Bug 690098

Summary:	Small miters do not render correctly
Product:	Ghostscript	Reporter:	Adrian Johnson <ajohnson>
Component:	PS Interpreter	Assignee:	Robin Watts <robin>
Status:	RESOLVED FIXED
Severity:	normal
Priority:	P4
Version:	8.63
Hardware:	PC
OS:	Linux
Customer:		Word Size:	---
Attachments:	The PostScript file The correct rendering of the PostScript file The output from Ghostscript photoshop.png patch

Description Adrian Johnson 2008-09-26 21:45:31 UTC

The attached PostScript file from the cairo test suite does not render the miter
in the lower left corner correctly. The miter has been chopped off making it
look more like a bevel join.

Also attached are the reference image from the cairo test suite showing the
miter correctly rendered and the image from Ghostscript showing the incorrect
rendering.

The PS file prints correctly on my LaserJet 4050.

Comment 1 Adrian Johnson 2008-09-26 21:46:51 UTC

Created attachment 4435 [details]
The PostScript file

The PostScript file.

Comment 2 Adrian Johnson 2008-09-26 21:47:49 UTC

Created attachment 4436 [details]
The correct rendering of the PostScript file

Comment 3 Adrian Johnson 2008-09-26 21:48:33 UTC

Created attachment 4437 [details]
The output from Ghostscript

Comment 4 Alex Cherepanov 2008-09-27 09:38:07 UTC

This is a matter of fixed point underflow that happen at gxstroke.c:730
at 84 dpi and lower. There are other underflows but at 169 dpi or higher the
rendering is close to the correct one. 

The sample file can be simplified to
%!
100000 setmiterlimit 10 setlinewidth
[0.001 0 0 -1 0 100] concat
10000 60 moveto 40000 60 lineto 10000 80 lineto stroke
showpage

Comment 5 Marcos H. Woehrmann 2008-09-27 20:27:33 UTC

I'm not sure that the "correct rendering" is correct.  Opening the PostScript file in Photoshop CS3 produces 
the same results as Ghostscript, as does printing the file on an an HP LaserJet P3005.

Comment 6 Marcos H. Woehrmann 2008-09-27 20:30:07 UTC

Created attachment 4439 [details]
photoshop.png

Comment 7 Henry Stiles 2009-11-19 07:50:30 UTC

Reassigning path and fill problems to Robin Watts.

Comment 8 Robin Watts 2009-12-01 09:09:03 UTC

> This is a matter of fixed point underflow that happen at gxstroke.c:730
> at 84 dpi and lower. There are other underflows but at 169 dpi or higher the
> rendering is close to the correct one. 

Looking back in the SVN history, this line is here:

726 :     		     * direction that corresponds to counter-
727 :     		     * clockwise in device space.
728 :     		     */ 
729 :     		    pl.e.cdelta.x = (fixed) (dpt.x * xx);
730 :     		    pl.e.cdelta.y = (fixed) (dpt.y * yy);
731 :     		    if (orient != orient_portrait)
732 :     			pl.e.cdelta.x += (fixed) (dpt.y * yx),
733 :     			    pl.e.cdelta.y += (fixed) (dpt.x * xy);
 
i.e. the cdelta values become so low that using them as directions in the 
mitering code means they look parallel, and so we don't mitre.

I have a fix that spots the deltas being small (sum of absolute lengths < an 
arbitrary value (16 currently)) and moving to use the floating point 'vector' 
stuff for directionality instead.

Testing now.

Comment 9 Robin Watts 2009-12-01 09:50:18 UTC

Created attachment 5729 [details]
patch

Proposed patch to solve the problem.

Comment 10 Robin Watts 2009-12-01 10:38:56 UTC

Fixed in revision 10425 using the attached patch.