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- /*
- * Copyright 2012 Google Inc.
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
- #include "src/core/SkMaskGamma.h"
- #include "include/core/SkColor.h"
- #include "include/core/SkTypes.h"
- #include "include/private/SkFloatingPoint.h"
- #include "include/private/SkTo.h"
- class SkLinearColorSpaceLuminance : public SkColorSpaceLuminance {
- SkScalar toLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luminance) const override {
- SkASSERT(SK_Scalar1 == gamma);
- return luminance;
- }
- SkScalar fromLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luma) const override {
- SkASSERT(SK_Scalar1 == gamma);
- return luma;
- }
- };
- class SkGammaColorSpaceLuminance : public SkColorSpaceLuminance {
- SkScalar toLuma(SkScalar gamma, SkScalar luminance) const override {
- return SkScalarPow(luminance, gamma);
- }
- SkScalar fromLuma(SkScalar gamma, SkScalar luma) const override {
- return SkScalarPow(luma, SkScalarInvert(gamma));
- }
- };
- class SkSRGBColorSpaceLuminance : public SkColorSpaceLuminance {
- SkScalar toLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luminance) const override {
- SkASSERT(0 == gamma);
- //The magic numbers are derived from the sRGB specification.
- //See http://www.color.org/chardata/rgb/srgb.xalter .
- if (luminance <= 0.04045f) {
- return luminance / 12.92f;
- }
- return SkScalarPow((luminance + 0.055f) / 1.055f,
- 2.4f);
- }
- SkScalar fromLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luma) const override {
- SkASSERT(0 == gamma);
- //The magic numbers are derived from the sRGB specification.
- //See http://www.color.org/chardata/rgb/srgb.xalter .
- if (luma <= 0.0031308f) {
- return luma * 12.92f;
- }
- return 1.055f * SkScalarPow(luma, SkScalarInvert(2.4f))
- - 0.055f;
- }
- };
- /*static*/ const SkColorSpaceLuminance& SkColorSpaceLuminance::Fetch(SkScalar gamma) {
- static SkLinearColorSpaceLuminance gSkLinearColorSpaceLuminance;
- static SkGammaColorSpaceLuminance gSkGammaColorSpaceLuminance;
- static SkSRGBColorSpaceLuminance gSkSRGBColorSpaceLuminance;
- if (0 == gamma) {
- return gSkSRGBColorSpaceLuminance;
- } else if (SK_Scalar1 == gamma) {
- return gSkLinearColorSpaceLuminance;
- } else {
- return gSkGammaColorSpaceLuminance;
- }
- }
- static float apply_contrast(float srca, float contrast) {
- return srca + ((1.0f - srca) * contrast * srca);
- }
- void SkTMaskGamma_build_correcting_lut(uint8_t table[256], U8CPU srcI, SkScalar contrast,
- const SkColorSpaceLuminance& srcConvert, SkScalar srcGamma,
- const SkColorSpaceLuminance& dstConvert, SkScalar dstGamma) {
- const float src = (float)srcI / 255.0f;
- const float linSrc = srcConvert.toLuma(srcGamma, src);
- //Guess at the dst. The perceptual inverse provides smaller visual
- //discontinuities when slight changes to desaturated colors cause a channel
- //to map to a different correcting lut with neighboring srcI.
- //See https://code.google.com/p/chromium/issues/detail?id=141425#c59 .
- const float dst = 1.0f - src;
- const float linDst = dstConvert.toLuma(dstGamma, dst);
- //Contrast value tapers off to 0 as the src luminance becomes white
- const float adjustedContrast = SkScalarToFloat(contrast) * linDst;
- //Remove discontinuity and instability when src is close to dst.
- //The value 1/256 is arbitrary and appears to contain the instability.
- if (fabs(src - dst) < (1.0f / 256.0f)) {
- float ii = 0.0f;
- for (int i = 0; i < 256; ++i, ii += 1.0f) {
- float rawSrca = ii / 255.0f;
- float srca = apply_contrast(rawSrca, adjustedContrast);
- table[i] = SkToU8(sk_float_round2int(255.0f * srca));
- }
- } else {
- // Avoid slow int to float conversion.
- float ii = 0.0f;
- for (int i = 0; i < 256; ++i, ii += 1.0f) {
- // 'rawSrca += 1.0f / 255.0f' and even
- // 'rawSrca = i * (1.0f / 255.0f)' can add up to more than 1.0f.
- // When this happens the table[255] == 0x0 instead of 0xff.
- // See http://code.google.com/p/chromium/issues/detail?id=146466
- float rawSrca = ii / 255.0f;
- float srca = apply_contrast(rawSrca, adjustedContrast);
- SkASSERT(srca <= 1.0f);
- float dsta = 1.0f - srca;
- //Calculate the output we want.
- float linOut = (linSrc * srca + dsta * linDst);
- SkASSERT(linOut <= 1.0f);
- float out = dstConvert.fromLuma(dstGamma, linOut);
- //Undo what the blit blend will do.
- float result = (out - dst) / (src - dst);
- SkASSERT(sk_float_round2int(255.0f * result) <= 255);
- table[i] = SkToU8(sk_float_round2int(255.0f * result));
- }
- }
- }
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