Improve horizontal baseline detection.

src/core/SkScalerContext.cpp

Index: src/core/SkScalerContext.cpp
diff --git a/src/core/SkScalerContext.cpp b/src/core/SkScalerContext.cpp
index caef2d137721254b865e0f0b29f5fac9af8fadf4..c67ca3e9fa992a4b6da0a921315e19ff070491bc 100644
--- a/src/core/SkScalerContext.cpp
+++ b/src/core/SkScalerContext.cpp
@@ -827,13 +827,21 @@ void SkScalerContextRec::computeMatrices(PreMatrixScale preMatrixScale, SkVector
     }
 }
 
-SkAxisAlignment SkComputeAxisAlignmentForHText(const SkMatrix& matrix) {
-    SkASSERT(!matrix.hasPerspective());
-
-    if (0 == matrix[SkMatrix::kMSkewY]) {
+SkAxisAlignment SkScalerContext::computeAxisAlignmentForHText() {
+    // Why fPost2x2 can be used here.
+    // getSingleMatrix multiplies in getLocalMatrix, which consists of
+    // * fTextSize (a scale, which has no effect)
+    // * fPreScaleX (a scale in x, which has no effect)
+    // * fPreSkewX (has no effect, but would on vertical text alignment).
+    // In other words, making the text bigger, stretching it along the
+    // horizontal axis, or fake italicizing it does not move the baseline.

[herb_g, 2016/02/29 18:37:10]

italicizing; it does...

[bungeman-skia, 2016/02/29 19:21:20]

it actually has no effect on the horizontal baseline (which is the implied
baseline, since this is For'H'Text.

+
+    if (0 == fRec.fPost2x2[1][0]) {
+        // The x axis is mapped onto the x axis.
         return kX_SkAxisAlignment;
     }
-    if (0 == matrix[SkMatrix::kMSkewX]) {
+    if (0 == fRec.fPost2x2[0][0]) {
+        // The x axis is mapped onto the y axis.
         return kY_SkAxisAlignment;
     }
     return kNone_SkAxisAlignment;