Fix advances for aliased text with DirectWrite.

The existing code assumes that GetGdiCompatibleGlyphMetrics transform
parameter acts the same as the GetGlyphOutlineW lpmat2 parameter.
However, it appears this is very much not true.

BUG=skia:6499

Change-Id: I88c762b27acd09cc746da4d6e2b901540876908a
Reviewed-on: https://skia-review.googlesource.com/13479
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

2 files changed, 9 insertions, 26 deletions

diff --git a/src/ports/SkScalerContext_win_dw.cpp b/src/ports/SkScalerContext_win_dw.cpp
index 81238a6088..1dfe8e08ee 100644
--- a/src/ports/SkScalerContext_win_dw.cpp
+++ b/src/ports/SkScalerContext_win_dw.cpp
@@ -243,9 +243,7 @@ SkScalerContext_DW::SkScalerContext_DW(sk_sp<DWriteFontTypeface> typefaceRef,
     // horizontal glyphs and the subpixel flag should not affect glyph shapes.
 
     SkVector scale;
-    SkMatrix GsA;
-    fRec.computeMatrices(SkScalerContextRec::kVertical_PreMatrixScale,
-                         &scale, &fSkXform, &GsA, &fG_inv);
+    fRec.computeMatrices(SkScalerContextRec::kVertical_PreMatrixScale, &scale, &fSkXform);
 
     fXform.m11 = SkScalarToFloat(fSkXform.getScaleX());
     fXform.m12 = SkScalarToFloat(fSkXform.getSkewY());
@@ -254,13 +252,6 @@ SkScalerContext_DW::SkScalerContext_DW(sk_sp<DWriteFontTypeface> typefaceRef,
     fXform.dx = 0;
     fXform.dy = 0;
 
-    fGsA.m11 = SkScalarToFloat(GsA.get(SkMatrix::kMScaleX));
-    fGsA.m12 = SkScalarToFloat(GsA.get(SkMatrix::kMSkewY)); // This should be ~0.
-    fGsA.m21 = SkScalarToFloat(GsA.get(SkMatrix::kMSkewX));
-    fGsA.m22 = SkScalarToFloat(GsA.get(SkMatrix::kMScaleY));
-    fGsA.dx = 0;
-    fGsA.dy = 0;
-
     // realTextSize is the actual device size we want (as opposed to the size the user requested).
     // gdiTextSize is the size we request when GDI compatible.
     // If the scale is negative, this means the matrix will do the flip anyway.
@@ -402,7 +393,9 @@ void SkScalerContext_DW::generateAdvance(SkGlyph* glyph) {
         HRVM(this->getDWriteTypeface()->fDWriteFontFace->GetGdiCompatibleGlyphMetrics(
                  fTextSizeMeasure,
                  1.0f, // pixelsPerDip
-                 &fGsA,
+                 // This parameter does not act like the lpmat2 parameter to GetGlyphOutlineW.
+                 // If it did then GsA here and G_inv below to mapVectors.
+                 nullptr,
                  DWRITE_MEASURING_MODE_GDI_NATURAL == fMeasuringMode,
                  &glyphId, 1,
                  &gm),
@@ -420,20 +413,18 @@ void SkScalerContext_DW::generateAdvance(SkGlyph* glyph) {
     }
     SkScalar advanceX = fTextSizeMeasure * gm.advanceWidth / dwfm.designUnitsPerEm;
 
-    SkVector vecs[1] = { { advanceX, 0 } };
+    SkVector advance = { advanceX, 0 };
     if (DWRITE_MEASURING_MODE_GDI_CLASSIC == fMeasuringMode ||
         DWRITE_MEASURING_MODE_GDI_NATURAL == fMeasuringMode)
     {
         // DirectWrite produced 'compatible' metrics, but while close,
         // the end result is not always an integer as it would be with GDI.
-        vecs[0].fX = SkScalarRoundToScalar(advanceX);
-        fG_inv.mapVectors(vecs, SK_ARRAY_COUNT(vecs));
-    } else {
-        fSkXform.mapVectors(vecs, SK_ARRAY_COUNT(vecs));
+        advance.fX = SkScalarRoundToScalar(advance.fX);
     }
+    fSkXform.mapVectors(&advance, 1);
 
-    glyph->fAdvanceX = SkScalarToFloat(vecs[0].fX);
-    glyph->fAdvanceY = SkScalarToFloat(vecs[0].fY);
+    glyph->fAdvanceX = SkScalarToFloat(advance.fX);
+    glyph->fAdvanceY = SkScalarToFloat(advance.fY);
 }
 
 HRESULT SkScalerContext_DW::getBoundingBox(SkGlyph* glyph,
diff --git a/src/ports/SkScalerContext_win_dw.h b/src/ports/SkScalerContext_win_dw.h
index f186ea52dc..9b0f244417 100644
--- a/src/ports/SkScalerContext_win_dw.h
+++ b/src/ports/SkScalerContext_win_dw.h
@@ -60,14 +60,6 @@ private:
     SkMatrix fSkXform;
     /** The total matrix without the text height scale. */
     DWRITE_MATRIX fXform;
-    /** The non-rotational part of total matrix without the text height scale.
-     *  This is used to find the magnitude of gdi compatible advances.
-     */
-    DWRITE_MATRIX fGsA;
-    /** The inverse of the rotational part of the total matrix.
-     *  This is used to find the direction of gdi compatible advances.
-     */
-    SkMatrix fG_inv;
     /** The text size to render with. */
     SkScalar fTextSizeRender;
     /** The text size to measure with. */