I seem to be missing a basic concept. If I open an image with alpha channel, I can't find any way to edit the alpha channel. I end up using other software to save the alpha as a grayscale and editing that. That requires converting my 24 bit image to grayscale too so I can load the alpha as a layer for editing. What I want is to keep the 24 bit rgb image and edit the alpha as an 8 bit layer. FWIW, I generally use 32 bit targa format for editing image with alpha., because I don't have any filter for directX compressed formats (DXT1, DXT3).

scott s. .

Quoting "scott s." :

I seem to be missing a basic concept. If I open an image with alpha channel, I can't find any way to edit the alpha channel. I end up using other software to save the alpha as a grayscale and editing that. That requires converting my 24 bit image to grayscale too so I can load the alpha as a layer for editing. What I want is to keep the 24 bit rgb image and edit the alpha as an 8 bit layer. FWIW, I generally use 32 bit targa format for editing image with alpha., because I don't have any filter for directX compressed formats (DXT1, DXT3).

Perform a "Layers->Mask->Add Layer Mask" and select "Transfer layer's alpha channel" as the initialize option. You can then edit the "alpha channel" by editing the layer mask. When you wish to transfer your edited layer mask back to the alpha channel, perform a "Layers->Mask->Apply Layer Mask".

saulgoode@brickfilms.com wrote:

Perform a "Layers->Mask->Add Layer Mask" and select "Transfer layer's alpha channel" as the initialize option. You can then edit the "alpha channel" by editing the layer mask. When you wish to transfer your edited layer mask back to the alpha channel, perform a "Layers->Mask->Apply Layer Mask".

Quoting "scott s." :

The advantage to editing the channel mask is that you can set a color for the mask, which makes it easier to see just what you are doing.
When I edit the layer mask, other than modulating the transparency of the connected layer,
I can't precisely see the effect.

If you wish to see just the mask itself, you can use the "Layer->Mask->Show mask" command. This will show the "effect" of either painting or filtering the mask.

If you wish to have a partially transparent (tinted) view of the mask (with your image behind it), you can perform a "Layer->Mask->Mask to selection" and then use Quickmask mode to edit the mask. After you are done editing the Quickmask, you can delete the layer's mask and perform an Add Layer Mask and choose "From Selection" for the Initialize option. (There may be better ways to accomplish this, and you may still prefer the channel method you are currently using.)

there is one other thing: In the add layer mask there is an option

Layer's alpha channel but also
Transfer layer's alpha channel

What is the difference?

The first option copies the alpha channel to the layer mask but leaves the alpha channel intact. The second option not only copies the alpha channel to a mask, but setsthe alpha channel to WHITE as well (i.e., sets it to fully opaque).

If you try the two different options on the following image, the difference should become apparent.

http://flashingtwelve.brickfilms.com/GIMP/Temp/GIMP-challenge.png

I suspect you will find "transferring" the alpha channel to a mask to be more useful for your purposes.