Changing Render Settings From Pre Render Frame Mel

[failure]

Is there a way to change the render settings from pre render frame script? I'd like to be able to alter the filename prefix for each frame that is rendered from within a single batch session.

[failure]

Okay, the pre render script is definitely acting capriciously

In order to investigate, I've made a small test case.

Simple scene with only one object: nurbsSpehre1

all scripts are in external file: pre.py

CODE
import maya.cmds

def preFrame():
    maya.cmds.move( 3, 0, 0, "nurbsSphere1", r = True)
    x = maya.cmds.getAttr( "nurbsSphere1.tx" )
    file = open( "log.txt","a")
    file.write ( "x = " + x + "\r\n" )
    file.close()

pre render scripts are assigned in render globals:
Pre render MEL: python( "import pre" )
Pre render frame MEL: python( "pre.preFrame()" )

I render 3 frames from command line and all 3 frames are identical, i.e. the sphere is not moved. But values written from preFrame() to the log file indicate that value of translateX actually changed on each frame:
CODE
x = 0.0
x = 3.0
x = 6.0

I'm probably missing something about the way the pre render scripts operate scratches head
Any clues?

[failure]

Hm, the problem exists only when rendering with mr. Doing it with maya software, the sphere appears to be moving in output images, hunky dory.

The question then is: how to get the pre render frame script to function properly with mental ray?

[failure]

Solved it! The problem was caused by two mr optimization flags, namely Oprimize Animation Detection and Optimize Non-animated Display Visibility (in mr render settings under translation/performance). They should be turned off in order for mr not to "optimize out" changes made on objects by pre render frame script. I suspect this is only an issue if objects are not conventionally animated, as it was in my case.

I still didn't mange to change the filename prefix. That's probably not doable from within pre frame script. The simplest way to do it would likely be to first render out the frames using standard naming convention and then rename them via post render script. I'll opt for that.

Gosh, I love talking to myself on forums