
+  uint16x4x4_t colors;
+  colors.val[0] = vget_low_u16(pixels);
+  colors.val[1] = vget_high_u16(pixels);
+  colors.val[2] = vreinterpret_u16_s16(vget_low_s16(output));
+  colors.val[3] = vreinterpret_u16_s16(vget_high_s16(output));
+  return colors;
+}
+
+template <typename T>
+ALWAYS_INLINE uint8x8_t GetRemapIndices(int32x4_t dots,
+                                        int32x4_t half_point,
+                                        int32x4_t c0_point,
+                                        int32x4_t c3_point) {
+  // bits = (dot < half_point ? 4 : 0)
+  //      | (dot < c0_point   ? 2 : 0)
+  //      | (dot < c3_point   ? 1 : 0)
+  int32x4_t cmp0 = vreinterpretq_s32_u32(
+      vandq_u32(vcgtq_s32(half_point, dots), vdupq_n_u32(4)));
+  int32x4_t cmp1 = vreinterpretq_s32_u32(
+      vandq_u32(vcgtq_s32(c0_point, dots), vdupq_n_u32(2)));
+  int32x4_t cmp2 = vreinterpretq_s32_u32(
+      vandq_u32(vcgtq_s32(c3_point, dots), vdupq_n_u32(1)));
+  int32x4_t bits = vorrq_s32(vorrq_s32(cmp0, cmp1), cmp2);
+
+  // Narrow it down to unsigned 8 bits and return.
+  return vqmovn_u16(vcombine_u16(vqmovun_s32(bits), vdup_n_u16(0)));
+}
+
+// dots:   Dot products for each pixel.
+// points: Crossover points.
+template <typename T>
+ALWAYS_INLINE uint8x16_t GetColorIndices(int32x4x4_t dots, int32x4_t points) {
+  // Crossover points for "best color in top half"/"best in bottom half" and
+  // the same inside that subinterval.
+  int32x4_t c0_point = vdupq_lane_s32(vget_low_s32(points), 1);
+  int32x4_t half_point = vdupq_lane_s32(vget_low_s32(points), 0);
+  int32x4_t c3_point = vdupq_lane_s32(vget_high_s32(points), 0);
+
+  // Get kRemap table indices.
+  uint8x8x4_t ind;
+  ind.val[0] = GetRemapIndices<T>(dots.val[0], half_point, c0_point, c3_point);
+  ind.val[1] = GetRemapIndices<T>(dots.val[1], half_point, c0_point, c3_point);
+  ind.val[2] = GetRemapIndices<T>(dots.val[2], half_point, c0_point, c3_point);
+  ind.val[3] = GetRemapIndices<T>(dots.val[3], half_point, c0_point, c3_point);
+
+  // Combine indices.
+  uint8x8_t indices_lo =
+      vreinterpret_u8_u32(vzip_u32(vreinterpret_u32_u8(ind.val[0]),
+                                   vreinterpret_u32_u8(ind.val[1])).val[0]);
+  uint8x8_t indices_hi =
+      vreinterpret_u8_u32(vzip_u32(vreinterpret_u32_u8(ind.val[2]),
+                                   vreinterpret_u32_u8(ind.val[3])).val[0]);
+  // Do table lookup and return 2-bit color indices.
+  return vcombine_u8(vtbl1_u8(T::kRemap, indices_lo),
+                     vtbl1_u8(T::kRemap, indices_hi));
+}
+
+template <typename T>
+ALWAYS_INLINE uint8x16_t
+MatchColorsBlock(const uint8x16x4_t& pixels_scattered, uint16x4x4_t colors) {
+  // Get direction vector and expand to 32-bit.
+  int32x4_t dir = vsubl_s16(vreinterpret_s16_u16(colors.val[0]),
+                            vreinterpret_s16_u16(colors.val[1]));
+  // Duplicate r g b elements of direction into different registers.
+  int32x4_t dir_r = vdupq_lane_s32(vget_low_s32(dir), 0);
+  int32x4_t dir_g = vdupq_lane_s32(vget_low_s32(dir), 1);
+  int32x4_t dir_b = vdupq_lane_s32(vget_high_s32(dir), 0);
+
+  // Transpose to separate red, green, blue and alpha channels into 4 different
+  // registers. Alpha is ignored.
+  uint16x4x2_t trn_lo = vtrn_u16(colors.val[0], colors.val[1]);
+  uint16x4x2_t trn_hi = vtrn_u16(colors.val[2], colors.val[3]);
+  uint32x4x2_t transposed_colors = vtrnq_u32(
+      vreinterpretq_u32_u16(vcombine_u16(trn_lo.val[0], trn_lo.val[1])),
+      vreinterpretq_u32_u16(vcombine_u16(trn_hi.val[0], trn_hi.val[1])));
+
+  // Expand to 32-bit.
+  int32x4_t colors_r =
+      vmovl_s16(vget_low_s16(vreinterpretq_s16_u32(transposed_colors.val[0])));
+  int32x4_t colors_g =
+      vmovl_s16(vget_high_s16(vreinterpretq_s16_u32(transposed_colors.val[0])));
+  int32x4_t colors_b =
+      vmovl_s16(vget_low_s16(vreinterpretq_s16_u32(transposed_colors.val[1])));
+
+  // Get dot products.
+  int32x4_t stops =
+      DotProduct(colors_r, colors_g, colors_b, dir_r, dir_g, dir_b);
+
+  // Build a register containing 4th, 2nd and 3rd elements of stops respectively
+  // in each 32 bits element.
+  int32x4_t points1 = vsetq_lane_s32(vgetq_lane_s32(stops, 3), stops, 0);
+  // Build a register containing 3rd, 4th and 1st elements of stops respectively
+  // in each 32 bits element.
+  int32x4_t points2 = vreinterpretq_s32_s64(
+      vextq_s64(vreinterpretq_s64_s32(stops), vreinterpretq_s64_s32(stops), 1));
+  // Add and divide by 2.
+  int32x4_t points = vshrq_n_s32(vaddq_s32(points1, points2), 1);
+
+  // Expand all pixels to signed 32-bit integers.
+  int32x4x4_t r = ExpandRGBATo32(pixels_scattered.val[0]);
+  int32x4x4_t g = ExpandRGBATo32(pixels_scattered.val[1]);
+  int32x4x4_t b = ExpandRGBATo32(pixels_scattered.val[2]);
+
+  int32x4x4_t dots = CalculateDots(r, g, b, dir);
+
+  // Get 2-bit color indices.
+  return GetColorIndices<T>(dots, points);
+}
+
+template <typename T>
+ALWAYS_INLINE uint32x4x2_t DoProdsTableLookup(uint8x8_t indices) {
+  // Do table lookup for each color index. The values in the table are 3 bytes
+  // big so we do it in 3 steps.
+  uint16x8_t lookup1 = vmovl_u8(vtbl1_u8(vcreate_u8(T::kProds[0]), indices));
+  uint16x8_t lookup2 = vmovl_u8(vtbl1_u8(vcreate_u8(T::kProds[1]), indices));
+  uint16x8_t lookup3 = vmovl_u8(vtbl1_u8(vcreate_u8(T::kProds[2]), indices));
+  // Expand to 32-bit.
+  uint32x4_t lookup1_lo = vmovl_u16(vget_low_u16(lookup1));
+  uint32x4_t lookup1_hi = vmovl_u16(vget_high_u16(lookup1));
+  uint32x4_t lookup2_lo = vmovl_u16(vget_low_u16(lookup2));
+  uint32x4_t lookup2_hi = vmovl_u16(vget_high_u16(lookup2));
+  uint32x4_t lookup3_lo = vmovl_u16(vget_low_u16(lookup3));
+  uint32x4_t lookup3_hi = vmovl_u16(vget_high_u16(lookup3));
+  // Combine results by shifting and or-ing to obtain the actual table value.
+  uint32x4x2_t result;
+  result.val[0] = vorrq_u32(lookup3_lo, vorrq_u32(vshlq_n_u32(lookup2_lo, 8),
+                                                  vshlq_n_u32(lookup1_lo, 16)));
+  result.val[1] = vorrq_u32(lookup3_hi, vorrq_u32(vshlq_n_u32(lookup2_hi, 8),
+                                                  vshlq_n_u32(lookup1_hi, 16)));
+  return result;
+}
+
+// Tries to optimize colors to suit block contents better.
+// Done by solving a least squares system via normal equations+Cramer's rule.
+template <typename T>
+ALWAYS_INLINE int RefineBlock(const uint8x16x4_t& pixels_scattered,
+                              uint16x4_t sum_rgb,
+                              uint16x8_t& base_colors,
+                              uint8x16_t indices) {
+  uint16x8_t old_base_colors = base_colors;
+
+  if (ElementsEqual(indices)) {  // Do all pixels have the same index?
+    // Yes, linear system would be singular; solve using optimal single-color
+    // match on average color.
+
+    // Get the average of the 16 pixels with rounding.
+    uint16x4_t average_rgb = vrshr_n_u16(sum_rgb, 4);
+
+    ALIGNAS(8) uint16_t rgb[4];
+    vst1_u16(rgb, average_rgb);
+    // Look up optimal values instead of trying to calculate.
+    uint16_t colors[8] = {g_o_match55[rgb[0]][0],
+                          MatchSingleGreenMax<typename T::BASE_TYPE>(rgb[1]),
+                          g_o_match55[rgb[2]][0],
+                          0,
+                          g_o_match55[rgb[0]][1],
+                          MatchSingleGreenMin<typename T::BASE_TYPE>(rgb[1]),
+                          g_o_match55[rgb[2]][1],
+                          0};
+    base_colors = vld1q_u16(colors);
+  } else {
+    // Expand to 16-bit.
+    int16x8x2_t r = ExpandRGBATo16(pixels_scattered.val[0]);
+    int16x8x2_t g = ExpandRGBATo16(pixels_scattered.val[1]);
+    int16x8x2_t b = ExpandRGBATo16(pixels_scattered.val[2]);
+
+    // Do table lookup for each color index.
+    int8x16_t w1 = DoW1TableLookup<T>(indices);
+    // Expand to 16-bit.
+    int16x8_t w1_lo = vmovl_s8(vget_low_s8(w1));
+    int16x8_t w1_hi = vmovl_s8(vget_high_s8(w1));
+    // Multiply and accumulate.
+    int16x8x4_t at1_rgb;
+    at1_rgb.val[0] = vmulq_s16(w1_lo, r.val[0]);
+    at1_rgb.val[0] = vmlaq_s16(at1_rgb.val[0], w1_hi, r.val[1]);
+    at1_rgb.val[1] = vmulq_s16(w1_lo, g.val[0]);
+    at1_rgb.val[1] = vmlaq_s16(at1_rgb.val[1], w1_hi, g.val[1]);
+    at1_rgb.val[2] = vmulq_s16(w1_lo, b.val[0]);
+    at1_rgb.val[2] = vmlaq_s16(at1_rgb.val[2], w1_hi, b.val[1]);
+    // [r][g][b][]
+    int32x4_t at1 = SumRGB(at1_rgb);
+
+    // [r][g][b][]
+    int32x4_t at2 = vreinterpretq_s32_u32(vmovl_u16(sum_rgb));
+    // at2 = 3 * at2 - at1;
+    at2 = vsubq_s32(vmulq_s32(at2, vdupq_n_s32(3)), at1);
+
+    // Do table lookup for each color index.
+    uint32x4x2_t akku1 = DoProdsTableLookup<T>(vget_low_u8(indices));
+    uint32x4x2_t akku2 = DoProdsTableLookup<T>(vget_high_u8(indices));
+    uint32x4_t sum_akku = vaddq_u32(
+        vaddq_u32(vaddq_u32(akku1.val[0], akku1.val[1]), akku2.val[0]),
+        akku2.val[1]);
+    // Pairwise add and accumulate.
+    uint64x1_t akku = vpaddl_u32(vget_low_u32(sum_akku));
+    akku = vpadal_u32(akku, vget_high_u32(sum_akku));
+
+    // Extract solutions and decide solvability.
+
+    // [akku >> 16]x4
+    int32x4_t xx =
+        vdupq_lane_s32(vreinterpret_s32_u64(vshr_n_u64(akku, 16)), 0);
+    // [(akku >> 8) & 0xff]x4
+    const uint64x1_t kFF = vdup_n_u64(0xff);
+    int32x4_t yy = vdupq_lane_s32(
+        vreinterpret_s32_u64(vand_u64(vshr_n_u64(akku, 8), kFF)), 0);
+    // [akku & 0xff]x4
+    int32x4_t xy = vdupq_lane_s32(vreinterpret_s32_u64(vand_u64(akku, kFF)), 0);
+
+    // ((3.0f * 31.0f) / 255.0f) / (xx * yy - xy * xy)
+    float32x4_t frb = Divide<2>(
+        vdupq_n_f32((3.0f * 31.0f) / 255.0f),
+        vcvtq_f32_s32(vsubq_s32(vmulq_s32(xx, yy), vmulq_s32(xy, xy))));
+    // frb * 63.0f / 31.0f
+    float32x4_t fg = vmulq_f32(vmulq_f32(frb, vdupq_n_f32(63.0f)),
+                               vdupq_n_f32(1.0f / 31.0f));
+
+    // Solve.
+
+    // [frb][fg][frb][]
+    float32x4_t frb_fg_frb = vsetq_lane_f32(vgetq_lane_f32(fg, 0), frb, 1);
+    // [31][63][31][]
+    const int32x4_t kClamp565_vec = {31, 63, 31, 0};
+
+    // (at1_r * yy - at2_r * xy) * frb + 0.5f
+    int32x4_t base0_rgb32 = vcvtq_s32_f32(vaddq_f32(
+        vmulq_f32(
+            vcvtq_f32_s32(vsubq_s32(vmulq_s32(at1, yy), vmulq_s32(at2, xy))),
+            frb_fg_frb),
+        vdupq_n_f32(0.5f)));
+    // Clamp and saturate.
+    uint16x4_t base0_rgb16 = vqmovun_s32(vbslq_s32(
+        vcgeq_s32(base0_rgb32, kClamp565_vec), kClamp565_vec, base0_rgb32));
+
+    // (at2_r * xx - at1_r * xy) * frb + 0.5f
+    int32x4_t base1_rgb32 = vcvtq_s32_f32(vaddq_f32(
+        vmulq_f32(
+            vcvtq_f32_s32(vsubq_s32(vmulq_s32(at2, xx), vmulq_s32(at1, xy))),
+            frb_fg_frb),
+        vdupq_n_f32(0.5f)));
+    // Clamp and saturate.
+    uint16x4_t base1_rgb16 = vqmovun_s32(vbslq_s32(
+        vcgeq_s32(base1_rgb32, kClamp565_vec), kClamp565_vec, base1_rgb32));
+
+    base_colors = vcombine_u16(base0_rgb16, base1_rgb16);
+  }
+
+  return !Equal(old_base_colors, base_colors);
+}
+
+template <typename T, Quality QUALITY>
+ALWAYS_INLINE void CompressColorBlock(uint8_t* dst,
+                                      const uint8x16x4_t& pixels_linear,
+                                      const uint8x16x4_t& pixels_scattered,
+                                      uint8x8_t min_rgba,
+                                      uint8x8_t max_rgba) {
+  // Take a shortcut if the block is constant (disregarding alpha).
+  uint32_t min32 = vget_lane_u32(vreinterpret_u32_u8(min_rgba), 0);
+  uint32_t max32 = vget_lane_u32(vreinterpret_u32_u8(max_rgba), 0);
+  if ((min32 & 0x00ffffff) == (max32 & 0x00ffffff)) {
+    // Swapping r and b channels to match Skia.
+    int b = min32 & 0xff;
+    int g = (min32 >> 8) & 0xff;
+    int r = (min32 >> 16) & 0xff;
+
+    uint16_t max16 = MatchSingleColorMax<typename T::BASE_TYPE>(r, g, b);
+    uint16_t min16 = MatchSingleColorMin<typename T::BASE_TYPE>(r, g, b);
+    uint32_t indices = T::kConstantColorIndices;
+    FormatFixup_Generic<typename T::BASE_TYPE>(&max16, &min16, &indices);
+
+    uint32_t* dst32 = reinterpret_cast<uint32_t*>(dst);
+    dst32[0] = max16 | (min16 << 16);
+    dst32[1] = indices;
+  } else {
+    uint16x4_t sum_rgb;
+    uint16x8_t base_colors;
+
+    if (QUALITY == kQualityLow) {
+      base_colors = GetApproximateBaseColors(pixels_linear, pixels_scattered,
+                                             min_rgba, max_rgba);
+    } else {
+      sum_rgb = SumRGB(pixels_scattered);
+      // Do Primary Component Analysis and map along principal axis.
+      base_colors = OptimizeColorsBlock(pixels_linear, pixels_scattered,
+                                        sum_rgb, min_rgba, max_rgba);
+    }
+
+    // Check if the two base colors are the same.
+    uint8x16_t indices;
+    if (!Equal(vget_low_u16(base_colors), vget_high_u16(base_colors))) {
+      // Calculate the two intermediate colors as well.
+      uint16x4x4_t colors = EvalColors(base_colors);
+
+      // Do a first pass to find good index candicates for all 16 of the pixels
+      // in the block.
+      indices = MatchColorsBlock<T>(pixels_scattered, colors);
+    } else {
+      // Any indices can be used here.
+      indices = vdupq_n_u8(0);
+    }
+
+    if (QUALITY == kQualityHigh) {
+      // Refine the base colors and indices multiple times if requested.
+      for (int i = 0; i < kNumRefinements; ++i) {
+        uint8x16_t lastIndices = indices;
+
+        if (RefineBlock<T>(pixels_scattered, sum_rgb, base_colors, indices)) {
+          if (!Equal(vget_low_u16(base_colors), vget_high_u16(base_colors))) {
+            uint16x4x4_t colors = EvalColors(base_colors);
+            indices = MatchColorsBlock<T>(pixels_scattered, colors);
+          } else {
+            // We ended up with two identical base colors, can't refine this
+            // further.
+            indices = vdupq_n_u8(0);
+            break;
+          }
+        }
+
+        if (Equal(indices, lastIndices)) {
+          // There's no need to do another refinement pass if we didn't get any
+          // improvements this pass.
+          break;
+        }
+      }
+    }
+
+    // Prepare the final block by converting the base colors to 16-bit and
+    // packing the pixel indices.
+    uint16x4_t base_colors_16 = PackBaseColors(base_colors);
+    uint64x1_t indices_2x16 = PackIndices<2>(indices);
+    FormatFixup<T>(&base_colors_16, &indices_2x16);
+    uint64x1_t output = vorr_u64(vshl_n_u64(indices_2x16, 32),
+                                 vreinterpret_u64_u16(base_colors_16));
+    vst1_u64(reinterpret_cast<uint64_t*>(dst), output);
+  }
+}
+
+// alpha: 8x8-bit alpha values.
+// dist:  Distance between max and min alpha in the color block.
+// bias:  Rounding bias.
+ALWAYS_INLINE uint8x8_t
+GetAlphaIndices(uint8x8_t alpha, int16x8_t dist, int16x8_t bias) {
+  // Expand to signed 16-bit.
+  int16x8_t alpha_16 = vreinterpretq_s16_u16(vmovl_u8(alpha));
+
+  // Multiply each alpha value by 7 and add bias.
+  int16x8_t a = vaddq_s16(vmulq_s16(alpha_16, vdupq_n_s16(7)), bias);
+
+  int16x8_t dist4 = vmulq_s16(dist, vdupq_n_s16(4));
+  int16x8_t dist2 = vmulq_s16(dist, vdupq_n_s16(2));
+
+  // Select index. This is a "linear scale" lerp factor between 0 (val=min)
+  // and 7 (val=max).
+  // t = (a >= dist4) ? -1 : 0
+  int16x8_t t =
+      vandq_s16(vreinterpretq_s16_u16(vcgeq_s16(a, dist4)), vdupq_n_s16(-1));
+  // ind1 = t & 4;
+  int16x8_t ind1 = vandq_s16(t, vdupq_n_s16(4));
+  // a1 = a - (dist4 & t);
+  int16x8_t a1 = vsubq_s16(a, vandq_s16(dist4, t));
+
+  // t = (a1 >= dist2) ? -1 : 0;
+  t = vandq_s16(vreinterpretq_s16_u16(vcgeq_s16(a1, dist2)), vdupq_n_s16(-1));
+  // ind2 = t & 2;
+  int16x8_t ind2 = vandq_s16(t, vdupq_n_s16(2));
+  // a2 = a1 - (dist2 & t);
+  int16x8_t a2 = vsubq_s16(a1, vandq_s16(dist2, t));
+
+  // ind3 = (a2 >= dist)
+  int16x8_t ind3 =
+      vandq_s16(vreinterpretq_s16_u16(vcgeq_s16(a2, dist)), vdupq_n_s16(1));
+
+  // indices = ind1 + ind2 + ind3
+  int16x8_t indices = vaddq_s16(ind1, vaddq_s16(ind2, ind3));
+
+  // Turn linear scale into alpha index (0/1 are extremal pts).
+  // ind = -indices & 7
+  int16x8_t ind = vandq_s16(vnegq_s16(indices), vdupq_n_s16(7));
+  // indices = ind ^ (2 > ind)
+  indices = veorq_s16(
+      ind, vandq_s16(vreinterpretq_s16_u16(vcgtq_s16(vdupq_n_s16(2), ind)),
+                     vdupq_n_s16(1)));
+  // Narrow it down to unsigned 8 bits and return.
+  return vqmovun_s16(indices);
+}
+
+ALWAYS_INLINE void CompressAlphaBlock(uint8_t* dst,
+                                      uint8x16_t pixels_alpha,
+                                      uint8x8_t min_rgba,
+                                      uint8x8_t max_rgba) {
+  // Take a shortcut if the block is constant.
+  uint8_t min_alpha = vget_lane_u8(min_rgba, 3);
+  uint8_t max_alpha = vget_lane_u8(max_rgba, 3);
+  if (min_alpha == max_alpha) {
+    dst[0] = max_alpha;
+    dst[1] = min_alpha;
+    // All indices are the same, any value will do.
+    *reinterpret_cast<uint16_t*>(dst + 2) = 0;
+    *reinterpret_cast<uint32_t*>(dst + 4) = 0;
+  } else {
+    // [max - min]x8
+    int16x8_t dist = vdupq_lane_s16(
+        vreinterpret_s16_u16(vget_low_u16(vsubl_u8(max_rgba, min_rgba))), 3);
+    // bias = (dist < 8) ? (dist - 1) : (dist / 2 + 2)
+    int16x8_t bias = vbslq_s16(vcltq_s16(dist, vdupq_n_s16(8)),
+                               vsubq_s16(dist, vdupq_n_s16(1)),
+                               vaddq_s16(vshrq_n_s16(dist, 1), vdupq_n_s16(2)));
+    // bias -= min * 7;
+    bias = vsubq_s16(
+        bias,
+        vmulq_s16(
+            vdupq_lane_s16(
+                vreinterpret_s16_u16(vget_low_u16(vmovl_u8(min_rgba))), 3),
+            vdupq_n_s16(7)));
+
+    uint8x8_t indices_lo =
+        GetAlphaIndices(vget_low_u8(pixels_alpha), dist, bias);
+    uint8x8_t indices_hi =
+        GetAlphaIndices(vget_high_u8(pixels_alpha), dist, bias);
+
+    // Prepare the final block by combining the base alpha values and packing
+    // the alpha indices.
+    uint8x8_t max_min_alpha = vzip_u8(max_rgba, min_rgba).val[0];
+    uint64x1_t indices = PackIndices<3>(vcombine_u8(indices_lo, indices_hi));
+    uint64x1_t output =
+        vorr_u64(vshl_n_u64(indices, 16),
+                 vshr_n_u64(vreinterpret_u64_u8(max_min_alpha), 48));
+    vst1_u64(reinterpret_cast<uint64_t*>(dst), output);
+  }
+}
+
+template <typename T, bool OPAQUE, Quality QUALITY>
+void CompressImage(const uint8_t* src, uint8_t* dst, int width, int height) {
+  for (int y = 0; y < height; y += 4, src += width * 4 * 4) {
+    for (int x = 0; x < width; x += 4) {
+      // Load the four rows of pixels.
+      uint8x16x4_t pixels_linear;
+      pixels_linear.val[0] = vld1q_u8(src + (x + 0 * width) * 4);
+      pixels_linear.val[1] = vld1q_u8(src + (x + 1 * width) * 4);
+      pixels_linear.val[2] = vld1q_u8(src + (x + 2 * width) * 4);
+      pixels_linear.val[3] = vld1q_u8(src + (x + 3 * width) * 4);
+
+      // Transpose/scatter the red, green, blue and alpha channels into
+      // separate registers.
+      ALIGNAS(8) uint8_t block[64];
+      vst1q_u8(block + 0 * 16, pixels_linear.val[0]);
+      vst1q_u8(block + 1 * 16, pixels_linear.val[1]);
+      vst1q_u8(block + 2 * 16, pixels_linear.val[2]);
+      vst1q_u8(block + 3 * 16, pixels_linear.val[3]);
+      uint8x16x4_t pixels_scattered = vld4q_u8(block);
+
+      // We need the min and max values both to detect solid blocks and when
+      // computing the base colors.
+      uint8x8_t min_rgba = FoldRGBA<vec_ops::Min>(pixels_scattered);
+      uint8x8_t max_rgba = FoldRGBA<vec_ops::Max>(pixels_scattered);
+
+      if (!OPAQUE) {
+        CompressAlphaBlock(dst, pixels_scattered.val[3], min_rgba, max_rgba);
+        dst += 8;
+      }
+
+      CompressColorBlock<T, QUALITY>(dst, pixels_linear, pixels_scattered,
+                                     min_rgba, max_rgba);
+      dst += 8;
+    }
+  }
+}
+
+}  // namespace
+
+void CompressATC_NEON(const uint8_t* src, uint8_t* dst, int width, int height) {
+  CompressImage<TYPE_ATC_NEON, true, kQualityHigh>(src, dst, width, height);
+}
+
+void CompressATCIA_NEON(const uint8_t* src,
+                        uint8_t* dst,
+                        int width,
+                        int height) {
+  CompressImage<TYPE_ATC_NEON, false, kQualityHigh>(src, dst, width, height);
+}
+
+void CompressDXT1_NEON(const uint8_t* src,
+                       uint8_t* dst,
+                       int width,
+                       int height) {
+  CompressImage<TYPE_DXT_NEON, true, kQualityHigh>(src, dst, width, height);
+}
+
+void CompressDXT5_NEON(const uint8_t* src,
+                       uint8_t* dst,
+                       int width,
+                       int height) {
+  CompressImage<TYPE_DXT_NEON, false, kQualityHigh>(src, dst, width, height);
+}
+
+}  // namespace texture_compress
+}  // namespace cc
+
+#endif  // __ARM_NEON__