videoplane masking challenge

Hello community, I'm using the tex_scale_x, tex_scale_y, tex_offset_x and tex_offset_y messages to jit.gl.videoplane to view portions of an incoming image as a texture. I need to be able to apply a mask to a number of these videoplanes to control the shapes of the final images. If I add a mask to the alpha channel of the incoming image it gets scaled and offset with the rest of the image. Is there a way to mask the videoplane image independently of these scalings and offsets? For instance, is it possible to grab the altered texture from the videoplane, add a mask and apply this to a second videoplane.

Any suggestions are most welcome.

Thanks,

Kenneth.