On Sunday 23 October 2005 12:16 am, Mirko Raner wrote:
> Hi all,
>
> I have a problem for which i haven't found a solution in GIMP, but
> I'm sure there is some easy way to achieve this.
> I have a 24-bit image file and a separate 256-gray-level image of
> the same size. The gray-level image is supposed to act as the alpha
> channel for the 24-bit image. I managed to transfer the gray-level
> information into the alpha channel of the grey image, and then
> switched the image mode from 256 gray levels to full RGB. Then I
> just selected the other 24-bit image and copied and pasted it into
> the modified gray level image. My idea was that, as the original
> 24-bit image does not have an alpha channel, the copy/paste would
> transfer the R, G, and B channels but leave the alpha channel as it
> was. Unfortunately, pasting the other 24-bit image also replaced
> the contents of the alpha channel with a 100% opaque setting.
> So, how can I apply a separate 8-bit gray-level image as the alpha
> channel for a 24-bit RGB image of the same size?
>
> Thanks in advance,
>
> Mirko
>

Indeed, there is an easy way!

You have to learn about layers (if you already had not) and masks - 
rightclick on alayer preview on the layer dialog - you will options 
for the mask - a layers masks acts roughly as a "second" alpha 
channel to it, but it is a separate drawable wich can be changed 
indepently of the layer itself.

FOr what you want: just add a mask for your RGB  image, go to your 
grey image, edit->copy, go back to the RGB image, select the mask (it 
has to get a white border 1px border in the layers dialog), and 
paste.

        JS
        -><-
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