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Hmmm... I see.  I don't know GameMaker well enough for doing tricks but if I were to do this in Photoshop, I would just change my noise layer to Multiply or Luminous and use the clipping to force it to the shape of the main sprite.  Couldn't you use something like:  sprite_set_alpha_from_sprite() to clip the noise sprite to the same shape of the main sprite and use gpu_set_blendmode_ext() to achieve the blending mode by drawing the main sprite with it's alpha setting and then the noise sprite with an alpha setting of 0.1 to combine them?  Or is that too slow?