Hello,
I have a problem displaying an image with some transparent pixels over various
colored backgrounds. We are masking out parts of an image by setting the alpha
channel to 0 for some pixels, and want to allow the user to display the image
over different background colors to see where the transparent pixels are. Each
pixel has the alpha channel value set to either 0x0 or 0xFF.
Our code works perfectly on Cocoa. In GNUstep, it displays correctly over a
white background. Over a black background the image appears entirely opaque,
and over other colors it displays a modified version of the image color for the
transparent pixels. Here are screenshots of the same image displayed over
white, magenta, and black:
The image is drawn using this code in my drawRect: method:
[image setCacheMode:NSImageCacheNever];
[image drawAtPoint:imageDrawOrigin
fromRect:imageRect
operation:NSCompositeSourceOver
fraction:1.0];
I traced the code in gdb to try to see where the NSCompositeSourceOver operator
was used, but there are a lot of complexities in the code that make it hard to
follow exactly what's going on. I hope maybe someone more familiar with the
code can see what might be going wrong. Here is a backtrace from the call shown
above down to the point where the cairo_paint() function is called.
(gdb) bt 6
#0 -[CairoGState drawGState:fromRect:toPoint:op:fraction:] (self=0x7b866e0,
_cmd=0x645ceb60 <.objc_selector_list+480>,
source=0xcf5eea8, aRect=..., aPoint=..., op=2, delta=1) at
CairoGState.m:1463
#1 0x6458761a in -[GSContext(Ops) GSdraw:toPoint:fromRect:operation:fraction:]
(self=0xe67c658,
_cmd=0x654b92d0 <.objc_selector_list+512>, gstateNum=61, aPoint=...,
srcRect=..., op=2, delta=1) at GSContext.m:858
#2 0x651a730d in -[NSImageRep nativeDrawInRect:fromRect:operation:fraction:]
(self=0x6496c70,
_cmd=0x654b9188 <.objc_selector_list+184>, dstRect=..., srcRect=..., op=2,
delta=1) at NSImageRep.m:632
#3 0x651a964c in -[NSImageRep
drawInRect:fromRect:operation:fraction:respectFlipped:hints:] (self=0x6496c70,
_cmd=0x654b7bd8 <.objc_selector_list+1288>, dstRect=..., srcRect=..., op=2,
delta=1, respectFlipped=0 '\000', hints=0x0)
at NSImageRep.m:867
#4 0x65198143 in -[NSImage
drawInRect:fromRect:operation:fraction:respectFlipped:hints:] (self=0x6495bd8,
_cmd=0x654b7bd0 <.objc_selector_list+1280>, dstRect=..., srcRect=..., op=2,
delta=1, respectFlipped=0 '\000', hints=0x0)
at NSImage.m:970
#5 0x651979ee in -[NSImage drawAtPoint:fromRect:operation:fraction:]
(self=0x6495bd8, _cmd=0xec4b38 <.objc_selector_list+56>,
point=..., srcRect=..., op=2, delta=1) at NSImage.m:878
(More stack frames follow...)
(gdb)
This was on Windows, but we're seeing the same problem on Linux.
Thanks for any pointers (or better yet, code fixes!).
Cheers,
Doug
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