Someone posted something which reminded me of an old problem. In gimp when you save alpha, to works in blender for color, but if you try to use noise or anything else, it sees the original image and ignores the alpha. What is the quick fix for this?
use a different map for your noise, and actually clear/ solid fill areas which have no bump required. Blender is not unique in ignoring alpha information in bump maps, as any solid area will render un-bumped.
This is not a good solution for me
I think ice-man one time told of a quick fix for this but I forget what he said. It was some operation in gimp I think.
It been long time and still no one knows solution?
Here is Iceman’s GIMP tutorial. I’m not sure if this is what you are after.
That is a nice tutorial though not the thing I was looking for. The thing is gimp saves the alpha in an alpha channel, but does not change the rgb channels. This is why alpha works fine when viewing color in blender, (if you hit view alpha), but nor will not work with alphas, you will see all the rgb channels.
There is a fix to get rgb channels to follow alpha in gimp, someone answeres this a long time ago at NaN.