On Tue, 4 Mar 2003 16:01:41 -0600, Judy Wilson wrote:
> * Malcolm Turnbull <[EMAIL PROTECTED]> [030304 09:27]:
> 
> > But I was wondering if it was possible for GIMP to save the background 
> > colour as transparent ? (It seems to allow only solid colours)
> 
> It will save as tranparent if you erase the part you want to be
> transparent. Open the layer dialog box and right click on the
> backgroud layer. Add an alpha channel. It won't work if you don't have
> an alpha channel. Then make your selection and be sure the part you
> want transparent is the selection. Use the erase tool of the
> selection. It should reveal the checkerboard underneath. That's
> transparency. Then save as a PNG. I do this all the time.
> 

There's something else to keep in mind with respect to the PNG format.  PNG 
supports transparency (both binary or full alpha channel), but can also support 
a default background color (through the bKGD chunk) for use when the application 
viewing the image doesn't have a natural background over which to lay the image.  
I believe this is what's being saved when you toggle "save background color".  

If your image has transparency and it's being overlayed on a background (like on 
a web page), and the browser supports transparent PNGs, then this background 
color is ignored (as the web page's background is used).  But if you're just 
viewing this image standalone in some application, or you're using an older 
browser on a web page which doesn't fully support PNGs transparency (e.g. 
Netscape 4.7x), then this background color you specify can be used instead of 
having the application pick it's own background for the image (e.g. black).

You can also tweak the background color of a PNG image with the netpbm utilities 
after you've created it in the gimp.


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