Any image can be selected as the current Alpha Channel. This acts as a mask affecting all drawing operations to all other images. Dark areas on the Alpha Channel make the paint transparent, light areas of the Alpha Channel make the paint opaque.
Here we have an example image loaded and selected as our current buffer, we create a new black image of the same size as our first image and, by selecting white as our current colour, we have drawn a white circle in the middle of the new black page and fixed the image.
Now, if we select the white circle image as the Alpha Channel by dragging the icon into the 'alpha' box in the images window, and start painting on the first image, we see that we can only paint inside the circle.
If we now invert the Alpha Channel (use the menu item 'invert alpha' from the Alpha/Paint layer menu) we see that the paint we applied inside the circle has gone and the paint outside the circle (that we couldn't previously see) reappears.
If we invert the Alpha Channel again, we again see only the paint inside the circle. As you can see, this makes it easy for us to mask an area to prevent our painting operations from affecting the image outside it.
The Alpha Channel is more than just a simple mask, however, the edges do not have to be sharp. If we select the alpha channel image, open the Effects window and apply a Blur effect (settings Gaussian, power 2) to blur the edges of our circle, once it is finished the paint on our previous picture is automatically adjusted as the new, smooth, Alpha Channel is used.
Now, where the paint we drew goes over the edge of our circular mask, it smoothly fades out. As we haven't fixed any of the paint on the first image, we can clear the image and draw over the image again. We still get smooth edges where our lines go over the circular mask.
Alpha-Channel masks need not be hand-drawn, some of the special effect loaders (Plasma, for example) make good Alpha Channels.
Here we have created a plasma image to use as our Alpha Channel. Now, where we paint the texture of the plasma affects our paint, which now has a variable transparency - it's translucent in some areas, opaque in others - based on our Alpha Channel.
Sometimes you will want to create an Alpha Channel to cover a specific area of the image (to mask out a face, for example, so it doesn't get affected by an image processing operation). Creating Alpha Channels for this is very easy with Photogenics.
First, paint over the area you wish to mask on your image. You don't have to be too accurate doing this, but do NOT fix the paint.
Now, select the 'Move paintlayer to Alpha' option from the 'Alpha /Paint-Layer' menu. This will create an Alpha Channel.
You will have to double click on the Alpha icon to open the window for this new buffer if you want to display it as we have done here. You do not have to have the Alpha Channel displayed to work with it.
Now, painting on the original image is limited within the area of the mask.
If we invert the Alpha Channel ('Invert Alpha' menu option) then the reverse happens, and our paint is outside the area.
Now, if we fill the screen with paint, the spaceman is left on a white background. We can now paint on the Alpha Channel to adjust it to fit the spaceman better, FIX changes to the alpha channel to make the new alpha channel active on the spaceman image.
We could easily now use RubThru or Compose to place the spaceman on a completely different background! Here, as the saying goes, is one we prepared earlier...