For motion blur, why does Krakatoa render multipass? Could you not render lines instead? Particle moves from point A to point B over the frame, so draw a line connecting those points?
Even if you needed multiple segments for the spline, you could just connect them together to form say an 8 segment spline?
I suppose matte objects and shadows would be an issue, but you currently have to re-evaluate them anyway per segment, right? Couldn’t you just apply the color per-segment to the lines?
We did attempt to use lines for the motion blur at one point, but ensuring the correct brightness level based on the blur was tricky, and didn’t produce a consistent look with parts that weren’t motion blurred. The interaction with the matte objects and density would also make the result technically incorrect without doing some much slower calculations.
The lighting is only calculated once during the render. If you have 20 lights and 20 motion blur passes, it will basically do 40 passes through the particles, one for each light and one for each motion blur pass.
We do intend to extend Krakatoa to use more shapes in a future version.