HDR Image Motion Blur (mental ray)

This shader applies fast, grain-free motion blur as a post process.

HDR Image Motion Blur works by using pixel motion vectors stored in the rendering phase and “smearing" these into a visual simulation of motion blur. Like using the rasterizer, this means that features such as mirror images or even objects seen through foreground transparent object will “streak" together with the foreground object. Furthermore, since the motion frame buffer only stores one segment, the “streaks" are always straight, never curved.

The major advantage of this method is rendering speed. Scene or shader complexity has no impact. The blur is applied as a mental ray “output shader" that is executed after the main rendering pass. The execution time of the output shader depends on how many pixels need to be blurred, and how far each pixel needs to be “smeared.”

Note: Because the shader works in post, it does not blur shadows or reflections of moving objects. If these effects are necessary, use standard motion-blur methods.

Interface

Shutter Duration (frames)

The amount of time the shutter is “open.” In practice this means that after the image has been rendered the pixels are smeared into streaks in both the forward and backward direction. Each smear length is half the distance the object moves during the shutter time.

Shutter Falloff (Blur Softness)

The drop-off speed of the smear; that is, how quickly it fades to transparent. This setting controls the “softness" of the blur.

Note: The perceived length of the motion blur diminishes as the Falloff value increases, so it might be necessary to compensate by increasing the Shutter Duration value slightly.

Thus, falloff is especially useful for creating the effect of over-bright highlights “streaking" convincingly: By using an inflated shutter length (above the cinematic default of 0.5) and a higher falloff, over-brights have the potential to smear in a pleasing manner.

Blur Environment/Background

Determines whether the camera environment (that is, the background) should be blurred by the cameras movement or not. When on, pixels from the environment are blurred, and when off they are not.

Note: This option does not work when Rasterizer / Scanline mode is active.
Calculation Color Space (Gamma)

Defines the gamma color space in which blur calculations occur. Because mental ray output shaders act on written frame buffers, and these buffers (unless floating point) already have any gamma correction applied, it is important to apply post effects with the appropriate gamma.

Min. Motion Threshold (pixels)

The minimum motion-vector length (measured in pixels) an object must move for blur to be added. If set to 0.0, it has no effect, and every object with even sub-pixel movement will have a slight amount of blur. While this is technically accurate, it might cause the image to be perceived as overly blurry.

Background Distance

The distance to the background, which helps the algorithm calculate the depth layout of the scene. This value should be about the same as the scene depth; anything farther from the camera than this distance would be considered “far away" by the algorithm.

Blur More Objects Near Camera

When on, the blurring of objects closer to the camera is more opaque than that of more-distant objects. Because this can result in the blurs of objects very close to the camera to be unrealistically opaque, use this option only when necessary; that is, when the blurs of more-distant objects overwrite those of closer objects.

Parent topic: Output Shaders