miker,
Yes, i did grasp your explanation
Unfortunately that was not what i meant to say/ask. .. sorry for that, let me try rephrase
Back in the day when you required a mask when using the blitter, and taking into account that we were quite lazy back in those days, we created our mask-bitmaps by checking individual pixels in the image and identifying which pixel belonged to the graphical image, and which pixel is/was part of the 'background'.
This can be a fully automated process, as long as you know which pixel belongs to what part of the image.
Now, doing that with 24-bit color-pixels with alpha channel can be a bit daunting but the process is similar. You only have to know which pixel(colours) belong to the background and which ones to the image (or you can do/combine that using pixel coordinates that belong to the overlay area in case it is consistent). That way you do not have to manually create the mask for the overlay image (your whitespace). Similar can be done for the 'glow' area. If the position of the glow is consistent then you can automate that process as well.
I ask, because as i can imagine, the creation of the mask must have taken away much of your time and is most probably a process that you do not wish to repeat (very) often.
That is why the original designer of those icons has a much easier job. (s)he has one drawer background and design the individual overlay-images. Then launches a macro or script to put all designed overlay images on top of the drawer icon. If you use modern tools like gimp or even TAD on native Amiga then creating such a bunch of icons is a peace of cake (not including designing the overlay images though, that is an art form in itself
).
edit: I recalled another trick from back in the day: When using planar image data, sometimes an overlay was just using 2 or 3 upper/lower planes and the background used the other planes. That way you only had to watch carefully with providing the correct colour information for the combined planar pixels. You could do some nice tricks at runtime with that as well ... especially when playing with the copper.