Move DistanceAdjustTable to its own file

src/gpu/GrDistanceFieldAdjustTable.cpp

+/*
+ * 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;
+            }
+        }
+    }
+}