Motion Blur

Use Motion Blur to simulate the blur created by fast-moving objects.

To create a Motion Blur, combustion takes samples of previous and subsequent frames and composites them over the current frame. combustion uses sub-frame sampling to create a smooth effect.

You control the number of samples, the transparency (or weight) of the samples, and the duration over which the samples are taken.

Note: You must nest a layer before you use Motion Blur on that layer. See Blurring the Movement of a Nested Layer.

Motion Blur Controls

The Motion Blur controls are described as follows.

Samples

Determines the quality of the Motion Blur. Increasing the number of samples improves the quality of the Motion Blur, creating a softer effect.

Note: Increasing the Samples value causes the processing time to increase linearly. In many cases, you do not need anti-aliasing on a blurred layer, so to save processing time, you can turn it off.

Phase

Specifies whether the Motion Blur is based on the movement before or after the current frame. A value of -100 places the Motion Blur before the current frame while a value of 100 places the Motion Blur after. A value of 0 is centered, which evenly distributes the Motion Blur.

Shutter

Determines the size of the Motion Blur. It is essentially the number of frames over which the shutter is open. Increasing the Shutter value does not increase processing time.

Motion Blur Graph

Determines the weight distribution of the samples. The vertical axis represents the weight (or opacity) of the samples, while the horizontal axis represents the position of the samples. Each sample is represented by a vertical green line. The red line represents the current frame.

To change the weight distribution of the samples:

Click the curve to add a control point, then drag the control point up or down to change the weight of the samples at that point. Click and drag a green line to shift the samples along the curve.

In the following illustration, samples in the center of the blur carry more weight than samples at either end of the effect.

In the next illustration, the samples at the front of the effect carry more weight and the object appears to be moving faster.

Blurring the Movement of a Nested Layer

Use Motion Blur to add realism to animated layers in your composite. To do so, first nest the layer, then apply the Motion Blur. You nest the layer so the blur is created for the movement of the layer, not the movement of items in the images.

In the following procedure, the front layer, a still image of a 16-ton iron weight, moves from the top to the bottom of the frame. The procedure describes how to use Motion Blur to create the illusion that the layer is falling fast.

To blur the movement of a layer:

1. Select the animated layer.



2. Choose Object | Nesting, and then click Selected Layers in the Nesting Options dialog.



3. In the Workspace panel, select the new nested layer.
4. Choose Operators | Blur/Sharpen | Motion Blur.

A Motion Blur filter is applied to the output of the nested composite. As a result, the Motion Blur filter can detect the movement (transforms) of the animated layer.



5. Adjust the Motion Blur settings.

The moving layer is blurred.

For more information on nesting layers, see Working with Nested Composites.

Applying a Motion Blur to a Single Layer

If you apply a Motion Blur to a layer that is not nested, the blur affects any movement in the clip. This can be useful if you want to blur a CG animation that is not in RPF format. If the animation is an RPF file, you should use the Motion Blur in the 3D Post filter menu. See RPF Motion Blur.

When you apply a Motion Blur to a single layer, combustion does not have any sub-frame motion information, therefore it uses the frame-to-frame motion to create the blur.

As a result, the Motion Blur is smooth on slow-moving objects; for fast moving objects, the samples will be farther apart and visible. Also remember, the blur is applied to all the motion in the layer, including any movement in the background.

In the following example, a Motion Blur is applied to the footage of a dancer shot in front of a blue screen.