1 /*
2 * Copyright 2012 Google Inc.
3 *
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
6 */
7
8 #include "src/core/SkMaskGamma.h"
9
10 #include "include/core/SkTypes.h"
11 #include "include/private/base/SkDebug.h"
12 #include "include/private/base/SkFloatingPoint.h"
13 #include "include/private/base/SkTo.h"
14
15 #include <cmath>
16
17 class SkLinearColorSpaceLuminance : public SkColorSpaceLuminance {
toLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luminance) const18 SkScalar toLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luminance) const override {
19 SkASSERT(SK_Scalar1 == gamma);
20 return luminance;
21 }
fromLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luma) const22 SkScalar fromLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luma) const override {
23 SkASSERT(SK_Scalar1 == gamma);
24 return luma;
25 }
26 };
27
28 class SkGammaColorSpaceLuminance : public SkColorSpaceLuminance {
toLuma(SkScalar gamma,SkScalar luminance) const29 SkScalar toLuma(SkScalar gamma, SkScalar luminance) const override {
30 return SkScalarPow(luminance, gamma);
31 }
fromLuma(SkScalar gamma,SkScalar luma) const32 SkScalar fromLuma(SkScalar gamma, SkScalar luma) const override {
33 return SkScalarPow(luma, SkScalarInvert(gamma));
34 }
35 };
36
37 class SkSRGBColorSpaceLuminance : public SkColorSpaceLuminance {
toLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luminance) const38 SkScalar toLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luminance) const override {
39 SkASSERT(0 == gamma);
40 //The magic numbers are derived from the sRGB specification.
41 //See http://www.color.org/chardata/rgb/srgb.xalter .
42 if (luminance <= 0.04045f) {
43 return luminance / 12.92f;
44 }
45 return SkScalarPow((luminance + 0.055f) / 1.055f,
46 2.4f);
47 }
fromLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luma) const48 SkScalar fromLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luma) const override {
49 SkASSERT(0 == gamma);
50 //The magic numbers are derived from the sRGB specification.
51 //See http://www.color.org/chardata/rgb/srgb.xalter .
52 if (luma <= 0.0031308f) {
53 return luma * 12.92f;
54 }
55 return 1.055f * SkScalarPow(luma, SkScalarInvert(2.4f))
56 - 0.055f;
57 }
58 };
59
Fetch(SkScalar gamma)60 /*static*/ const SkColorSpaceLuminance& SkColorSpaceLuminance::Fetch(SkScalar gamma) {
61 static SkLinearColorSpaceLuminance gSkLinearColorSpaceLuminance;
62 static SkGammaColorSpaceLuminance gSkGammaColorSpaceLuminance;
63 static SkSRGBColorSpaceLuminance gSkSRGBColorSpaceLuminance;
64
65 if (0 == gamma) {
66 return gSkSRGBColorSpaceLuminance;
67 } else if (SK_Scalar1 == gamma) {
68 return gSkLinearColorSpaceLuminance;
69 } else {
70 return gSkGammaColorSpaceLuminance;
71 }
72 }
73
apply_contrast(float srca,float contrast)74 static float apply_contrast(float srca, float contrast) {
75 return srca + ((1.0f - srca) * contrast * srca);
76 }
77
SkTMaskGamma_build_correcting_lut(uint8_t * table,U8CPU srcI,SkScalar contrast,const SkColorSpaceLuminance & dstConvert,SkScalar dstGamma)78 void SkTMaskGamma_build_correcting_lut(uint8_t* table, U8CPU srcI, SkScalar contrast,
79 const SkColorSpaceLuminance& dstConvert, SkScalar dstGamma) {
80 const SkColorSpaceLuminance& srcConvert = dstConvert;
81 const SkScalar srcGamma = dstGamma;
82 const float src = (float)srcI / 255.0f;
83 const float linSrc = srcConvert.toLuma(srcGamma, src);
84 //Guess at the dst. The perceptual inverse provides smaller visual
85 //discontinuities when slight changes to desaturated colors cause a channel
86 //to map to a different correcting lut with neighboring srcI.
87 //See https://code.google.com/p/chromium/issues/detail?id=141425#c59 .
88 const float dst = 1.0f - src;
89 const float linDst = dstConvert.toLuma(dstGamma, dst);
90
91 //Contrast value tapers off to 0 as the src luminance becomes white
92 const float adjustedContrast = contrast * linDst;
93
94 //Remove discontinuity and instability when src is close to dst.
95 //The value 1/256 is arbitrary and appears to contain the instability.
96 if (fabs(src - dst) < (1.0f / 256.0f)) {
97 float ii = 0.0f;
98 for (int i = 0; i < 256; ++i, ii += 1.0f) {
99 float rawSrca = ii / 255.0f;
100 float srca = apply_contrast(rawSrca, adjustedContrast);
101 table[i] = SkToU8(sk_float_round2int(255.0f * srca));
102 }
103 } else {
104 // Avoid slow int to float conversion.
105 float ii = 0.0f;
106 for (int i = 0; i < 256; ++i, ii += 1.0f) {
107 // 'rawSrca += 1.0f / 255.0f' and even
108 // 'rawSrca = i * (1.0f / 255.0f)' can add up to more than 1.0f.
109 // When this happens the table[255] == 0x0 instead of 0xff.
110 // See http://code.google.com/p/chromium/issues/detail?id=146466
111 float rawSrca = ii / 255.0f;
112 float srca = apply_contrast(rawSrca, adjustedContrast);
113 SkASSERT(srca <= 1.0f);
114 float dsta = 1.0f - srca;
115
116 //Calculate the output we want.
117 float linOut = (linSrc * srca + dsta * linDst);
118 SkASSERT(linOut <= 1.0f);
119 float out = dstConvert.fromLuma(dstGamma, linOut);
120
121 //Undo what the blit blend will do.
122 float result = (out - dst) / (src - dst);
123 SkASSERT(sk_float_round2int(255.0f * result) <= 255);
124
125 table[i] = SkToU8(sk_float_round2int(255.0f * result));
126 }
127 }
128 }
129