[OpenJDK 2D-Dev] More incompatibilities

Roman Kennke roman.kennke at aicas.com
Tue Mar 3 15:23:53 UTC 2009


Sorry, wrong patch. Here it comes.

/Roman

Am Dienstag, den 03.03.2009, 16:17 +0100 schrieb Roman Kennke:
> Hi there,
> 
> > 4. StrokeShapeTest: createStrokedShape() behaves differently.
> 
> It turns out that there is an arithmetic overflow here. The pisces
> stroker does a stupid thing here. First it initializes the
> scaledLineWidth like this:
> 
>         this.scaledLineWidth2 = ((long)transform.m00*lineWidth2);
> 
> which is infact wrong, because in fixed point arithmetics you need to
> apply >> 16, because the decimal moves.
> 
> However, in another place it uses this factor like this:
> 
>                 dx = (int)( (ly*scaledLineWidth2)/ilen >> 16);
>                 dy = (int)(-(lx*scaledLineWidth2)/ilen >> 16);
> 
> which makes it ok in the end. The only problem is, that we start with an
> incredibly large number, then multiply another incredibly large number
> and _after_ that (when the overflow already occured) do the >> 16. The
> patch moves the >> 16 to the initialization of scaledLineWidth, which
> makes it clearer, more correct and even a tad faster, because we do the
> second calculation at least 2x.
> 
> Of course, the real fix here would be to turn the whole implementation
> into floating point. This particular testcase is fixed by this patch,
> and the 'range of correct behaviour' is much large now, but if you deal
> with very large numbers in your shapes, then you will get trouble again.
> 
> /Roman
-- 
Dipl.-Inform. (FH) Roman Kennke, Software Engineer, http://kennke.org
aicas Allerton Interworks Computer Automated Systems GmbH
Haid-und-Neu-Straße 18 * D-76131 Karlsruhe * Germany
http://www.aicas.com   * Tel: +49-721-663 968-48
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Geschäftsführer: Dr. James J. Hunt

-------------- next part --------------
diff --git a/src/share/classes/sun/java2d/pisces/Stroker.java b/src/share/classes/sun/java2d/pisces/Stroker.java
--- a/src/share/classes/sun/java2d/pisces/Stroker.java
+++ b/src/share/classes/sun/java2d/pisces/Stroker.java
@@ -181,7 +181,7 @@
                               Transform4 transform) {
         this.lineWidth = lineWidth;
         this.lineWidth2 = lineWidth >> 1;
-        this.scaledLineWidth2 = (long)transform.m00*lineWidth2;
+        this.scaledLineWidth2 = ((long)transform.m00*lineWidth2) >> 16;
         this.capStyle = capStyle;
         this.joinStyle = joinStyle;
         this.miterLimit = miterLimit;
@@ -243,8 +243,8 @@
             if (ilen == 0) {
                 dx = dy = 0;
             } else {
-                dx = (int)( (ly*scaledLineWidth2)/ilen >> 16);
-                dy = (int)(-(lx*scaledLineWidth2)/ilen >> 16);
+                dx = (int)( (ly*scaledLineWidth2)/ilen);
+                dy = (int)(-(lx*scaledLineWidth2)/ilen);
             }
         } else {
             double dlx = x1 - x0;


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