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/*
* 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;)
SkScalar* build_distance_adjust_table(SkScalar paintGamma, SkScalar deviceGamma) {
// 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
size = SkScalerContext::GetGammaLUTSize(contrast, paintGamma, deviceGamma,
&width, &height);
SkASSERT(kExpectedDistanceAdjustTableSize == height);
SkScalar* table = 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;
table[row] = d;
break;
}
}
}
return table;
}
void GrDistanceFieldAdjustTable::buildDistanceAdjustTables() {
fTable = build_distance_adjust_table(SK_GAMMA_EXPONENT, SK_GAMMA_EXPONENT);
fGammaCorrectTable = build_distance_adjust_table(SK_Scalar1, SK_Scalar1);
}
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