This was another of those 'How do we deal with a complex system' problem.
If you were to lay down large blobs of paint tube and glitter on a layer, add a layer and airbrush overtop... what would you expect to happen to the bump underneath?
Your choices are: a) The bump underneath is ignored totally so you have what appears to be flat airbrush obscuring the bump underneath.
b) The bump from beneath is carried on through to the layer above, and the airbrush apears to be applied to the bumpy surface.
Now... the tricky bit... Merge the airbrush down, and think about the conseqences of choices A) and B) above. We'll deal with B first...
B) The result is the same as when the layers were separate - the airbrush still follows the contours of the bump.
Now A)... If we use the bump from the layer beneath and add it to the airbrush, the result can be very different to prior to merging the layers. If we try to use the bump from the airbrush layer, you have a transition area where the bump goes from tall mounds of glitter and paint to the flat airbrush. So there appears to be a 'crater' - the bump map transition area appears to have sloping sides from the high areas down to the flat airbrushed area. In a single layer, we cant replicate the look of having the airbrush 'floating' overtop the bumpy surface.
It's another of those cases where we had to make a decision about what users would probably most want.
Note that you still have the option of using the eraser on the layer beneath to flatten out the bump if you need to.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"