[fpc-devel] Rect drawing inconsistency in TFPCanvas

[Michael Van Canneyt]

On Thu, 7 Feb 2019, Ondrej Pokorny wrote:

> Hello,
>
> when working with TFPCanvas, I face an annoying inconsistency between 
> fcl-image and all other image libraries I know (including LCL Graphics).
>
> The problem is that TFPCustomCanvas.FillRect and all other rect-aware methods 
> (FillEllipse, etc...) fill 1 more pixel to the right and bottom.
>
> E.g.
>
>       MyCanvas.FillRect(0, 0, 50, 50);
>
> - FP canvas draws a rectangle with 51x51 pixels.
>
> - LCL Graphics draws a rectangle with 50x50 pixels. (And all other libraries 
> like Delphi Graphics, BGRABitmap etc. do the same.)
>
> ---
>
> IMO TFPCanvas is wrong because
>
> 1.) The rectangle should be filled up until the right/bottom coordinate - 
> that means excluding the pixel at (50, 50). And not including it.
>
> 2.) Rect(0, 0, 50, 50) has a width/height of 50 pixels - see "function 
> TRect.getWidth" in FPC sources. In FPCanvas if you draw Rect(0, 0, 50, 50) it 
> is 51x51 pixels, which is inconsistent.
>
> 3.) In FPCanvas if you draw Rect(0, 0, 50, 50) and Rect(50, 0, 100, 50) there 
> is a 1px overlay between them! (That is wrong - Seen e.g. with dmAlphaBlend 
> painting as in https://bugs.freepascal.org/view.php?id=34266 ).
>
> 4.) In FPCanvas if you draw Rect(0, 0, 0, 0) you get a 1px dot. (That is 
> wrong.)
>
> ---
>
> See the attached example project.
>
> Should I report it in Mantis?

Yes, please.

Michael.