Topic: Motion blur not rendering correctly?
posted by Eivind Nag, Vlado
archived on 23.5.2003
Have a look at this scene: link not needed
Its so a simple scene as you can get it, this is the error which made us have heart attacks on out last job. The error seems to be that if you have one or more volume selects in the stack with a tesselation inbetween each one vray doesnt get the geometry correctly for each frame. What is worse is that it is completely random if there is motion blur, if there is not or if there is 10x to much motion blur.
Comment: Later tests ...look close at the sferical gizmo in the second selection, during the animation, it bounces up and down so the selection area varies.
Comment: That the selection area is changing is the whole point of the setup, this is a standard way of tesselating geometry according to either camera view or an object. For example if you are making water, you dont want 1km squared with a tesselation that is really really small, therefore you add a volume select, link either the gizmo or a sphere to the object or camera and tesselate by a frame by frame basis. This way you only get crude geometry for everything but whats in the view or focus area. My problem with this is that if you try to render the scene i sent enough times the same frame will render 100% correctly with motion blur and sometimes not correctly so clearly vray has got some random problem with it. Max is always generating the correct geometry at the correct frame so it has to be some way that vray tries to "pull" this info out of max that fails in some cases. Just for fun, try to up the geometry samples to about 10, it is pretty clear that vray gets all the geometry at every possible time, its just that with 2 geometry samples its random if it manages to interpolate or not. As for just adding geometry samples to render this, its not funny to have 10 geometry samples when the water plane is 500k faces.
A: Eivind, the problem is that VRay can't generate motion blur for objects which change their toplogy (face/vertex count) during the motion blur interval. You need to make sure that the object has consistent topology when VRay evaluates it for motion blur. You can probably do this with an Expression or a MaxScript expression controller, which makes sure the volume select gizmo stays constant within one frame, but changes between successive frames.
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