Snap GDI matrix when snapping height.

src/ports/SkFontHost_win.cpp

Index: src/ports/SkFontHost_win.cpp
===================================================================
--- a/src/ports/SkFontHost_win.cpp	(revision 11569)
+++ b/src/ports/SkFontHost_win.cpp	(working copy)
@@ -665,15 +665,22 @@
 
     // textSize is the actual device size we want (as opposed to the size the user requested).
     // If the scale is negative, this means the matrix will do the flip anyway.
-    SkScalar textSize = SkScalarAbs(SkScalarRoundToScalar(GA.get(SkMatrix::kMScaleY)));
+    SkScalar realTextSize = SkScalarAbs(GA.get(SkMatrix::kMScaleY));
+    SkScalar textSize = SkScalarRoundToScalar(realTextSize);

[reed1, 2013/10/03 14:21:07]

can we rename this something more distinct from realTextSize, like
"integralTextSize" or something?

Alternatively, can we just defer the rounding til the actual calculation for
scale, and place the round() call right there?

scale = (some predicate) ? invert(textSize) : invert(round(textSize));

[bungeman-skia, 2013/10/03 15:48:31]

I'd hate to have two separate calls to SkScalarInvert, as that makes these two
look unrelated, though they are very much related. Also, we need to make sure
that 'textSize' doesn't end up 0, if it does bad things tend to happen in GDI.
Also, the abs is still needed. A better name for 'textSize' here would probably
be 'gdiTextSize'.

     if (textSize == 0) {
         textSize = SK_Scalar1;
     }
 
+    // When not hinting, remove only the textSize scale which will be applied by GDI.
+    // When GDI hinting, remove the entire Y scale to prevent 'subpixel' metrics.
+    SkScalar scale = (fRec.getHinting() == SkPaint::kNo_Hinting ||
+                      fRec.getHinting() == SkPaint::kSlight_Hinting)
+                   ? SkScalarInvert(textSize)
+                   : SkScalarInvert(realTextSize);
+
     // sA is the total matrix A without the textSize (so GDI knows the text size separately).
     // When this matrix is used with GetGlyphOutline, no further processing is needed.
     SkMatrix sA(A);
-    SkScalar scale = SkScalarInvert(textSize);
     sA.preScale(scale, scale); //remove text size
 
     // GsA is the non-rotational part of A without the text height scale.