Thanks to all for the quick responses on interpolation. Here's a quick example showing what I was concerned about: arrows.bld ------------------ .color red .arrow 0 0 0 1 0 0 .color green .arrow 0 0 0 0 1 0 .color blue .arrow 0 0 0 0 0 1 ------------------ matrix2 ----------------- Model 0.0 0.505641 0.433743 -0.745785 -2.58211 -0.807506 0.542275 -0.232105 2.90884 0.303746 0.719587 0.624446 -0.194334 ----------------- matrix3 ---------------- Model 0.0 0.60158 -0.458982 0.653787 0.653219 0.190507 -0.712397 -0.675423 5.48737 0.775763 0.530872 -0.341124 0.0103466 ---------------- The first matrix is just unity. The 'reset'-based interpolation works great for going between 1 and 3 or 1 and 2 in both directions, as well as 3 to 2, but fails from 2 to 3. Since all this is new to me, I can't say for certain that this is due to gimbal lock, but that was my first guess. I haven't had the opportunity yet to try out Tom's axis-angle approach, but that should rule out any Euler problems. The other problem for me is that all my current methods are based upon a stepwise approach via commands (ungainly, but very functional when combined with shell scripts, imagemagick etc), and this is incompatible with the 'reset' interpolation. Perhaps it was misleading to mention the use of 'matrixset'. I appreciate that it is more trouble to deal with external files, but if it is not possible to easily implement the interpolation I'm looking for dynamically within Chimera's Python (due to required math libraries, etc) then I would have to hard-code the interpolation matrices into the .py script file anway, defeating the purpose. Thanks for the help: I'd love to hear about any future development in this area.