VCL drawPolygon() off-by-one without line color

Luboš Luňák l.lunak at
Thu Nov 7 11:37:41 UTC 2019

On Monday 04 of November 2019, Thorsten Behrens wrote:
> Luboš Luňák wrote:
> > Is this some kind of a strange rule that I'm not aware of? Does
> > somebody know a good reason why all graphics libraries would do
> > this?
> Yes, essentially all graphics systems fill polygons like that
> (including 3D ones). The reason is that otherwise you cannot render
> pixel-perfect adjacent polygons without occasional overlap (especially
> for non-straight connecting lines).
> So really drawRect() is the odd one out, as Caolan suggested.
> ...and now you know why the code is so littered with this +1/-1
> corrections for rectangles. The (now gone) software renderer in
> basebmp had some comments about those subtleties, and some explicit
> adjustments in the svp backend layer above.

 I see. That's really non-obvious (and the usual pathetic style of VCL's 
documentation doesn't help either), but it kind of makes sense, so I'll make 
Skia do the same and add a note to VCL backendtest pointing to this for 
whoever will feel like cleaning that mess up somehow.

 Luboš Luňák
 l.lunak at

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