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 "GrDistanceFieldAdjustTable.h"
9
10#include "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    SkScalerContext::GetGammaLUTData(contrast, paintGamma, deviceGamma, data.get());
67
68    // find the inverse points where we cross 0.5
69    // binsearch might be better, but we only need to do this once on creation
70    for (int row = 0; row < height; ++row) {
71        uint8_t* rowPtr = data.get() + row*width;
72        for (int col = 0; col < width - 1; ++col) {
73            if (rowPtr[col] <= 127 && rowPtr[col + 1] >= 128) {
74                // compute point where a mask value will give us a result of 0.5
75                float interp = (127.5f - rowPtr[col]) / (rowPtr[col + 1] - rowPtr[col]);
76                float borderAlpha = (col + interp) / 255.f;
77
78                // compute t value for that alpha
79                // this is an approximate inverse for smoothstep()
80                float t = borderAlpha*(borderAlpha*(4.0f*borderAlpha - 6.0f) + 5.0f) / 3.0f;
81
82                // compute distance which gives us that t value
83                const float kDistanceFieldAAFactor = 0.65f; // should match SK_DistanceFieldAAFactor
84                float d = 2.0f*kDistanceFieldAAFactor*t - kDistanceFieldAAFactor;
85
86                table[row] = d;
87                break;
88            }
89        }
90    }
91
92    return table;
93}
94
95void GrDistanceFieldAdjustTable::buildDistanceAdjustTables() {
96    fTable = build_distance_adjust_table(SK_GAMMA_EXPONENT, SK_GAMMA_EXPONENT);
97    fGammaCorrectTable = build_distance_adjust_table(SK_Scalar1, SK_Scalar1);
98}
99