height on transparent images/parts

A while ago I made a surface filter that needed to be transparent (see image: blue would be transparent). But when rendered the transparent part would simply show a hole (no height). My solution was to render the transparent part as plain blue (#0000FF) and use a second filter to turn the blue into transparency. A bit of a pain, since the second filter would not filter out the blue as nicely as I wanted to, but acceptable.

Is there any reason the surface does not work on transparent parts? Is it deliberate or just FF's insides work? (as in: it does work, but does not change the alpha value)

Are you just trying to get the highlight and shadow shadings from the height map and compose it with any image in a different program?

(If you let FF render it over your original image, it would be a simpler matter, but I assume you're not looking at doing that.)

As far as I can tell, this would be impossible with a single surface filter, you'd have to create a simple filter and fake the highlights and shadows some other way like Offset or Refraction.

Or do it with two different filters, one surface filter that would create a grayscale version of you image and another simple filter that would extract the transparency information for shadows and highlights. Pretty similar to what you were describing. The trick here is that the shadows would have to be full black with the alpha mask creating the shading strenght variations and the highlights full white with the alpha mask creating the highlight intensity variations.