Factor text size device mapping in SkScalerContext.

src/core/SkScalerContext.h

Index: src/core/SkScalerContext.h
diff --git a/src/core/SkScalerContext.h b/src/core/SkScalerContext.h
index b78efef9c23727049eaf55e7c321b7d10c3ddc72..88b52860fb88503fe69e7a64db7cc66dcbcb92cf 100644
--- a/src/core/SkScalerContext.h
+++ b/src/core/SkScalerContext.h
@@ -86,6 +86,50 @@ struct SkScalerContextRec {
     void    getSingleMatrix(SkMatrix*) const;
     void    getSingleMatrixWithoutTextSize(SkMatrix*) const;
 
+    /** The kind of scale which will be applied by the underlying port (pre-matrix). */
+    enum PreMatrixScale {
+        kFull_PreMatrixScale,  // The underlying port can apply both x and y scale.
+        kVertical_PreMatrixScale,  // The underlying port can only apply a y scale.
+        kVerticalInteger_PreMatrixScale  // The underlying port can only apply an integer y scale.
+    };
+    /**
+     *  Compute useful matrices for use with sizing in underlying libraries.
+     *
+     *  There are two kinds of text size, a 'requested/logical size' which is like asking for size
+     *  '12' and a 'real' size which is the size after the matrix is applied. The matrices produced
+     *  by this method are based on the 'real' size. This method effectively finds the total device
+     *  matrix and decomposes it in various ways.
+     *
+     *  The most useful decomposition is into 'scale' and 'remaining'. The 'scale' is applied first
+     *  and then the 'remaining' to fully apply the total matrix. This decomposition is useful when
+     *  the text size ('scale') may have meaning apart from the total matrix. This is true when
+     *  hinting, and sometimes true for other properties as well.
+     *
+     *  The second (optional) decomposition is of 'remaining' into a non-rotational part
+     *  'remainingWithoutRotation' and a rotational part 'remainingRotation'. The 'scale' is applied
+     *  first, then 'remainingWithoutRotation', then 'remainingRotation' to fully apply the total
+     *  matrix. This decomposition is helpful when only horizontal metrics can be trusted, so the
+     *  'scale' and 'remainingWithoutRotation' will be handled by the underlying library, but
+     *  the final rotation 'remainingRotation' will be handled manually.
+     *
+     *  The 'total' matrix is also (optionally) available. This is useful in cases where the
+     *  underlying library will not be used, often when working directly with font data.
+     *
+     *  The parameters 'scale' and 'remaining' are required, the other pointers may be NULL.
+     *
+     *  @param preMatrixScale the kind of scale to extract from the total matrix.
+     *  @param scale the scale extracted from the total matrix (both values positive).
+     *  @param remaining apply after scale to apply the total matrix.
+     *  @param remainingWithoutRotation apply after scale to apply the total matrix sans rotation.
+     *  @param remainingRotation apply after remainingWithoutRotation to apply the total matrix.
+     *  @param total the total matrix.
+     */
+    void computeMatrices(PreMatrixScale preMatrixScale,
+                         SkVector* scale, SkMatrix* remaining,
+                         SkMatrix* remainingWithoutRotation = NULL,
+                         SkMatrix* remainingRotation = NULL,
+                         SkMatrix* total = NULL);
+
     inline SkPaint::Hinting getHinting() const;
     inline void setHinting(SkPaint::Hinting);