experiment with zero-length round capped line segments

include/core/SkPaint.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 SkPaint_DEFINED
 #define SkPaint_DEFINED
 
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         behavior of miter joins when the joins angle is sharp. This value must
         be >= 0.
         @param miter    set the miter limit on the paint, used whenever the
                         paint's style is Stroke or StrokeAndFill.
     */
     void setStrokeMiter(SkScalar miter);
 
     /** Cap enum specifies the settings for the paint's strokecap. This is the
         treatment that is applied to the beginning and end of each non-closed
         contour (e.g. lines).
+
+        If the cap is round or square, the caps are drawn when the contour has
+        a zero length. Zero length contours can be created by following moveTo
+        with a lineTo at the same point, or a moveTo followed by a close.
+
+        A dash with an on interval of zero also creates a zero length contour.
+
+        The zero length contour draws the square cap without rotation, since
+        the no direction can be inferred.
     */
     enum Cap {
         kButt_Cap,      //!< begin/end contours with no extension
         kRound_Cap,     //!< begin/end contours with a semi-circle extension
         kSquare_Cap,    //!< begin/end contours with a half square extension
 
         kCapCount,
         kDefault_Cap = kButt_Cap
     };
 
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     friend class GrStencilAndCoverTextContext;
     friend class GrPathRendering;
     friend class GrTextContext;
     friend class GrGLPathRendering;
     friend class SkScalerContext;
     friend class SkTextToPathIter;
     friend class SkCanonicalizePaint;
 };
 
 #endif