Velocity Motion Blur is an “artificial blur”, when it is impossible to calculate the Deformation blur in the case when the sorting of points changes and the topology is not constant. Ideally, you should always try and use Deformation blur
From the developers #
Velocity blur is typically used when topology is changing (which happens most often with particle systems). For velocity blur, karma only looks at a single time sample on the stage and uses the velocity attribute to compute motion (or acceleration blur).
Deformation blur takes multiple time samples from the stage and computes the blur between the samples. Ideally, samples stored on the stage should line up with the shutter times defined by the camera. If they don’t align, USD/Karma will do some interpolation between the nearest frames to get the position at the given time.
If the particle count remains constant, deformation blur can work just fine for particles.
The style of motion blur (velocity, deformation etc.) can be defined on a per-object basis. So some objects might use deformation blur, while others use velocity blur. This is set using the Render Geometry Settings node. The Karma node provides the default behaviour, but that can be overridden on a per-object basis.
The motion blur LOP can be used to author time samples on the stage which can be used for deformation blur.
more info: https://www.sidefx.com/docs/houdini/solaris/kug/motionblur.html
Trail SOP settings: Backward difference: For each point, subtract the previous position from the current position
Central difference: For each point, subtract the previous position from the next frame’s position
Forward Difference: For each point, subtract the current point’s position from the next frame’s position
Backward difference will give you the velocity leading up to the current frame.
Forward difference will give you the velocity leaving the current frame toward the next frame.
Central difference may seem weird since you’re ignoring the position of the current frame, but it’s actually the average of forward and backward difference: fwd(next - current) - bkwd(current - prev) -> next - current + current - prev -> next - prev
Which to use with a real camera? #
So, the type of velocity you want depends on the shutter you have when rendering. If you’re shutter is centered (-.25 to 0.25), then central differencing will likely give you the most similar type of blur.