Blur Tool UI

The Blur tool UI is composed of five tabs:

Directional Gaussian and Radial Blur

The Directional Gaussian blur filter lets you scale and rotate its elliptical shape. This allows you to blur an image by a certain amount in a given direction, and by a different amount in a direction perpendicular to it. Each pixel is blurred by the same amount.

The Radial Blur simulates the effect of motion blur that would be generated by the movement of the camera due to zooming and spinning around a given point. The zooming and spinning movements are simply converted into motion vectors representing the local displacement at each pixel. These motion vectors are then used to construct blur ellipses aligned with each vector. The size of the major axis matches the length of the displacement vector. The size of the minor axis is kept fixed at a small value. That value is chosen to keep aliasing artifacts at an acceptable level while not introducing too much blurriness.

Modulation Blur

A particular color channel (red, green, blue, alpha or luminance) of the modulation input image can be used to modulate a modulated motion blur. The end effect is that the modulation image will apply a varying amount of scaling at each pixel. The elliptical shape of the blur filter is scaled uniformly by this modulation amount.

The Modulation blur tool has the following parameters:

Although the Modulation Blur tool can simulate depth of field, artifacts may appear for a number of reasons and are inherent 2.5D motion blur and depth of field.

Depth of field is a process that occurs in 3D. Out-of-focus objects blur on top of objects that stand further away. They never blur on top of objects in front of them. The modulated blur does not make this distinction about objects that are in front or behind thus causing artifacts every time a silhouette edge appears in an image. Rendering depth of field therefore requires segmentation of the image along the silhouette edges. Please note that it is not always possible to partition a scene into separate objects using silhouette edges.

Anti-aliased images also cause some problems in that the Z-channel cannot be used to determine which portion of the pixel belongs to the background object and which part belongs to the foreground. When an object becomes out of focus, one starts to see parts of the scene that were not visible before through the blurred edge. To replicate that effect as a 2D post-process, the depth of field tool must guess what the background pixels might look behind foreground objects. To do this, segmenting and matting out foreground objects' then reconstructing the background using in-filling techniques become necessary and which can be a difficult task.

To correct some of the Z-channel problems, the 3D elements of a scene can be rendered as separate layers. The scene would be manually segmented into objects all having approximately the same Z. A single layer would not have any significant Z discontinuities. Each layer can be properly anti-aliased. The Z-information of the foreground element and the background element would be available at partially covered edge pixels and the color of the background objects behind foreground elements would also be available. However, the Z-information is not defined at the transparent pixels of each layer. A renderer is likely to assign them some very far-away value. To the modulated blur tool, this represents a huge discontinuity in its blur modulation and the Z-discontinuity ends up as a discontinuity in the blurred image. To help remove the resulting artifacts, the parameters (Min. Alpha and Max Radius) under the Extend label can be set to generate the missing Z information.

Vectors Blur

Use the Vector Blur tool to specify the length and the width of the blur ellipses. The blur ellipses are then oriented independently for each pixel in the direction of the vectors of the Forward Vector input image and uniformly scaled by the length of the vectors. You can also apply an extra constant rotation to all the blur ellipses.

Using this definition of vector blurs you can blur only in the direction of the vector field. To do this, set the amount of blur in the perpendicular vector direction to zero. No matter how much it is scaled, it will always stay null. Then, use the minimum blur radius control to clamp to a finite value to avoid sampling aliasing.

The Vectors blur tool has the following parameters:

Using the Modulation blur tool to simulate motion blur can be difficult due to some of the same inherent issues as when trying to simulate depth of field using the Modulation blur tool;

• Need to segment out at silhouette edges.
• Can not deal with anti-aliased images at silhouette edges.
• Needs the background pixels behind foreground elements.
• Discontinuity in the motion vector field ends-up as discontinuity in the blurred image.
• Needs to out-fill motion vectors over transparent region.

These limitations can be worked around, if the 3D department has already partitioned the image into layers of constant movement. You can then use the Min. Alpha and Max Radius parameters of the Vectors blur tool to extend the motion vectors field to get more realistic motion blur effects.

Output Controls

The Output UI allows you to control the following parameters: