Rendering particles: tips

[philmc]

Saw this over at CGTalk and figured it was too valuable to let disappear. The original link is http://forums.cgsociety.org/showthread. ... 6&t=589761.

ok.. maybe a take a minute to share my findings with particles over the past few years. maybe some of the info can help.

first off: software rendering particles in maya is a pain in some cases. i logged several bugs about the caching and the rendering but most is still in the current version.

so: there is one very easy way to check in a command line rendering, if the cache files are properly used: go to any of the objects in your scene (no matter which one) and create an expression for it (e.g. in the x translation slot-> add expression):

 print ("\n Current Time: ");
  print (`currentTime -q`);

if you now render from the command line the correct display should be:
after loading the scene, the current frame gets displayed. that is the frame you had, when saving the file.
the next output now should be the frame you want to render. right after that the rendering should start.
as mentioned: that is the way it should be.

if you have no cache files, maya does a runup. so it will (e.g. when rendering frame 10) display: 1,2,3,4,5,6,7,8,9,10 and then start rendering.
still ok but not wanted.

now comes the terrible part you all seem to talk about. if maya has problems with the cache files it will start with the rendering output like this:
1,2,3,4,5,6,7,8,9,10
2,3,4,5,6,7,8,9,10
3,4,5,6,7,8,9,10
sometimes it now starts to render, sometimes it continues this loop until frame 10 is really reached. this is the situation where it all turns terrible slow. if you have some serious particle action in your scene this can take hours.

changing the workspace.mel file like i shown above usually prevents this (at least for me) from happening. i am not really sure why, because the original workspace.mel commands should be correct, but this is happening since maya 3.0 to me.

BUT:
there are also some other things to be sure of:
- be 101% sure that there are NO uncached particle systems in your scene file. even a static particle system, that is not connected to time and is set to be non dynamic will mess up the cache situation!!
- hiding uncached particle systems does not help. so be sure there are none somewhere down in your hirarchy!
- sometimes cached particles seem to be unsure what to do. runtime/creation expressions sometimes seem to be being calculated even if there is a cache present. so try deleting them all after the caching.
- breaking cache creation, deleting and redoing caches messes maya up sometimes. so always have a clean scene to start with. that scene should be ready to have a cache created. now save it under a DIFFERENT name and now create the cache. if something goes wrong, delete all cache files and the scene the cache came from, load the clean scene, save under different name and THEN create the new cache.
- if all still fails - dont use a cache file but do the following: use the "seed" command in the creation expressions of all of your particle systems to make sure the random values are the same all the time. if you command line render this on several machines, render with a chunk size of 1. higher amounts of rendering consecutive frames can cause problems again.

and one final issue i nearly forgot: not really rendering but hell anoying (maya 8.0+)
- if you have a file in node in your scene (especially one with "use image sequence") and try to render your particles with hardware render buffer, you will for sure get huge amounts of memory usage. maya caches all the file nodes to RAM in that case.. bad one.. really bad one.