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AlphaImageTutorial

Display a gaussian masked image, locked to the cursor position,
using the Screen(‘DrawTexture’) command.

This illustrates an application of OpenGL Alpha blending by displaying
an image masked with a gaussian transparency mask.

In each frame, first the image is drawn. Then a texture acting as a
transparency mask is drawn “over” the image, leaving only selected
parts of the image.

Please note that we only use two textures: One holding the image,
the other defining the mask.

see also: PsychDemos, MovieDemo, DriftDemo