Error message "could not allocate matrix" using Viddl engine

hi experts,
another one of those irritating posts where I can't yet send a condensed proof-of-concept example. I'm running a patch which does the following things:
-- Records video to a file whenever movement is detected
-- Reads one of the recorded files every minute
-- Every hour, reads up to 12 files, mixes them into overlapping strips using an alpha gradient
I'm doing everything in textures. Using jit.qt.movie to output straight into textures. Using jit.qt.slab with alphaglue.jxs to do the alpha overlay. Using iit.qt.videoplane to blend the layers. I've got a couple of other things going on like a slab slide.jxs.

I will try to share a test patch soon, but prior to that: big glaring question - what does "could not allocate matrix" mean? What objects throw this error? Can anyone confirm that this is a GPU memory issue -- too many textures perhaps?

More specifically. I'm currently working at 1280 x 720. 25fps. Video files are h264 high. By my sums, I'm pushing the following texture data to the GPU:

Up to 13 x RGB images (12 slices plus background)
Up to 12 x alpha masks, one for each of the images
(The result of combining the RGB with the alpha mask -- I don't know, does this allocated a new 4-layer matrix?)
Occasional recording video to disk - not sure what GPU usage this has

Thoughts welcome.

Ollie