Rendering Slowdown & Comp Node Questions

Hello all, I have a few questions and was hoping someone can answer any or all of them.

  1. Rendering slow down issue:
    I’m doing some particle experiments, and seem to be having an odd performance issue that I have not run into before. Basically on certain frames the rendering will take many many times long to finish for no apparent reason. I was able to get around this by turning off either Enable Shadow Calculations, or Enable Ray tracing in the Render panel. However, I’m still wondering if anyone can tell me what causes this bottleneck, and if there are other ways around it.

The file I’m seeing this in is below.
FYI I’m on an intel imac with 2gb ram.

  1. Can composite node properties be animated? If so how, and if not is this planned for the future ala 2.50?

  2. Is it possible in the composite nodes to get the previous frame rendered. Basic usage would be to mix the previous frame with the current frame, for things like ghosting / echo effects.

Don’t have time to check #1, but here’s for 2 and 3.

#2: You can animate values like “Factor” for the Mix nodes, etc., by adding a Time node and mapping the output to the range and value you want to animate.

#3: In theory, yes – though I’ve never done it. You would need to render in two passes, though, using the first pass of renders as an Image Input node for your second pass. Of course, the second time through, you don’t need to re-render – just set up a node network (or Sequence) that uses two instances of the first pass of renders, offset by one (or however many) frames. If you try to do this in one pass, you’ll end up recursively altering your frames, which would be bad.