pictctl knob-making tool in jitter seems to uncover some flaws in jit.rota?

I made a tool to create a knob image for pictctl from a single knob image. However, it really doesn't work right, and I'm mystified. The knob comes out wobbly and clipped. I've looked over my patch and math too much at this point - it shouldn't be this difficult! I suspect the culprit is jit.rota's interpolation, however, is there something else I'm missing?
Another possible source of weirdness is that the image exported from jit.matrix appears as "72.01 dpi" rather than 72 dpi, which is odd.
The attached zip has the patch and the knob image I'm using as a "seed" if anyone wants to confirm...

Peter.