Motion Blur

What is Motion Blur?

Motion blur is an effect caused when either the camera or the subject moves during the exposure. A similar effect can be obtained by stacking a series of images each one slightly offset from the previous one and averaging them together. Thus, the output image is computed at each pixel by averaging pixel values computed at a sequence of offsets from the input pixel.

When the offsets are clustered more densely at some location, the image at that offset is weighted more heavily than the ones from other areas and it is more distinct in the result.

Where the offsets are farther apart, there is more blurring and the motion trail is wispier. When the offsets are even further apart, the result image starts to break down into a series of multiple exposures as though you used a flashing strobe to photograph a moving object.

How Motion Blur Works

The Motion Blur transformation lets you draw one or more curves called blur paths in an area in the Motion Blur dialog box that looks a little like graph paper. The center of the graph paper represents an offset of (0,0) with respect to the input image. From the time you first click in the blur path area until you release the mouse button, Picture Window samples the cursor location at regular intervals. By default, this is once every 20 milliseconds or 50 times per second, but you can adjust the slider to vary the sampling interval to speed it up or slow it down according to your personal preference. Each time the cursor location is sampled, the offset corresponding to that location is recorded and a small red circle is displayed to mark the location in the blur path. Thus, as you drag the cursor across the blur path area, red circles are left behind every time a sample is taken. The slower you drag, the closer the samples are to each other and the more distinct the image corresponding to that offset will appear in the output image. This transformation is unique in that it depends on how fast you drag the mouse and therefore it takes a little practice to learn to control the speed to get the effect you want. Once you master it however, you can achieve many subtle effects.

You can build a complex blur path by combining multiple strokes or you can press the Erase to clear the blur path and start over at any time. When it comes time to update the preview or compute the final image, the images at every single stored offset in the blur path are averaged together. For this reason, the more offsets you specify, the longer it takes to compute the result.

The Amount control acts as a scale factor on all the offsets so the smaller the amount, the less the motion blur. If you use an amount mask, you can vary the amount of motion blur in different parts of the image.

The Opt menu lets you reset the settings to defaults, load a saved settings file or save the current settings in a file.

There can be something of a problem at the edges of the result image where there is no offset image data available to shift into the frame. Since pixels outside the image are treated as black, the edges of the result image are darkened by an amount that depends on the direction and amount of the offsets in the blur path. If you don't like this effect you can simply crop out the dark edges.