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/*
* Copyright 2016 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef SkSRGB_DEFINED
#define SkSRGB_DEFINED
#include "SkNx.h"
/** Components for building our canonical sRGB -> linear and linear -> sRGB transformations.
*
* Current best practices:
* - for sRGB -> linear, lookup R,G,B in sk_linear_from_srgb;
* - for linear -> sRGB, call sk_linear_to_srgb() for R,G,B;
* - the alpha channel is linear in both formats, needing at most *(1/255.0f) or *255.0f.
*
* sk_linear_to_srgb() will run a little faster than usual when compiled with SSE4.1+.
*/
extern const float sk_linear_from_srgb[256];
extern const uint16_t sk_linear12_from_srgb[256];
extern const uint8_t sk_linear12_to_srgb[4096];
// [0.0f, 1.0f] -> [0, 255].
static inline Sk4i sk_linear_to_srgb(const Sk4f& x) {
// Approximation of the sRGB gamma curve (within 1 when scaled to 8-bit pixels).
//
// Constants tuned by brute force to minimize (in order of importance) after truncation:
// 1) the number of bytes that fail to round trip (0 of 256);
// 2) the number of points in [FLT_MIN, 1.0f] that are non-monotonic (0 of ~1 billion);
// 3) the number of points halfway between bytes that hit the wrong byte (131 of 255).
auto rsqrt = x.rsqrt(),
sqrt = rsqrt.invert(),
ftrt = rsqrt.rsqrt();
auto lo = (13.0471f * 255.0f) * x;
auto hi = SkNx_fma(Sk4f{+0.412999f * 255.0f}, ftrt,
SkNx_fma(Sk4f{+0.687999f * 255.0f}, sqrt,
Sk4f{-0.0974983f * 255.0f}));
auto s = (x < 0.0048f).thenElse(lo, hi);
// Now clamp and truncate.
// The order of the arguments is important here. We want to make sure that NaN
// clamps to zero. Note that max(NaN, 0) = 0, while max(0, NaN) = NaN.
return SkNx_cast<int>(Sk4f::Min(Sk4f::Max(s, 0.0f), 255.0f));
}
#endif//SkSRGB_DEFINED
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