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Diffstat (limited to 'src/core/SkSRGB.h')
-rw-r--r-- | src/core/SkSRGB.h | 50 |
1 files changed, 50 insertions, 0 deletions
diff --git a/src/core/SkSRGB.h b/src/core/SkSRGB.h new file mode 100644 index 0000000000..de80596de4 --- /dev/null +++ b/src/core/SkSRGB.h @@ -0,0 +1,50 @@ +/* + * 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 |