1 /* 2 * Copyright 2016 Google Inc. 3 * 4 * Use of this source code is governed by a BSD-style license that can be 5 * found in the LICENSE file. 6 */ 7 8 #ifndef SkSRGB_DEFINED 9 #define SkSRGB_DEFINED 10 11 #include "SkNx.h" 12 13 /** Components for building our canonical sRGB -> linear and linear -> sRGB transformations. 14 * 15 * Current best practices: 16 * - for sRGB -> linear, lookup R,G,B in sk_linear_from_srgb; 17 * - for linear -> sRGB, call sk_linear_to_srgb() for R,G,B; 18 * - the alpha channel is linear in both formats, needing at most *(1/255.0f) or *255.0f. 19 * 20 * sk_linear_to_srgb() will run a little faster than usual when compiled with SSE4.1+. 21 */ 22 23 extern const float sk_linear_from_srgb[256]; 24 extern const uint16_t sk_linear12_from_srgb[256]; 25 extern const uint8_t sk_linear12_to_srgb[4096]; 26 27 // [0.0f, 1.0f] -> [0, 255]. 28 static inline Sk4i sk_linear_to_srgb(const Sk4f& x) { 29 // Approximation of the sRGB gamma curve (within 1 when scaled to 8-bit pixels). 30 // 31 // Constants tuned by brute force to minimize (in order of importance) after truncation: 32 // 1) the number of bytes that fail to round trip (0 of 256); 33 // 2) the number of points in [FLT_MIN, 1.0f] that are non-monotonic (0 of ~1 billion); 34 // 3) the number of points halfway between bytes that hit the wrong byte (131 of 255). 35 auto rsqrt = x.rsqrt(), 36 sqrt = rsqrt.invert(), 37 ftrt = rsqrt.rsqrt(); 38 39 auto lo = (13.0471f * 255.0f) * x; 40 41 auto hi = SkNx_fma(Sk4f{+0.412999f * 255.0f}, ftrt, 42 SkNx_fma(Sk4f{+0.687999f * 255.0f}, sqrt, 43 Sk4f{-0.0974983f * 255.0f})); 44 auto s = (x < 0.0048f).thenElse(lo, hi); 45 46 // Now clamp and truncate. 47 // The order of the arguments is important here. We want to make sure that NaN 48 // clamps to zero. Note that max(NaN, 0) = 0, while max(0, NaN) = NaN. 49 return SkNx_cast<int>(Sk4f::Min(Sk4f::Max(s, 0.0f), 255.0f)); 50 } 51 52 #endif//SkSRGB_DEFINED 53
