Bug in Alpha compositing

[Georg Fleischmann]

Hi,

I realized a new bug with alpha compositing. I found out that the problem  
appeared with the changes in XGGState.m -setColor:state: and -setAlphaColor: in 
 
May. If I use an earlier version of XGGState.m it works.
As I don't have an idea of the changes and not much time at the moment, I  
thought I just post what I know.


Here is what I'm doing:
I use a window which I fill transparent, and then I draw opaque graphics  
(Line, Arc...) to it, like this.

    [betaView lockFocus];
        [[NSColor colorWithDeviceWhite:1.0 alpha:0.0] set];
        NSRectFill(rect_start);
        [obj draw:self];
    [betaView unlockFocus];


I then composite this window using NSCompositeSourceOver. All I get is a white  
opaque rectangle.

PScomposite(NSMinX(rect_start), NSMinY(rect_start),
            NSWidth(rect_start), NSHeight(rect_start),
            [betaView gState], NSMinX(rect_now), NSMinY(rect_now),
            NSCompositeSourceOver);


If I use a composite with NSCompositeCopy instead, the graphic is copied (but  
on a white opaque background):

NSCopyBits([betaView gState], rect_start, rect_now.origin);


Georg