SkFontHost_gamma.cpp revision 0910916c0f7b951ee55c4b7c6358295b9bca0565 
1#include "SkFontHost.h"
2#include <math.h>
3
4// define this to use pre-compiled tables for gamma. This is slightly faster,
5// and doesn't create any RW global memory, but means we cannot change the
6// gamma at runtime.
7#define USE_PREDEFINED_GAMMA_TABLES
8
9#ifndef USE_PREDEFINED_GAMMA_TABLES
10    // define this if you want to spew out the "C" code for the tables, given
11    // the current values for SK_BLACK_GAMMA and SK_WHITE_GAMMA.
12    #define DUMP_GAMMA_TABLESx
13#endif
14
15///////////////////////////////////////////////////////////////////////////////
16
17#ifdef USE_PREDEFINED_GAMMA_TABLES
18
19#include "sk_predefined_gamma.h"
20
21#else   // use writable globals for gamma tables
22
23static bool gGammaIsBuilt;
24static uint8_t gBlackGamma[256], gWhiteGamma[256];
25
26#define SK_BLACK_GAMMA     (1.4f)
27#define SK_WHITE_GAMMA     (1/1.4f)
28
29static void build_power_table(uint8_t table[], float ee)
30{
31    //    printf("------ build_power_table %g\n", ee);
32    for (int i = 0; i < 256; i++)
33    {
34        float x = i / 255.f;
35        //   printf(" %d %g", i, x);
36        x = powf(x, ee);
37        //   printf(" %g", x);
38        int xx = SkScalarRound(SkFloatToScalar(x * 255));
39        //   printf(" %d\n", xx);
40        table[i] = SkToU8(xx);
41    }
42}
43
44#ifdef DUMP_GAMMA_TABLES
45
46#include "SkString.h"
47
48static void dump_a_table(const char name[], const uint8_t table[],
49                         float gamma) {
50    SkDebugf("\n");
51    SkDebugf("\/\/ Gamma table for %g\n", gamma);
52    SkDebugf("static const uint8_t %s[] = {\n", name);
53    for (int y = 0; y < 16; y++) {
54        SkString line, tmp;
55        for (int x = 0; x < 16; x++) {
56            tmp.printf("0x%02X, ", *table++);
57            line.append(tmp);
58        }
59        SkDebugf("    %s\n", line.c_str());
60    }
61    SkDebugf("};\n");
62}
63
64#endif
65
66#endif
67
68///////////////////////////////////////////////////////////////////////////////
69
70void SkFontHost::GetGammaTables(const uint8_t* tables[2])
71{
72#ifndef USE_PREDEFINED_GAMMA_TABLES
73    if (!gGammaIsBuilt)
74    {
75        build_power_table(gBlackGamma, SK_BLACK_GAMMA);
76        build_power_table(gWhiteGamma, SK_WHITE_GAMMA);
77        gGammaIsBuilt = true;
78
79#ifdef DUMP_GAMMA_TABLES
80        dump_a_table("gBlackGamma", gBlackGamma, SK_BLACK_GAMMA);
81        dump_a_table("gWhiteGamma", gWhiteGamma, SK_WHITE_GAMMA);
82#endif
83    }
84#endif
85    tables[0] = gBlackGamma;
86    tables[1] = gWhiteGamma;
87}
88
89// If the luminance is <= this value, then apply the black gamma table
90#define BLACK_GAMMA_THRESHOLD   0x40
91
92// If the luminance is >= this value, then apply the white gamma table
93#define WHITE_GAMMA_THRESHOLD   0xC0
94
95int SkFontHost::ComputeGammaFlag(const SkPaint& paint)
96{
97    if (paint.getShader() == NULL)
98    {
99        SkColor c = paint.getColor();
100        int r = SkColorGetR(c);
101        int g = SkColorGetG(c);
102        int b = SkColorGetB(c);
103        int luminance = (r * 2 + g * 5 + b) >> 3;
104
105        if (luminance <= BLACK_GAMMA_THRESHOLD)
106        {
107        //    printf("------ black gamma for [%d %d %d]\n", r, g, b);
108            return SkScalerContext::kGammaForBlack_Flag;
109        }
110        if (luminance >= WHITE_GAMMA_THRESHOLD)
111        {
112        //    printf("------ white gamma for [%d %d %d]\n", r, g, b);
113            return SkScalerContext::kGammaForWhite_Flag;
114        }
115    }
116    return 0;
117}
118
119