[mapserver-dev] mapcache: background color

Daniele Debernardi daniele at geosar.ch
Tue Jan 8 06:30:16 PST 2013

working with mapcache I had a problem with the request of tiles that are 
half-in and half-out the extent.
The outside part of the image has a white background color instead of 
transparent as I was expecting (using png).

After looking at the code I found that function 
"mapcache_image_copy_resampled_bilinear" in file lib/image.c
using PIXMAN library, do a "pixman_image_composite (PIXMAN_OP_SRC, ..." 
call and this sets the
white background color.
Changing it to PIXMAN_OP_ADD solves my problem and I finally get the 
transparent background.

Is there a specific reason why the PIXMAN_OP_SRC is used?

I tried with others operations too, like PIXMAN_OP_OVER and others, but 
I noticed performance problems.

Which is the best operation to do in this case?

Thanks in advance,

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