Super-imposing images by making background transparent (case 429)

[allonv]

I need to super-impose two polygons, one polygon on top of another. To do this, I need to make the background transparent. This can be done for a pgn file, but apparently DirectRT cannot take pgn files.

A possibility would be to do the super-imposition outside of DirectRT, but the polygons are made of 2 sets of 30, which would require pre-randomization of 900 trials.

I would be grateful for any advice.