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 "SkTypes.h"
9
10 #include "SkColor.h"
11 #include "SkFloatingPoint.h"
12 #include "SkMaskGamma.h"
13
14 class SkLinearColorSpaceLuminance : public SkColorSpaceLuminance {
toLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luminance) const15 SkScalar toLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luminance) const override {
16 SkASSERT(SK_Scalar1 == gamma);
17 return luminance;
18 }
fromLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luma) const19 SkScalar fromLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luma) const override {
20 SkASSERT(SK_Scalar1 == gamma);
21 return luma;
22 }
23 };
24
25 class SkGammaColorSpaceLuminance : public SkColorSpaceLuminance {
toLuma(SkScalar gamma,SkScalar luminance) const26 SkScalar toLuma(SkScalar gamma, SkScalar luminance) const override {
27 return SkScalarPow(luminance, gamma);
28 }
fromLuma(SkScalar gamma,SkScalar luma) const29 SkScalar fromLuma(SkScalar gamma, SkScalar luma) const override {
30 return SkScalarPow(luma, SkScalarInvert(gamma));
31 }
32 };
33
34 class SkSRGBColorSpaceLuminance : public SkColorSpaceLuminance {
toLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luminance) const35 SkScalar toLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luminance) const override {
36 SkASSERT(0 == gamma);
37 //The magic numbers are derived from the sRGB specification.
38 //See http://www.color.org/chardata/rgb/srgb.xalter .
39 if (luminance <= 0.04045f) {
40 return luminance / 12.92f;
41 }
42 return SkScalarPow((luminance + 0.055f) / 1.055f,
43 2.4f);
44 }
fromLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luma) const45 SkScalar fromLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luma) const override {
46 SkASSERT(0 == gamma);
47 //The magic numbers are derived from the sRGB specification.
48 //See http://www.color.org/chardata/rgb/srgb.xalter .
49 if (luma <= 0.0031308f) {
50 return luma * 12.92f;
51 }
52 return 1.055f * SkScalarPow(luma, SkScalarInvert(2.4f))
53 - 0.055f;
54 }
55 };
56
Fetch(SkScalar gamma)57 /*static*/ const SkColorSpaceLuminance& SkColorSpaceLuminance::Fetch(SkScalar gamma) {
58 static SkLinearColorSpaceLuminance gSkLinearColorSpaceLuminance;
59 static SkGammaColorSpaceLuminance gSkGammaColorSpaceLuminance;
60 static SkSRGBColorSpaceLuminance gSkSRGBColorSpaceLuminance;
61
62 if (0 == gamma) {
63 return gSkSRGBColorSpaceLuminance;
64 } else if (SK_Scalar1 == gamma) {
65 return gSkLinearColorSpaceLuminance;
66 } else {
67 return gSkGammaColorSpaceLuminance;
68 }
69 }
70
apply_contrast(float srca,float contrast)71 static float apply_contrast(float srca, float contrast) {
72 return srca + ((1.0f - srca) * contrast * srca);
73 }
74
SkTMaskGamma_build_correcting_lut(uint8_t table[256],U8CPU srcI,SkScalar contrast,const SkColorSpaceLuminance & srcConvert,SkScalar srcGamma,const SkColorSpaceLuminance & dstConvert,SkScalar dstGamma)75 void SkTMaskGamma_build_correcting_lut(uint8_t table[256], U8CPU srcI, SkScalar contrast,
76 const SkColorSpaceLuminance& srcConvert, SkScalar srcGamma,
77 const SkColorSpaceLuminance& dstConvert, SkScalar dstGamma) {
78 const float src = (float)srcI / 255.0f;
79 const float linSrc = srcConvert.toLuma(srcGamma, src);
80 //Guess at the dst. The perceptual inverse provides smaller visual
81 //discontinuities when slight changes to desaturated colors cause a channel
82 //to map to a different correcting lut with neighboring srcI.
83 //See https://code.google.com/p/chromium/issues/detail?id=141425#c59 .
84 const float dst = 1.0f - src;
85 const float linDst = dstConvert.toLuma(dstGamma, dst);
86
87 //Contrast value tapers off to 0 as the src luminance becomes white
88 const float adjustedContrast = SkScalarToFloat(contrast) * linDst;
89
90 //Remove discontinuity and instability when src is close to dst.
91 //The value 1/256 is arbitrary and appears to contain the instability.
92 if (fabs(src - dst) < (1.0f / 256.0f)) {
93 float ii = 0.0f;
94 for (int i = 0; i < 256; ++i, ii += 1.0f) {
95 float rawSrca = ii / 255.0f;
96 float srca = apply_contrast(rawSrca, adjustedContrast);
97 table[i] = SkToU8(sk_float_round2int(255.0f * srca));
98 }
99 } else {
100 // Avoid slow int to float conversion.
101 float ii = 0.0f;
102 for (int i = 0; i < 256; ++i, ii += 1.0f) {
103 // 'rawSrca += 1.0f / 255.0f' and even
104 // 'rawSrca = i * (1.0f / 255.0f)' can add up to more than 1.0f.
105 // When this happens the table[255] == 0x0 instead of 0xff.
106 // See http://code.google.com/p/chromium/issues/detail?id=146466
107 float rawSrca = ii / 255.0f;
108 float srca = apply_contrast(rawSrca, adjustedContrast);
109 SkASSERT(srca <= 1.0f);
110 float dsta = 1.0f - srca;
111
112 //Calculate the output we want.
113 float linOut = (linSrc * srca + dsta * linDst);
114 SkASSERT(linOut <= 1.0f);
115 float out = dstConvert.fromLuma(dstGamma, linOut);
116
117 //Undo what the blit blend will do.
118 float result = (out - dst) / (src - dst);
119 SkASSERT(sk_float_round2int(255.0f * result) <= 255);
120
121 table[i] = SkToU8(sk_float_round2int(255.0f * result));
122 }
123 }
124 }
125