This is the latest in a number of adaptations from the new Inspired series published by Premier Press. Comprised of four titles and edited by Kyle Clark and Michael Ford, these books are designed to provide animators and curious moviegoers with tips and tricks from Hollywood veterans. The following is excerpted from Lighting and Compositing. Read Part 1 of this article.

[Figures 1 & 2] Block with no motion blur (left) composited over a background plate. Block with motion blur and a shutter speed setting of 0.1 (right).

3D CG Motion Blur Comparisons
The amount of motion blur depends on the speed of the motion, and this factor can have a large impact on how computer graphics elements are viewed. With the simple composite shown in Figure 1, the block is rendered with no motion blur. The block is actually moving from the top right to the bottom left of the picture plane, but this image gives no indication of that animation. The clarity of the render, along with its size in the frame, gives away the lack of detail in the block’s textures. The sharp edges of the block are a dead giveaway that it has not been thrown across the view of the camera, even though the 3D computer graphics scene indicates that it has.

The image in Figure 2 shows the block with the same animation as in Figure 1, but this time motion blur has been turned on in the renderer with a shutter speed setting of 0.1. The block, when in motion, now appears as if it was tossed gently in front of the camera. The shutter speed is set lower than a normal film camera would record. Because the normal setting would be 0.5, this image produces 1/5 the amount of blur one would expect to see with a block moving this speed and recorded with a 180-degree shutter. Although the motion blur is slight, it still makes quite a difference in the look of the image. The interior of the block is blurred, but note the edges. A small range of pixels around the outside of the image is now semi-transparent, allowing portions of the background scene to be viewed through the block’s edges. 3D motion blur calculates the opacity of the object by making it proportional to the amount of time the object resides in that pixel. With the shutter setting at 0.1, the moving block is exposed to the “film” for only 1/240 of a second, so the amount of motion blur and transparency are small.

[Figures 3 & 4] Block with motion blur and a shutter speed setting of 0.25 (left). Block with motion blur and a shutter speed setting of 0.5 (right).

Increasing the shutter speed to 0.25 yields the result shown in Figure 3. This amount of blur creates the perception that the block is moving much faster. The transparent area around the edges is much larger, as the two frames combined to create this image are now farther apart. The blurring has also reduced the contrast and saturation of the block, because the values are mixed together.

If the block is moving very fast across the picture frame (which it is in this particular animation), an image like that in Figure 4 is appropriate. The shutter speed setting for this frame is 0.5, which matches a typical film camera with a 180-degree shutter. At this setting, the block has moved a great distance between frames 22 and 22.5, creating large areas of transparency. The blur is so great that the object is now barely recognizable as a block. If this block only appears in shots with a great deal of motion blur, making its color and texture almost indiscernible, the texture painting and shader departments should be alerted ahead of time. It does not make sense to spend time painting detailed textures and developing complex shaders for a computer graphics object only viewed as a blur. If the object is to be used in other scenes in which it is moving less or not at all, attention to detail in the textures and shaders is appropriate. If the block is viewed only as in Figure 4, the time is better spent elsewhere.