/*
* 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/SkTypes.h"
#include "include/private/base/SkDebug.h"
#include "include/private/base/SkFloatingPoint.h"
#include "include/private/base/SkTo.h"
#include <cmath>
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, U8CPU srcI, SkScalar contrast,
const SkColorSpaceLuminance& dstConvert, SkScalar dstGamma) {
const SkColorSpaceLuminance& srcConvert = dstConvert;
const SkScalar srcGamma = 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 = 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));
}
}
}
