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 12 SkDEBUGCODE(static const int kExpectedDistanceAdjustTableSize = 8;) 13 14 void GrDistanceFieldAdjustTable::buildDistanceAdjustTable() { 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 SkScalar paintGamma = SK_GAMMA_EXPONENT; 59 SkScalar deviceGamma = SK_GAMMA_EXPONENT; 60 61 size = SkScalerContext::GetGammaLUTSize(contrast, paintGamma, deviceGamma, 62 &width, &height); 63 64 SkASSERT(kExpectedDistanceAdjustTableSize == height); 65 fTable = new SkScalar[height]; 66 67 SkAutoTArray<uint8_t> data((int)size); 68 SkScalerContext::GetGammaLUTData(contrast, paintGamma, deviceGamma, data.get()); 69 70 // find the inverse points where we cross 0.5 71 // binsearch might be better, but we only need to do this once on creation 72 for (int row = 0; row < height; ++row) { 73 uint8_t* rowPtr = data.get() + row*width; 74 for (int col = 0; col < width - 1; ++col) { 75 if (rowPtr[col] <= 127 && rowPtr[col + 1] >= 128) { 76 // compute point where a mask value will give us a result of 0.5 77 float interp = (127.5f - rowPtr[col]) / (rowPtr[col + 1] - rowPtr[col]); 78 float borderAlpha = (col + interp) / 255.f; 79 80 // compute t value for that alpha 81 // this is an approximate inverse for smoothstep() 82 float t = borderAlpha*(borderAlpha*(4.0f*borderAlpha - 6.0f) + 5.0f) / 3.0f; 83 84 // compute distance which gives us that t value 85 const float kDistanceFieldAAFactor = 0.65f; // should match SK_DistanceFieldAAFactor 86 float d = 2.0f*kDistanceFieldAAFactor*t - kDistanceFieldAAFactor; 87 88 fTable[row] = d; 89 break; 90 } 91 } 92 } 93 } 94
