Walsh Family wrote:
>
> What I'd really like to do in the end is to be able to take an RGB
image,
> desaturate it and then manipulate the gray scale image in GIMP, save
that as
> the alpha layer back into the original RGB image for ex****t to Unreal.
But
> it doesn't seem to be that simple!
It's actually quite simple. What you want to do is to copy the grayscale
image that you've created to a layer mask associated with the target
image, and convert the layer mask into an alpha channel. Here are the
detailed steps:
1. At the window for the grayscale image, do a "Select All" and then
copy the image to the clipboard.
2. Add a layer mask to the target image. (Manipulating layers can be
done using the menus of the image window, but the "Layers" dialog is
more convenient. From the "Layers" dialog, select the layer to which you
want to add the layer mask. Right-click and select "Add Layer Mask..."
Initialize the layer mask to all white (full opacity).)
3. Select the layer mask for editing. From the "Layers" dialog, click on
the (newly-created) layer mask to select it for editing. Your editing
actions will now apply to the layer mask instead of the image.
4. Paste the image in the clipboard onto the (newly-created) layer mask.
5. Convert the layer mask into an alpha channel. From the "Layers"
dialog, select the desired layer of the target image, right-click and
select "Apply Layer Mask". The layer mask will be converted into an
alpha channel. (After the operation, the layer will no longer have a
layer mask).
> What happens with multiple layers - can they each have a different alpha
> channel/ layer?
Yes.
> Do the effect of each layers' alpha
> channel interact up the layer stack or are they only active on that
> particular layer?
The alpha channel of a layer controls the transparency of only those
pixels in that layer. However, an image can have multiple layers, each
of which can have (semi-)transparent regions. In that sense, alpha
channels in different layers can interact, much like when you look
through two slides stacked together.
> Can you make a "master" alpha layer (at the top/ bottom
> of the layer stack???) that will have ultimate control over the final
alpha
> of the image but still leave the layer structure intact or do you have
to
> compress the image first?
I don't think that can be done in GIMP.
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