Rendering Motion blur
Film cameras capture blurred images of objects moving rapidly. Without motion blur in a rendered image, moving objects will strobe from frame to frame and not match live action.
There are two types of motion, transformation blur and geometry blur. Transformation blur occurs when multiple transformations are specified for a single object for a single frame. Geometry blur occurs when multiple geometry objects are specified for an object over a single frame.
When generating motion blur in mantra, you must turn on the Allow Motion Blur checkbox on the Sampling sub-tab of the Properties tab.
Motion is evaluated at sub-frame intervals. Therefore, expressions which use $F will not evaluate properly for motion blur. Use $FF to get floating point frame values.
Each object may have a number of transformation blur segments. The more segments an object has, the more accurately the motion will be captured by mantra.
Increasing the Xform Time Samples parameter on the Sampling sub-tab of the Properties tab of the render node will allow you to see the motion blur as an object transforms along a curve. This is used to increase segment counts at the object level.
Geometry blur is often used for deforming geometry. For example, if a character is deforming or particles are moving, object level transformations are insufficient for capturing the motion of the deforming geometry.
For mantra to be able to determine the correct motion for deforming geometry, the each segment of the geometry must have the same topology. The primitive must also appear in the same order. In some cases, such as rendering particle systems, this is impossible. In cases like this, you can turn on Geometry Velocity Blur on the Sampling sub-tab of the Render tab in the parameter editor at the object level. Instead of sending down multiple sets of geometry, mantra is able to use the velocity attribute to determine how the geometry deforms.
When the velocity blur is used, the full geometry is saved at the first time segment using a
ray_detail statement. The second geometry is saved using a special form of the
ray_detail statement which gives a timestep (in seconds) and tells mantra to compute the velocity using the attribute attached to the first segment’s geometry. This sets the
geometry:timescale property on the detail.
Turn on the Geometry Velocity Blur checkbox on the object (Render tab > Sampling sub-tab), in addition to the Allow Motion Blur checkbox on the mantra output driver (Properties tab > Sampling sub-tab).
Increasing the Geo Time Samples parameter on the Sampling sub-tab of the Properties tab of the render node will allow you to see the motion blur as the geometry deforms. This is used to increase segment counts at the geometry level.
When objects are moving extremely fast, detail is lost in the blurring. Increasing the Motion Factor on the Sampling sub-tab of the Properties tab in the parameter editor will cause the Shading Quality parameter to be adjusted based on how quickly a primitive is moving in screen space. Increasing the Motion Factor can speed up renders by decreasing the amount of shading performed.
Increasing the number of Pixel Samples on the Sampling sub-tab of the Properties tab of the render node creates a smoother blurred look when used with motion blur.
IFD blur controls
The two controls that determine if the file is baked with motion blur are Enable Motion Blur and Enable Raytraced Motion Blur, which can be added to the Render sub-tab of the Properties tab of the render node. For more information on adding parameters to nodes see: What’s new in Houdini 9: Parameter Interface. By default both of these checkboxes are turned on.
Perspective based sampling
You can specify (on a per-object basis) to perform motion blur after primitives have been projected to the screen. This is basically equivalent to doing an occlusion corrected 2D motion blur. It is slightly faster than true 3D sampling (which mantra does by default) but it is not as accurate. This is controlled by the Perspective Correct Blur checkbox, which can be added to the Sampling sub-tab of the Properties tab of the render node. For more information on adding parameters to nodes see: What’s new in Houdini 9: Parameter Interface.
You cannot perform perspective projected blur when ray-tracing.