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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     virtual SkScalar toLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luminance) const SK_OVERRIDE {
16         SkASSERT(SK_Scalar1 == gamma);
17         return luminance;
18     }
fromLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luma) const19     virtual SkScalar fromLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luma) const SK_OVERRIDE {
20         SkASSERT(SK_Scalar1 == gamma);
21         return luma;
22     }
23 };
24 
25 class SkGammaColorSpaceLuminance : public SkColorSpaceLuminance {
toLuma(SkScalar gamma,SkScalar luminance) const26     virtual SkScalar toLuma(SkScalar gamma, SkScalar luminance) const SK_OVERRIDE {
27         return SkScalarPow(luminance, gamma);
28     }
fromLuma(SkScalar gamma,SkScalar luma) const29     virtual SkScalar fromLuma(SkScalar gamma, SkScalar luma) const SK_OVERRIDE {
30         return SkScalarPow(luma, SkScalarInvert(gamma));
31     }
32 };
33 
34 class SkSRGBColorSpaceLuminance : public SkColorSpaceLuminance {
toLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luminance) const35     virtual SkScalar toLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luminance) const SK_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 <= SkFloatToScalar(0.04045f)) {
40             return luminance / SkFloatToScalar(12.92f);
41         }
42         return SkScalarPow((luminance + SkFloatToScalar(0.055f)) / SkFloatToScalar(1.055f),
43                         SkFloatToScalar(2.4f));
44     }
fromLuma(SkScalar SkDEBUGCODE (gamma),SkScalar luma) const45     virtual SkScalar fromLuma(SkScalar SkDEBUGCODE(gamma), SkScalar luma) const SK_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 <= SkFloatToScalar(0.0031308f)) {
50             return luma * SkFloatToScalar(12.92f);
51         }
52         return SkFloatToScalar(1.055f) * SkScalarPow(luma, SkScalarInvert(SkFloatToScalar(2.4f)))
53                - SkFloatToScalar(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