don't bother - i solved this for the time being by grabbing the signal before loading it on the gpu and then recreating the whole slabchain in cpu-land. not pretty but since i can work with a tiny, tiny matrix it does not affect performance... k

some last remark on this if someone else should stumble across this problem: i did not try this in any way but i am just browsing through the filters of jit.gl.imageunit ( https://developer.apple.com/library/mac/#documentation/GraphicsImaging/Reference/CoreImageFilterReference/Reference/reference.html ) and there are a bunch of filters which return culminated values as single pixels which could probably be helpful with this sort of stuff... to get a value (not a pixel) you would still need to readback though. k