1/*
2 * Copyright 2015 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/gpu/text/GrDistanceFieldAdjustTable.h"
9
10#include "src/core/SkScalerContext.h"
11
12SkDEBUGCODE(static const int kExpectedDistanceAdjustTableSize = 8;)
13
14SkScalar* build_distance_adjust_table(SkScalar paintGamma, SkScalar deviceGamma) {
15 // This is used for an approximation of the mask gamma hack, used by raster and bitmap
16 // text. The mask gamma hack is based off of guessing what the blend color is going to
17 // be, and adjusting the mask so that when run through the linear blend will
18 // produce the value closest to the desired result. However, in practice this means
19 // that the 'adjusted' mask is just increasing or decreasing the coverage of
20 // the mask depending on what it is thought it will blit against. For black (on
21 // assumed white) this means that coverages are decreased (on a curve). For white (on
22 // assumed black) this means that coverages are increased (on a a curve). At
23 // middle (perceptual) gray (which could be blit against anything) the coverages
24 // remain the same.
25 //
26 // The idea here is that instead of determining the initial (real) coverage and
27 // then adjusting that coverage, we determine an adjusted coverage directly by
28 // essentially manipulating the geometry (in this case, the distance to the glyph
29 // edge). So for black (on assumed white) this thins a bit; for white (on
30 // assumed black) this fake bolds the geometry a bit.
31 //
32 // The distance adjustment is calculated by determining the actual coverage value which
33 // when fed into in the mask gamma table gives us an 'adjusted coverage' value of 0.5. This
34 // actual coverage value (assuming it's between 0 and 1) corresponds to a distance from the
35 // actual edge. So by subtracting this distance adjustment and computing without the
36 // the coverage adjustment we should get 0.5 coverage at the same point.
37 //
38 // This has several implications:
39 // For non-gray lcd smoothed text, each subpixel essentially is using a
40 // slightly different geometry.
41 //
42 // For black (on assumed white) this may not cover some pixels which were
43 // previously covered; however those pixels would have been only slightly
44 // covered and that slight coverage would have been decreased anyway. Also, some pixels
45 // which were previously fully covered may no longer be fully covered.
46 //
47 // For white (on assumed black) this may cover some pixels which weren't
48 // previously covered at all.
49
50 int width, height;
51 size_t size;
52
53#ifdef SK_GAMMA_CONTRAST
54 SkScalar contrast = SK_GAMMA_CONTRAST;
55#else
56 SkScalar contrast = 0.5f;
57#endif
58
59 size = SkScalerContext::GetGammaLUTSize(contrast, paintGamma, deviceGamma,
60 &width, &height);
61
62 SkASSERT(kExpectedDistanceAdjustTableSize == height);
63 SkScalar* table = new SkScalar[height];
64
65 SkAutoTArray<uint8_t> data((int)size);
66 if (!SkScalerContext::GetGammaLUTData(contrast, paintGamma, deviceGamma, data.get())) {
67 // if no valid data is available simply do no adjustment
68 for (int row = 0; row < height; ++row) {
69 table[row] = 0;
70 }
71 return table;
72 }
73
74 // find the inverse points where we cross 0.5
75 // binsearch might be better, but we only need to do this once on creation
76 for (int row = 0; row < height; ++row) {
77 uint8_t* rowPtr = data.get() + row*width;
78 for (int col = 0; col < width - 1; ++col) {
79 if (rowPtr[col] <= 127 && rowPtr[col + 1] >= 128) {
80 // compute point where a mask value will give us a result of 0.5
81 float interp = (127.5f - rowPtr[col]) / (rowPtr[col + 1] - rowPtr[col]);
82 float borderAlpha = (col + interp) / 255.f;
83
84 // compute t value for that alpha
85 // this is an approximate inverse for smoothstep()
86 float t = borderAlpha*(borderAlpha*(4.0f*borderAlpha - 6.0f) + 5.0f) / 3.0f;
87
88 // compute distance which gives us that t value
89 const float kDistanceFieldAAFactor = 0.65f; // should match SK_DistanceFieldAAFactor
90 float d = 2.0f*kDistanceFieldAAFactor*t - kDistanceFieldAAFactor;
91
92 table[row] = d;
93 break;
94 }
95 }
96 }
97
98 return table;
99}
100
101const GrDistanceFieldAdjustTable* GrDistanceFieldAdjustTable::Get() {
102 static const GrDistanceFieldAdjustTable* dfat = new GrDistanceFieldAdjustTable;
103 return dfat;
104}
105
106GrDistanceFieldAdjustTable::GrDistanceFieldAdjustTable() {
107 fTable = build_distance_adjust_table(SK_GAMMA_EXPONENT, SK_GAMMA_EXPONENT);
108 fGammaCorrectTable = build_distance_adjust_table(SK_Scalar1, SK_Scalar1);
109}
110