I often use masks on flat objects (rectangle) to create complex "appearing" effects. example- Photo of leaves in Diffuse channel, black and white trans map made from same image in Mask channel. For some reason, I'm having trouble with rendered images having a light fringe around the mask boundaries, especially of the background is dark. Had it in 7.5 and also now in 8.2.2. A simple color in the Diffuse channel (green in this case) with the same transparency map in the Mask channel works great. No fringes, but of course, no details on the leaves. Any help would be greatly appreciated. iMac new, quad 3.4, retina screen,
A mask is not really the same as an alpha channel. I think this is usually handled by minimizing the transition from white to black in the Mask. The best way to do this is to double the size of the mask file and convert it ti bitmap, so there is no grey in the image.
Thanks so much for the information. I tried what you mentioned but the result was the same. Am I trying to achieve this effect through entirely the wrong method? I found no information about alpha channels in the guide (other than rendering dialog and backgrounds.) I tried making a simple square (B and W) map in PS so edges would be clean. converted to bit map also. Still came out the same. Thanks again for your help. T