Motion blur is active for the scene by default. To view the motion blur of objects in a scene, activate the motion blur setting in the Render Region Options Property Editor. To render the motion blur, activate the motion blur setting in the Render Pass Property Editor. As long as these options are on and you have a moving object in your scene, the motion blur is visible and rendered. After that, you can tweak the timing and quality of the blur effect with the various motion blur settings discussed in this section:
You can also create motion blur for a specific object in a scene by assigning it a motion blur property. This is useful when you want to force motion blur off for a given object, or if you want to optimize rendering performance by setting the more render-intensive deformation blur only for a specific object. This is discussed in Creating a Motion Blur Property.
Motion blur is active for the scene by default, but to tweak the effect you can set scene-based motion blur options such as shutter speed and offset, and specify where on the frame the blur is evaluated and rendered. These are the primary settings for defining the motion blur effect.
Specifying the Shutter Interval
1. Set the motion blur Speed for the scene. This setting specifies the time interval (usually between 0 and 1) during which the geometry and any motion transformations and motion vectors are evaluated for the frame. The motion data is then pushed to the renderer (by default mental ray).
- Setting Speed to 0 turns motion blur off.
- Longer (slow) shutter speeds (a difference of greater than 0.6) create a wider and/or longer motion blur effect, simulating a faster speed.
- Shorter (quicker) shutter speeds (a difference of less than 0.3) create subtler motion blurs.
- Shutter speeds higher than 1 produce intense motion blurs, but may cause artifacts to appear in a rendered image.
- If you’re using motion blur with particles, the Speed setting must be less than 1.
Of course, all shutter speeds are relative to the speed at which the blurred object is animated. Objects that move from one end of a scene to another in a few frames produce a large amount of motion blur, regardless of the shutter speed.
2. Specify an Offset for the shutter’s time interval which allows you to push the motion blur trails, even extend them into later frames. This can be a helpful option when you’re trying to match rendered 3D objects to live-action plates.
Setting On-Frame Shutter Modes
3. Decide where on the frame the blur should be evaluated and rendered by setting an “on-frame” shutter mode.
To illustrate the different on-frame behaviors, the images below use a set of still markers (representing frames) and an animated sphere that moves along a curved path such that on frame 0 it is over the first marker, on frame 1 it is over the second marker, and on frame 3 it is over the third marker. The sphere is animated linearly from -10 in X, at frame 1, to +10 in X, at frame 3, and you render frame 2.
- End on Frame: The blur starts prior to current frame and ends on the current frame. For example, if Speed is set to 0.5 and Offset is 0.0, the shutter opens half-way between the current frame and the previous frame, and closes on the current frame.
- Start on Frame: The blur starts on the current frame and ends after the current frame. For example, if Speed is set to 0.5 and Offset is 0.0, the shutter opens at the current frame and closes half-way between the current frame and the next frame.
- Center on Frame: The blur is centered on the current frame such that the shutter opens before the current frame and closes at exactly the same distance after the current frame. For example, if Speed is set to 0.5 and Offset is 0.0, the shutter opens three-quarters of the way between the previous frame and the current frame, and closes one-quarter of the way between the current frame and the next frame.
You may notice that the blur does not line up with the original object position. This is a limitation with Center on Frame that occurs when the blurred object moves along a curved path (as illustrated). The problem is that it is not possible to place the object exactly at the same position where it would appear if there was no motion blur, because the interpolation is linear between the two points on either end of the curve. In this case, you can do one of the following:
• Use the End on Frame shutter mode instead.
• If you are rendering motion vectors to be used in post-processing, enable a “motion” render channel for all passes so that all the images will line up when compositing later.
4. When you are rendering motion vectors (with motion or raster motion render channels) to be used in a post-processing tools pipeline, the on-frame placement of your blurred objects will be affected by the Compensate For Object Motion setting.
The Compensate For Object Motion option is enabled by default and it dynamically adjusts the shutter offset and close values so that the object is placed at (or as close to it as possible) the location where it would normally appear if there was no motion blur enabled. When off, there is no compensation and the object is always placed at the shutter close position.
Assuming the same example as above with the animated sphere, but this time the Speed is set to 1.0. You would get the following results:
- For Center on Frame with Compensate For Object Motion on, the sphere will appear at X=0. When Compensate For Object Motion is off, the sphere will appear at X=5, since the blur at frame 2 goes from -5 to 5 in X.
Again, notice that the blur does not line up with the original object position. The images (above) clearly show the limitation of Center on Frame when the blurred object moves along a curved path. Refer to the earlier note for an explanation and the possible work-arounds for this issue.
- For End on Frame there is no change, and the sphere will always show up at X=0, because the motion blur ends at that point (the blur goes from -10 to 0, for frame 2).
- For Start on Frame, with Compensate For Object Motion on, the sphere will appear at X=0, but with Compensate For Object Motion off, the sphere will appear at X=10 (the blur goes from 0 to 10, for frame 2).
To render the motion blur of objects in a scene, you must activate the motion blur settings for each render pass that requires blurring.
1. Set the Pass Renderer to mental ray (this is the default).
2. Enable Motion Blur for the pass.
3. The scene’s motion blur settings are used automatically for the pass. If you want to override them for a specific pass, select Override Scene Render Options and modify the motion blur settings as described in Scene Motion Blur Settings.
The mental ray renderer provides three different algorithms you can use to render motion blur.
1. Select a rendering algorithm to use when rendering motion blur from the Primary Rays > Type options:
• Raytracing correctly blurs attributes applied to moving objects, such as highlights, textures, shadows, and reflections as well as refractions, transparency, and intersecting objects.
• Scanline is much faster than raytracing motion blur, but it cannot handle reflections and refractions (and shadows must be approximated with shadow maps). However, motion blur of highlights, textures, transparency, and intersecting objects still work with scanline rendering.
• Rasterizer accelerates motion blur rendering in large and complex scenes with a lot of motion blur. Using this algorithm requires that you set additional parameters that are described in The Rasterizer.
You can use antialiasing techniques on motion blurred objects to heighten the level of realism. When you apply antialiasing to an object that has a blur on it, the antialiasing level determines whether more subdivisions of the pixel are needed during sampling.
2. Use Min Level and Max Level options only to set a hard sampling limit, and then you can further improve the motion blur effect by adding Motion Samples for every color sample taken.
3. If your scene contains rotating objects that require improved motion blur results, increase the Motion Steps > Transform value to control the interpolation between an object's motion data during the shutter interval.
When this value is set to 1, motion blur is calculated based on a single linear interpolation between the two values. This may not be adequate for objects that are rotating about an axis (propellers, for example). Increasing the interpolation value adds extra steps between the two points, and interpolates linearly between successive steps. This improves the blurred result considerably for rotating objects.
Increasing the number of steps does not work well for objects that are animated along a complex path.
4. Enable Motion Steps > Deformation if you want shape changes to be considered in the motion blurred results. This option should only be activated when objects in the scene change shape quickly enough to cause a motion blur. Deformation motion blur is computed for each animated vertex. This greatly increases rendering time.
5. If your scene contains animated deformations that require improved motion blur results, increase the Deformation steps value. Deformation motion blur is calculated based on a curvilinear interpolation between an object's motion data at the beginning and end of the shutter interval. Increasing the interpolation value adds extra steps between these two points, and creates curved motion between each successive interpolation step. The interpolation steps are placed randomly (using the Quasi-Monte Carlo method) along the object’s motion data.
Interpolated deformation motion blur is only supported for polygon meshes. If you enable deformation motion blur for NURBS surfaces, particles, or hair simulations, the Deformation steps option is ignored and the deformation blur is calculated linearly between the open and close shutter interval.
The Scene Motion Blur Settings define the point in time when the geometry and its motion transformations and motion vectors are evaluated: this data is then pushed to mental ray.
You can use the mental ray Shutter and Delay options to tweak the size and length of the motion blur trails (without re-evaluating the geometry and its motion data). This allows you to quickly “dial-in” settings, if needed, although they are not particularly accurate.
- When Shutter is set to 1.0 (default), mental ray uses the full length of the geometry’s motion transformations and motion vectors as specified by the shutter Speed interval for the pass/scene.
For moving objects with displacement, you can optimize the displacement approximation depending on how fast the object is moving by setting the Motion-Based Displacement Quality option. The automatic reduction of displacement detail on moving objects has a significant impact on rendering performance and memory consumption by cutting down on the typically huge amount of tessellation data that gets generated.
The displaced geometry should have a Geometry Approximation property applied to it and motion blur must be enabled for the render region and/or the render pass.
By default, mental ray clips color samples after each ray, before filtering them into the final pixel value. When you render motion blurred objects, this color clipping causes objects with varying intensity values (above 1.0) to appear as though they all have the same intensity value when blurred.
Deactivating the Color Channel Clipping option forces mental ray to not perform color clipping, allowing the object’s intensity to carry through to the blurred result.
• To deactivate Color Channel clipping, set it to No Clip.
Select the Shadowmaps > Motion Blur checkmark to activate the shadow mapping option for motion blur effects. Shadow maps are a scanline-based feature that produce shadows faster than raytracing. Activating them with motion blur slows render times but matches the shadow to the motion blur nicely. If you are running a quick render test, keep this option off.
