Re: Strange behaviour / Possible Bug with GC Function

[Colin Thomas <colin designresources co uk>]

Hi Havoc,

A picture is worth a thousand words (as they say) so I have attached a GIF.

I am drawing four rectangles, starting at the top left and moving to the
bottom right.

The colours I am drawing in are specificaly RED-BLUE-YELLOW-GREEN, but the 
last rectangle, although specified in GREEN is always drawn in BLUE.

The GDK_ORing seems to pick up the "overlapping" RED-BLUE rectangles ( I like
this), but NOT the BLUE-YELLOW or GREEN(now BLUE)-YELLOW overlaps.

As I am always drawing to the foreground, are you saying that the GDKFUNCTIONS
are not ORing the rectangle (I am adding to the foreground), with what I have
already drawn to the foreground? If this is the case, why is the RED-BLUE 
overlap marked?

I hope you can see what I am trying to achieve now :-) 

If we can only GDK_OR foreground to background, AFTER a rectangle has been 
applied, could it be that we "copy" the foreground to the background ( and
simultaneously clear the foreground)in readiness for the next rectangle to be 
applied (or something similar), to get around this??

I look forward to your thoughts..

Best regards and thanks for your reply...


Colin Thomas
> X-Authentication-Warning: icon.labs.redhat.com: hp set sender to hp redhat com 
using -f
> To: Colin Thomas <colin designresources co uk>
> Cc: gtk-app-devel-list gnome org
> Subject: Re: Strange behaviour / Possible Bug with GC Function
> From: Havoc Pennington <hp redhat com>
> Date: 13 Mar 2001 17:05:31 -0500
> User-Agent: Gnus/5.0807 (Gnus v5.8.7) Emacs/20.7
> MIME-Version: 1.0
>
>
> Colin Thomas <colin designresources co uk> writes:
> > I am running with gtk+ 1.2.8 on a Solaris platform.
> >
> > I have the requirement to draw to a window various shapes, and when
> > overlapping to show the overlaps.
> >
> > I have a pixmap which I draw to a window.
> >
> > I have located the GDK_FUNCTION associated with the graphics context,
> > i.e. GDK_COPY , GDK_AND GDK_OR etc etc .
> >
> > The GDK_OR gives me the ability to see overlapping shapes, but I seem
> > to get some strange behaviour. For example when I draw a green rectangle
> > it always paints as BLUE ?? (When I remove the call seting GDK_OR, Green
> > appears agains, but I look the overlapping).
>
> I don't understand what you want to happen - if you don't want green,
> which is the only color you've provided explicitly, what color do you
> want? Blue seems as rational as anything else, maybe you want the
> function GDK_READ_MY_MIND_AND_PICK_NICE_COLOR? ;-)
>
> Anyway, the graphics functions define how the GC foreground pixel will
> be combined with the GC background pixel, on a bitwise level.
>
> The resulting color is pretty much totally undefined. GDK_XOR will
> normally produce a contrasting color, and has the property that
> drawing the same thing twice "undraws" the result of the first draw,
> so it's the only GdkFunction anyone ever uses. Though even GDK_XOR is
> pretty useless.
>
> Havoc

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| Colin Thomas.        
| Email   : colin designresources co uk     
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