how to make a composite mask?

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That makes it simple, thanks for pointing it out. I couldn't find any description of this in the documentation.

I can't guarantee that the values are all the same, just that they are zero or nonzero. One mask might have 0 and 255 values but another may have 0 and 8bit grayscale values and another may have 0 and 1 values.

Am I correct in saying that if you were to bitwise_and a mask of all 128s with a mask of all 1s you would get a mask of all 0s, but pixelwise should return a mask of all 255 which is what I want since both masks contain all nonzero (i.e. true) pixels.

Sounds like there is no convenient command for this desired action.

You just need to make them 255, then bitwise_and.

Just call maskX > 0 and carry on.

what is maskX?

Anyhow, it would seem trivial for OpenCV to add two mask-level functions that treat mask elements pixelwise. Nice and clean.

i.e. for each array pixel, mask_or() would be zero, zero = 0 zero, nonzero = 255 nonzero, nonzero = 255

mask_and() would be zero, zero = 0 zero, nonzero = 0 nonzero, nonzero = 255

All it would do is maskX > 0 and then the normal bit-wise function. maskX is whichever mask you're using now. So mask = (mask1 > 0) & (mask2 > 0) & ... This is the same as using bitwise_and