1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/gfx/skia/trunk/src/core/SkMaskGamma.cpp Wed Dec 31 06:09:35 2014 +0100
1.3 @@ -0,0 +1,124 @@
1.4 +/*
1.5 + * Copyright 2012 Google Inc.
1.6 + *
1.7 + * Use of this source code is governed by a BSD-style license that can be
1.8 + * found in the LICENSE file.
1.9 + */
1.10 +
1.11 +#include "SkTypes.h"
1.12 +
1.13 +#include "SkColor.h"
1.14 +#include "SkFloatingPoint.h"
1.15 +#include "SkMaskGamma.h"
1.16 +
1.17 +class SkLinearColorSpaceLuminance : public SkColorSpaceLuminance {
1.18 + virtual SkScalar toLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luminance) const SK_OVERRIDE {
1.19 + SkASSERT(SK_Scalar1 == gamma);
1.20 + return luminance;
1.21 + }
1.22 + virtual SkScalar fromLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luma) const SK_OVERRIDE {
1.23 + SkASSERT(SK_Scalar1 == gamma);
1.24 + return luma;
1.25 + }
1.26 +};
1.27 +
1.28 +class SkGammaColorSpaceLuminance : public SkColorSpaceLuminance {
1.29 + virtual SkScalar toLuma(SkScalar gamma, SkScalar luminance) const SK_OVERRIDE {
1.30 + return SkScalarPow(luminance, gamma);
1.31 + }
1.32 + virtual SkScalar fromLuma(SkScalar gamma, SkScalar luma) const SK_OVERRIDE {
1.33 + return SkScalarPow(luma, SkScalarInvert(gamma));
1.34 + }
1.35 +};
1.36 +
1.37 +class SkSRGBColorSpaceLuminance : public SkColorSpaceLuminance {
1.38 + virtual SkScalar toLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luminance) const SK_OVERRIDE {
1.39 + SkASSERT(0 == gamma);
1.40 + //The magic numbers are derived from the sRGB specification.
1.41 + //See http://www.color.org/chardata/rgb/srgb.xalter .
1.42 + if (luminance <= 0.04045f) {
1.43 + return luminance / 12.92f;
1.44 + }
1.45 + return SkScalarPow((luminance + 0.055f) / 1.055f,
1.46 + 2.4f);
1.47 + }
1.48 + virtual SkScalar fromLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luma) const SK_OVERRIDE {
1.49 + SkASSERT(0 == gamma);
1.50 + //The magic numbers are derived from the sRGB specification.
1.51 + //See http://www.color.org/chardata/rgb/srgb.xalter .
1.52 + if (luma <= 0.0031308f) {
1.53 + return luma * 12.92f;
1.54 + }
1.55 + return 1.055f * SkScalarPow(luma, SkScalarInvert(2.4f))
1.56 + - 0.055f;
1.57 + }
1.58 +};
1.59 +
1.60 +/*static*/ const SkColorSpaceLuminance& SkColorSpaceLuminance::Fetch(SkScalar gamma) {
1.61 + static SkLinearColorSpaceLuminance gSkLinearColorSpaceLuminance;
1.62 + static SkGammaColorSpaceLuminance gSkGammaColorSpaceLuminance;
1.63 + static SkSRGBColorSpaceLuminance gSkSRGBColorSpaceLuminance;
1.64 +
1.65 + if (0 == gamma) {
1.66 + return gSkSRGBColorSpaceLuminance;
1.67 + } else if (SK_Scalar1 == gamma) {
1.68 + return gSkLinearColorSpaceLuminance;
1.69 + } else {
1.70 + return gSkGammaColorSpaceLuminance;
1.71 + }
1.72 +}
1.73 +
1.74 +static float apply_contrast(float srca, float contrast) {
1.75 + return srca + ((1.0f - srca) * contrast * srca);
1.76 +}
1.77 +
1.78 +void SkTMaskGamma_build_correcting_lut(uint8_t table[256], U8CPU srcI, SkScalar contrast,
1.79 + const SkColorSpaceLuminance& srcConvert, SkScalar srcGamma,
1.80 + const SkColorSpaceLuminance& dstConvert, SkScalar dstGamma) {
1.81 + const float src = (float)srcI / 255.0f;
1.82 + const float linSrc = srcConvert.toLuma(srcGamma, src);
1.83 + //Guess at the dst. The perceptual inverse provides smaller visual
1.84 + //discontinuities when slight changes to desaturated colors cause a channel
1.85 + //to map to a different correcting lut with neighboring srcI.
1.86 + //See https://code.google.com/p/chromium/issues/detail?id=141425#c59 .
1.87 + const float dst = 1.0f - src;
1.88 + const float linDst = dstConvert.toLuma(dstGamma, dst);
1.89 +
1.90 + //Contrast value tapers off to 0 as the src luminance becomes white
1.91 + const float adjustedContrast = SkScalarToFloat(contrast) * linDst;
1.92 +
1.93 + //Remove discontinuity and instability when src is close to dst.
1.94 + //The value 1/256 is arbitrary and appears to contain the instability.
1.95 + if (fabs(src - dst) < (1.0f / 256.0f)) {
1.96 + float ii = 0.0f;
1.97 + for (int i = 0; i < 256; ++i, ii += 1.0f) {
1.98 + float rawSrca = ii / 255.0f;
1.99 + float srca = apply_contrast(rawSrca, adjustedContrast);
1.100 + table[i] = SkToU8(sk_float_round2int(255.0f * srca));
1.101 + }
1.102 + } else {
1.103 + // Avoid slow int to float conversion.
1.104 + float ii = 0.0f;
1.105 + for (int i = 0; i < 256; ++i, ii += 1.0f) {
1.106 + // 'rawSrca += 1.0f / 255.0f' and even
1.107 + // 'rawSrca = i * (1.0f / 255.0f)' can add up to more than 1.0f.
1.108 + // When this happens the table[255] == 0x0 instead of 0xff.
1.109 + // See http://code.google.com/p/chromium/issues/detail?id=146466
1.110 + float rawSrca = ii / 255.0f;
1.111 + float srca = apply_contrast(rawSrca, adjustedContrast);
1.112 + SkASSERT(srca <= 1.0f);
1.113 + float dsta = 1.0f - srca;
1.114 +
1.115 + //Calculate the output we want.
1.116 + float linOut = (linSrc * srca + dsta * linDst);
1.117 + SkASSERT(linOut <= 1.0f);
1.118 + float out = dstConvert.fromLuma(dstGamma, linOut);
1.119 +
1.120 + //Undo what the blit blend will do.
1.121 + float result = (out - dst) / (src - dst);
1.122 + SkASSERT(sk_float_round2int(255.0f * result) <= 255);
1.123 +
1.124 + table[i] = SkToU8(sk_float_round2int(255.0f * result));
1.125 + }
1.126 + }
1.127 +}
