2. Blur Filters

2.1. Introduction

Figure 16.2. Original for demo

Original for demo

This is a set of filters that blur images, or parts of them, in various ways. If there is a selection, only the selected parts of an image will be blurred. There may, however, be some leakage of colors from the unblurred area into the blurred area. To help you pick the one you want, we will illustrate what each does when applied to the image shown at right. These are, of course, only examples: most of the filters have parameter settings that allow you to vary the magnitude or type of blurring.

Figure 16.3. Gaussian blur (radius 10)

Gaussian blur (radius 10)

The most broadly useful of these is the Gaussian blur. (Don't let the word "Gaussian" throw you: this filter makes an image blurry in the most basic way.) It has an efficient implementation that allows it to create a very blurry blur in a relatively short time.

Figure 16.4. Simple blur

Simple blur

If you only want to blur the image a little bit — to soften it, as it were — you might use the simple "Blur" filter. This filter runs automatically, without creating a dialog. The effect is subtle enough that you might not even notice it, but you can get a stronger effect by repeating it. In GIMP 2.0 the filter shows a dialog that allows you to set a "repeat count". If you want a strong blurring effect, this filter is too slow to be a good choice: use a Gaussian blur instead.

Figure 16.5. Selective blur

Selective blur

The Selective Blur filter allows you to set a threshold so that only pixels that are similar to each other are blurred together. It is often useful as a tool for reducing graininess in photos without blurring sharp edges. (In the example, note that the graininess of the background has been reduced.) The implementation is much slower than a Gaussian blur, though, so you should not use it unless you really need the selectivity.

Figure 16.6. Pixelize


The Pixelize filter produces the well-known "Abraham Lincoln" effect by turning the image into a set of large square pixels. (The Oilify filter, in the Artistic Filters group, has a similar effect, but with irregular blobs instead of perfectly square pixels.)

[Note] Note

You can find a nice explanation of the Abraham Lincoln effect at [BACH04]. You will see the Salvador Dali's painting Gala Contemplating the Mediterranean Sea turning to an Abraham Lincoln's portrait when looking at it from a distance.

Figure 16.7. Motion blur

Motion blur

The Motion Blur filter blurs in a specific direction at each point, which allows you to create a sense of motion: either linear, radial, or rotational.

Finally, the Tileable Blur filter is really the same thing as a Gaussian blur, except that it wraps around the edges of an image to help you reduce edge effects when you create a pattern by tiling multiple copies of the image side by side.

[Note] Note

Tileable Blur is actually implemented by a Script-Fu script that invokes the Gaussian blur plug-in.