[GM-help] png, transparency and composite

[florian i. <flo...@vi...>]

Hello everybody,

using Magick++ library, i'm trying to merge two png images, one of them 
has a transparency channel. I want to apply a global transparency value 
to this image before the composite operation. Here is an extract of my 
code :
Magick::Image blankImage( "/home/firagne/checker.png" );
if(prepareLogo("/tmp/logoPreview.png", PREVIEW_XSIZE, PREVIEW_YSIZE)){
	Magick::Image logoPreview( "/tmp/logoPreview.png" );
	logoPreview.opacity( transparency * TransparentOpacity / 100 );
		blankImage.composite( logoPreview, xpad, ypad,
			              Magick::OverCompositeOp );
}

It works perfectly if the logoPreview source has no transparency 
channel, and don't work if it has one, meaning that on the composite 
image, transparency is applied to the transparent channel and not to the 
global image.

The same result is observed if i use gif, and does not occur using jpeg, 
since jpeg doesn't handle transparency, as far as i know.

I'm probably doing something wrong but don't see what. Any help will be 
more than welcome!

thanks

Florian