| /* |
| * Copyright 2015 Google Inc. |
| * |
| * Use of this source code is governed by a BSD-style license that can be |
| * found in the LICENSE file. |
| */ |
| |
| #include "GrDistanceFieldAdjustTable.h" |
| |
| #include "SkScalerContext.h" |
| |
| SkDEBUGCODE(static const int kExpectedDistanceAdjustTableSize = 8;) |
| |
| void GrDistanceFieldAdjustTable::buildDistanceAdjustTable() { |
| // This is used for an approximation of the mask gamma hack, used by raster and bitmap |
| // text. The mask gamma hack is based off of guessing what the blend color is going to |
| // be, and adjusting the mask so that when run through the linear blend will |
| // produce the value closest to the desired result. However, in practice this means |
| // that the 'adjusted' mask is just increasing or decreasing the coverage of |
| // the mask depending on what it is thought it will blit against. For black (on |
| // assumed white) this means that coverages are decreased (on a curve). For white (on |
| // assumed black) this means that coverages are increased (on a a curve). At |
| // middle (perceptual) gray (which could be blit against anything) the coverages |
| // remain the same. |
| // |
| // The idea here is that instead of determining the initial (real) coverage and |
| // then adjusting that coverage, we determine an adjusted coverage directly by |
| // essentially manipulating the geometry (in this case, the distance to the glyph |
| // edge). So for black (on assumed white) this thins a bit; for white (on |
| // assumed black) this fake bolds the geometry a bit. |
| // |
| // The distance adjustment is calculated by determining the actual coverage value which |
| // when fed into in the mask gamma table gives us an 'adjusted coverage' value of 0.5. This |
| // actual coverage value (assuming it's between 0 and 1) corresponds to a distance from the |
| // actual edge. So by subtracting this distance adjustment and computing without the |
| // the coverage adjustment we should get 0.5 coverage at the same point. |
| // |
| // This has several implications: |
| // For non-gray lcd smoothed text, each subpixel essentially is using a |
| // slightly different geometry. |
| // |
| // For black (on assumed white) this may not cover some pixels which were |
| // previously covered; however those pixels would have been only slightly |
| // covered and that slight coverage would have been decreased anyway. Also, some pixels |
| // which were previously fully covered may no longer be fully covered. |
| // |
| // For white (on assumed black) this may cover some pixels which weren't |
| // previously covered at all. |
| |
| int width, height; |
| size_t size; |
| |
| #ifdef SK_GAMMA_CONTRAST |
| SkScalar contrast = SK_GAMMA_CONTRAST; |
| #else |
| SkScalar contrast = 0.5f; |
| #endif |
| SkScalar paintGamma = SK_GAMMA_EXPONENT; |
| SkScalar deviceGamma = SK_GAMMA_EXPONENT; |
| |
| size = SkScalerContext::GetGammaLUTSize(contrast, paintGamma, deviceGamma, |
| &width, &height); |
| |
| SkASSERT(kExpectedDistanceAdjustTableSize == height); |
| fTable = new SkScalar[height]; |
| |
| SkAutoTArray<uint8_t> data((int)size); |
| SkScalerContext::GetGammaLUTData(contrast, paintGamma, deviceGamma, data.get()); |
| |
| // find the inverse points where we cross 0.5 |
| // binsearch might be better, but we only need to do this once on creation |
| for (int row = 0; row < height; ++row) { |
| uint8_t* rowPtr = data.get() + row*width; |
| for (int col = 0; col < width - 1; ++col) { |
| if (rowPtr[col] <= 127 && rowPtr[col + 1] >= 128) { |
| // compute point where a mask value will give us a result of 0.5 |
| float interp = (127.5f - rowPtr[col]) / (rowPtr[col + 1] - rowPtr[col]); |
| float borderAlpha = (col + interp) / 255.f; |
| |
| // compute t value for that alpha |
| // this is an approximate inverse for smoothstep() |
| float t = borderAlpha*(borderAlpha*(4.0f*borderAlpha - 6.0f) + 5.0f) / 3.0f; |
| |
| // compute distance which gives us that t value |
| const float kDistanceFieldAAFactor = 0.65f; // should match SK_DistanceFieldAAFactor |
| float d = 2.0f*kDistanceFieldAAFactor*t - kDistanceFieldAAFactor; |
| |
| fTable[row] = d; |
| break; |
| } |
| } |
| } |
| } |