Hey guys,
I’m comping a frame without motion blur and velocity passa activated for testing purposes and I see some extrange results.
First, I don’t know if it’s normal that velocity pass contains white areas.
Second, the picture atteched shows the result of this velocity pass with the original frame.
Why is the best Draw Point Filter method or best settings to comp the motion blur on postproduction stage.
Maybe it’s a bug. or a problem with the alpha I don’t know.
I uploaded the pictures:
White is normal. The colors represent vectors, so that just means a vector that is something like 1,1,1 (or it may be larger, but because of how your display clamps the values, you wouldn’t know from the image).
It looks like you’ve already rendered motion blur though. Many of your particles are in the form of streaks.
Because of the nature of the filtering, you generally don’t want it to be active for passes like motion vectors. In the case of blur, it’s difficult for the viewer to tell the difference between a vector of 1,0,0 and -1,0,0. The blur will appear to run in a line between those vectors anyway. But if you were to filter two particles together who had those velocity vectors, the resulting pixel would be 0,0,0 and would receive no blur at all. So generally speaking, you don’t want to have any filtering between the particle values and the pixel values.
It should also be said that in general doing post motion blur will only look good in very rare cases. In cases where you have a lot of variety in the motion, you’ll get really bad artifacts.
Thanks Chad.
So I’m forced to do it on rendertime if I don’t want artifacts, such in case that the motion of the particles is complex.
It’s a bit frustrating. On 2k render with 150million of particles and motion blur the time it’s around 7 minutes, and without it is around 45 seconds…!
Best!
Hi!
As Chad mentioned, the Velocity pass makes very little sense in typical Volumetric rendering because it represents the velocity of only the particles closest to the camera, completely ignoring any particles partially or fully behind them. Also a single particle covers not only one, but a grid of 2x2, 3x3 or more pixels depending on the beauty pass’ filtering mode, so even more data is hidden.
The main reason Krakatoa can write out a Velocity pass is that people asked for it and we wanted them to shut up, not because it makes any sense 
We have logged a wish list item against improving the built in Motion Blur in the future - trying to provide better results (right now it produces linear streaks lit at the center of the shutter interval), allowing for adaptive passes (background particles drawn less often than foreground particles) and making it faster (less than linear dependency on the number of passes). This is a goal for Krakatoa 3, but there is no plan yet, so no promises.
Hi bobo
Great info. Completely understood,
Sounds very promising, will be a great feature.
Best!
