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