I have a rendered image (blender) for an iOS program and would like to copy/convert it into a version where color information is replaced with alpha values. For example to be used in TabBar. I tried ImageMagick base on past positive experience; but for some reasons the performance under Mac OS X is very poor even for simple resizing. Compiling from source didn't help so right now I gave up on this toolbox. In Pixelmator I lack the knowledge on doing that kind of conversion. Appreciate any hint how to achieve this kind of conversion.