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