Factor text size device mapping in SkScalerContext.

src/core/SkScalerContext.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkScalerContext_DEFINED
 #define SkScalerContext_DEFINED
 
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     // must be a multiple of 4. SkDescriptor requires that its arguments be
     // multiples of four and this structure is put in an SkDescriptor in
     // SkPaint::MakeRec.
 
     void    getMatrixFrom2x2(SkMatrix*) const;
     void    getLocalMatrix(SkMatrix*) const;
     void    getLocalMatrixWithoutTextSize(SkMatrix*) const;
     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);
 
     SkMask::Format getFormat() const {
         return static_cast<SkMask::Format>(fMaskFormat);
     }
 
     SkColor getLuminanceColor() const {
         return fLumBits;
     }
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     return static_cast<SkPaint::Hinting>(hint);
 }
 
 void SkScalerContextRec::setHinting(SkPaint::Hinting hinting) {
     fFlags = (fFlags & ~SkScalerContext::kHinting_Mask) |
                                 (hinting << SkScalerContext::kHinting_Shift);
 }
 
 
 #endif