Move DistanceAdjustTable to its own file

src/gpu/GrAtlasTextContext.cpp

Index: src/gpu/GrAtlasTextContext.cpp
diff --git a/src/gpu/GrAtlasTextContext.cpp b/src/gpu/GrAtlasTextContext.cpp
index 153d0fe5169461a16ee92f40f757f37ca65540ee..38b9c042d4433f71cf4ad1393748299191b09cc9 100644
--- a/src/gpu/GrAtlasTextContext.cpp
+++ b/src/gpu/GrAtlasTextContext.cpp
@@ -46,13 +46,11 @@ static const int kLargeDFFontLimit = 384;
 #else
 static const int kLargeDFFontLimit = 2 * kLargeDFFontSize;
 #endif
-
-SkDEBUGCODE(static const int kExpectedDistanceAdjustTableSize = 8;)
 };
 
 GrAtlasTextContext::GrAtlasTextContext(GrContext* context, const SkSurfaceProps& surfaceProps)
     : INHERITED(context, surfaceProps)
-    , fDistanceAdjustTable(new DistanceAdjustTable) {
+    , fDistanceAdjustTable(new GrDistanceFieldAdjustTable) {
     // We overallocate vertices in our textblobs based on the assumption that A8 has the greatest
     // vertexStride
     static_assert(GrAtlasTextBlob::kGrayTextVASize >= GrAtlasTextBlob::kColorTextVASize &&
@@ -62,87 +60,6 @@ GrAtlasTextContext::GrAtlasTextContext(GrContext* context, const SkSurfaceProps&
     fCache = context->getTextBlobCache();
 }
 
-void GrAtlasTextContext::DistanceAdjustTable::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;
-            }
-        }
-    }
-}
 
 GrAtlasTextContext* GrAtlasTextContext::Create(GrContext* context,
                                                const SkSurfaceProps& surfaceProps) {