Strange interpretation of some animated parameters with acitvated motion blur

[greymiura]

I thought I was going to lose my mind over a light that was flashing on one frame too early and it took me a while to understand that it had to do with the animated enable / disable parameter of the light in connection with motion blur. Something similar happens with an animated material tag. And maybe other parameters?

I tried to isolate these two examples in this little scene:

https://we.tl/t-53Go0nr7p5

It contains a V-Ray Rectangle Light with the following keys:

Frame 0 > Enabled deactivated

Frame 1 > Enabled activated

Frame 2 > Enabled deactivated


And there's also a sphere with an animated Material parameter in the material tag

Frame 0 > Material White

Frame 1 > Material Red

Frame 2 > Material White

If you render the three frames, once with motion blur and once without, you'll get the following results:



Is this an issue or some kind of inherent limitation? Did I choose a wrong setup? I know how to handle it now, but I'm curious about the cause for this frame shift...

[deyan.hadzhiev]

Hi greymiura, what you have observed is a known limitation. Basically, the motion blur interpolates between parameters of an object in its state in the beginning of the motion blur interval and the end of the motion blur interval. But when the parameters are not easily interpolated (e.g. enabled/disabled state, assigned material or other parameters that rely on true/false state) the value used for the rendering duration may only be one of the two states. Since your example with the motion blur picks the states in the end of the interval, I suppose that you are using a motion blur interval of 1 frame and interval center equal to or above 0.5. Modifying the interval center may help you overcome this limitation by making the motion blur avoid the changed state at the next frame, but note that it will also modify the sampled positions of objects and parameters.

[greymiura]

Hey deyan.hadzhiev,

thank you for your swift reply and your kind explanation! Unfortunately, I couldn't come to terms with your solution. Some things seemed to change after reducing the Interval Center to 0.4 or playing around with the Duration, but rather randomly and I wasn't able to produce the correct results for all three frames.

Curiously, V-Ray has no problem with some other non interpolation parameters like Render Visibility or the Switch value of the Switch Material.

Anyway, I just wanted to point it out and thought there was a simple solution, but I'm totally fine with some workarounds.

Thanks again and have a nice day!

[deyan.hadzhiev]

Curiously, V-Ray has no problem with some other non interpolation parameters like Render Visibility or the Switch value of the Switch Material.

Yes, as long as the parameter is constrained in a single object. Integer value interpolation in most cases ends up truncating the value so an ID interpolation is fine. Material links are something that is harder to interpolate.