Motionblur in cycles calculates drivers' scripted expressions' multiple times per frame #37734

New Issue

levon hudson · 2013-12-08T14:24:42+01:00

levon hudson commented

2013-12-08 14:24:42 +01:00

when using scripted expression drivers, and rendering in cycles with motion blur, the actual expression is evaluated 4 times per frame.

in the attached blend file:

run the script, im using the default driver_function template 'slow_value', except ive modified the script the print out the value to the console each time it is run.
render an animation with motion blur on. in the console it prints out 4 values for each frame.
turn motionblur off and render animation. only 1 value per frame is printed to the console.

when using scripted expression drivers, and rendering in cycles with motion blur, the actual expression is evaluated 4 times per frame. in the attached blend file: 1) run the script, im using the default driver_function template 'slow_value', except ive modified the script the print out the value to the console each time it is run. 2) render an animation with motion blur on. in the console it prints out 4 values for each frame. 3) turn motionblur off and render animation. only 1 value per frame is printed to the console.

levon hudson commented

2013-12-08 14:24:42 +01:00

Changed status to: 'Open'

levon hudson commented

2013-12-08 14:24:42 +01:00

Added subscriber: @LevonHudson

Brecht Van Lommel commented

2013-12-08 14:40:00 +01:00

Added subscriber: @brecht

Brecht Van Lommel commented

2013-12-08 14:40:00 +01:00

mblur_drivers.blend

[mblur_drivers.blend](https://archive.blender.org/developer/F37438/mblur_drivers.blend)

Brecht Van Lommel commented

2013-12-08 14:48:03 +01:00

Changed status from 'Open' to: 'Archived'

Brecht Van Lommel closed this issue

2013-12-08 14:48:03 +01:00

Brecht Van Lommel self-assigned this 2013-12-08 14:48:03 +01:00

Brecht Van Lommel commented

2013-12-08 14:48:03 +01:00

This is pretty much working as intended I think, for each frame it will evaluate the animation at the current frame, then two other frames to find the motion, and then restore the current frame again.

In principle this could be optimized to reduce computation time, but I'm not sure why it would be a bug? How many times are you expecting it to be evaluated? Going from 4 to 3 is possible in some cases but it's quite complex due to motion blur settings themselves possibly being animated, and going to 1 for animation renders would give extra memory usage, which I don't think is a good trade-off.

This is pretty much working as intended I think, for each frame it will evaluate the animation at the current frame, then two other frames to find the motion, and then restore the current frame again. In principle this could be optimized to reduce computation time, but I'm not sure why it would be a bug? How many times are you expecting it to be evaluated? Going from 4 to 3 is possible in some cases but it's quite complex due to motion blur settings themselves possibly being animated, and going to 1 for animation renders would give extra memory usage, which I don't think is a good trade-off.

levon hudson commented

2013-12-08 14:54:34 +01:00

ill try and get a better example. in some code im working on at the moment, it renders fine with out motion blur... but with motion blur on, and being evaluated 4 times per frame, it totally screws up the animation.

Im kinda doing frame sensitive calculations in my code, so each frame skips 4.

While im not actually looking at using motion blur at the moment, i just noticed that motion blur was causing my script to act differently.

this would obviously be avoided if i could bake the drivers to animation, but i dont think that is possible at the moment.

ill try and get a better example. in some code im working on at the moment, it renders fine with out motion blur... but with motion blur on, and being evaluated 4 times per frame, it totally screws up the animation. Im kinda doing frame sensitive calculations in my code, so each frame skips 4. While im not actually looking at using motion blur at the moment, i just noticed that motion blur was causing my script to act differently. this would obviously be avoided if i could bake the drivers to animation, but i dont think that is possible at the moment.

Brecht Van Lommel commented

2013-12-08 15:05:46 +01:00

Ok, with motion blur it's basically expected to jump around time to sample motion (and it will do subframe calculations as well soon), that's standard for renderers and motion blur I think. It can be tricky if you do physics or other things that only like to move forward in time, but there's no practical way to do that I think.

levon hudson commented

2013-12-08 15:11:49 +01:00

yep, fair enough. i think a driver baking tool would be a much better option anyway.

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Motionblur in cycles calculates drivers' scripted expressions' multiple times per frame #37734