When using long shutter speeds, more memory is required for the BSP render accelerations since the blurred triangles span across many voxels. In this case, you can use the scanline algorithm so that no BSP tree is activated during a render.
Motion blur is active for the scene by default. To view the motion blur of objects in a scene, activate the motion blur settings in the render region options and/or the render pass options. As long as these options are on and you have a moving object in your scene, the motion blur is visible.
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
1. Choose Render > Render > Render Manager from the Render toolbar.
2. 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.
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When using long shutter speeds, more memory is required for the BSP render accelerations since the blurred triangles span across many voxels. In this case, you can use the scanline algorithm so that no BSP tree is activated during a render. |
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.
3. 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 helpful when you’re matching rendered 3D objects to live-action plates.
4. Specify where on the frame the blur is evaluated and rendered.
- 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, 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 current frame and ends after the current frame. For example, if Speed is set to 0.5, 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, 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.
1. Click the Current Pass button in the left-pane of the Render Manager.
2. Set Current Pass to the render pass for which you want to define motion blur.
3. Set the Scene Renderer. By default the renderer is set to mental ray as defined in your Scene Globals.
4. Enable Motion Blur for the pass.
5. The scene’s motion blur settings (Scene Globals) 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 the following procedures.
1. Click the mental ray button in the left-pane of the Render Manager.
2. On the Rendering tab, specify a render algorithm and other rendering options as described in the following procedures.
mental ray Algorithms For Motion Blur Rendering
The mental ray renderer provides three different algorithms you can use to render motion blur.
3. 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.
4. On the Motion Blur tab, specify your sampling contrast, motion steps, and shutter settings as described in the following procedures.
5. Set the Motion Blur Sampling Contrast by adjusting the RGBA sliders. If the difference between neighboring samples is greater than this level, another level of sampling occurs.
The Motion Blur Sampling Contrast determines how often the motion-blurred object is sampled during the rendering process. Although each process is similar, the motion blur and antialiasing techniques are not computed at the same time.
The smaller the value for each color (R, G, B), the greater the amount of sampling that must be done to close the contrast gap between the rendered image and the contrast settings, and the smoother the motion blur. Render times are not greatly affected when using this parameter.
Antialiasing and Motion Blur Sampling
You can use antialiasing 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.
The Motion Blur > Sampling Contrast determines how often the motion-blurred object is sampled during the rendering process. Although each process is similar, the motion blur and antialiasing techniques are not computed at the same time.
If you’re also using antialiasing, set the Motion Blur > Sampling Contrast values higher than the Rendering > Sampling Contrast values (on the Rendering tab of the same property editor) to speed up motion blur rendering. This results in grainier images, but the quality of antialiasing is not degraded.
For more information on antialiasing, see Controlling Aliasing in the Rendering guide.
Transform Motion Steps for Rotational Blur
6. If your scene contains rotating objects (propellers, for example) 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.
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Increasing the number of steps does not work well for objects that are animated along a complex path. |
Deformation Motion Steps for Deformation Blur
7. 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.
8. 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.
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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. |
For information on how to optimize rendering with deformation motion blur, see Local Deformation Motion Blur.
The Scene Motion Blur Settings for motion blur, 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 settings 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.
- A non-zero Delay moves the motion blur trails.
mental ray Motion Blurred Shadowmaps
9. On the Shadows tab, select Shadowmaps > Motion Blur 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.
For more information about shadow maps, see Creating Shadow-Mapped Shadows.
Motion Blur and Intensity Clipping
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 Intensity Clipping option forces mental ray to not perform color clipping, allowing the object’s intensity to carry through to the blurred result.
To deactivate intensity clipping
• On the Framebuffer tab, set Color Channel Clipping to No Clip.
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