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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, clamp to 255, and round;
* - the alpha channel is linear in both formats, needing at most *(1/255.0f) or *255.0f.
*
* sk_linear_to_srgb()'s output requires rounding; it does not round for you.
*
* Given inputs in [0,1], sk_linear_to_srgb() will not underflow 0 but may overflow 255.
* The overflow is small enough that you can safely either clamp then round or round then clamp.
* (If you don't trust the inputs are in [0,1], you'd better clamp both sides immediately.)
*
* sk_linear_to_srgb() will run a little faster than usual when compiled with SSE4.1+.
*/
extern const float sk_linear_from_srgb[256];
static inline Sk4f sk_linear_to_srgb(const Sk4f& x) {
// Approximation of the sRGB gamma curve (within 1 when scaled to 8-bit pixels).
// For 0.00000f <= x < 0.00349f, 12.92 * x
// For 0.00349f <= x <= 1.00000f, 0.679*(x.^0.5) + 0.423*x.^(0.25) - 0.101
// Note that 0.00349 was selected because it is a point where both functions produce the
// same pixel value when rounded.
auto rsqrt = x.rsqrt(),
sqrt = rsqrt.invert(),
ftrt = rsqrt.rsqrt();
auto lo = (12.92f * 255.0f) * x;
auto hi = (-0.101115084998961f * 255.0f) +
(+0.678513029959381f * 255.0f) * sqrt +
(+0.422602055039580f * 255.0f) * ftrt;
return (x < 0.00349f).thenElse(lo, hi);
}
#endif//SkSRGB_DEFINED
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