Blur Filters

Gimp provides many ways to create blur effects. This chapter tells you how you can learn what filter to use and how to get the kind of blur you want.

| Blur filters are often used on certain parts of an image to shift the focus to the sharper parts, to soften hard edges or to create an illusion of depth or distance.

## Antialias

Antialias is an effective little filter for smoothing jagged edges. Antialias will find the edges and smooth the boundary between areas of different color or contrast, but it won't blur the entire image as other blur filters will. This filter is used by the `right-click|Filters|Blur|``Antialias...` script that is described later in this chapter, so if you'd like to change any parameters, we suggest you use the Antialias... script instead.

## Blur

| Blur makes your image look soft and out of focus. Several settings can be adjusted in the Blur dialog.

Blur causes random displacement of pixels. The Randomization% can be adjusted from 0% to 100%. A high value will render in a high level of blur. You can also specify if you want the filter to be repeated only once, or up to 100 times. Don't use too high repeat values, because this will make your image very unfocused.

You can set the Randomization Seed to the current time, or you can set it yourself in the input field as an Other Value. With Other Value you will get the "same" initial random value for each repeat; with current time you will get a different value for each repeat step.

## Gaussian Blur

### Gaussian Blur (IIR)

Gaussian Blur (IIR) is a variable blurring method, based on the radius of the blur. Higher radiuses produce a higher amount of blur (values less than 1.0 are invalid).

You can also choose to blur vertically, horizontally or both by checking the appropriate checkboxes. These parameters make it possible to create a motion blur with this filter.

The Gaussian Blur (IIR) blur type is best for scanned images and other natural (photographic) images.

### Gaussian Blur (RLE)

Gaussian Blur (RLE) works the same as Gaussian Blur (IIR) and provides the same parameters in its dialog. Gaussian Blur (RLE) is the blurring technique you should use for computer-made images.

## Motion Blur

| Motion Blur simulates a snapshot of a moving object.

### Linear Motion Blur

If you check the Linear radio button in the Blur type field, the image will look as if the object was moving beside you. The Angle slider sets the direction of the motion and the Length slider controls the speed of the motion (the more Length the higher the speed).

### Radial Motion Blur

If you check the Radial Blur type radio button, the image or selection will appear to be rocking or spinning in front of you.

In this case, Angle determines the amount of radial motion. A high value for Angle will make the image spin. Length determines how fast the object appears to be rocking/spinning.

### Zoom Motion Blur

If you check the Zoom Blur type radio button, the objects in the middle of the image will look as if they are moving towards or away from you. Here, Length is the apparent speed at which the object is travelling. Angle seems to have no impact on this blur mode.

## Pixelize

| Pixelize makes your image look as if it was made of really large pixels. You have to set the new pixel size. In other words, a Pixel Width of 3 makes an area made up of a 3x3 pixel block look like a single large pixel.

## Selective Gaussian Blur

Selective Gaussian Blur is very useful for removing noise from scanned images. It works by adding blur to the parts of the image where focus is low.

The Max Delta parameter controls the size of the image area that will be blurred. A high value (>70) will blur larger areas, while low values (<40) will only blur small portions of the image. This means that using a high value for Max Delta will cause more details to be blurred.

The Radius parameter controls the amount of blur that is applied to selected parts. High values (>7) produce a high level of blur, and low values (<4) produce a weaker blur effect.

When you use this filter to remove noise, we recommend that you start with the default values (5, 50). If the default values prove too strong, then lower the Max Delta parameter until you are satisfied. If this still doesn't remove your noise, then raise the blur Radius. If a higher blur level should reduce details in the image, then lower the Max Delta value.

Another application for this filter is for accentuating the focused part of the image. Since Selective Gaussian Blur removes noise, it will also smooth less focused parts of the image, thus adding depth to the image, and highlighting the focused parts. This function is ideal when you work with amateur portrait photographs.

## Tileable Blur

Tileable Blur is an excellent tool for softening the tile seams in images you want to use in tiled backgrounds. Tileable Blur compares the pixels that will end up next to each other when the image is tiled, and compensates for this by blending/blurring those edges as if they had been adjacent.

The Blur Vertical/Horizontal and Radius functions are the same as described in the Gaussian Blur filter. The RLE and IIR options are for computer-generated and photographic images, respectively, just as with Gaussian Blur RLE/IIR.

To avoid blurring the entire image, use `right-click|Select|``Feather` to select the important parts of the image (the parts that you don't want to blur) and `right-click|Select|``Invert` the selection before you apply the filter.

Note that Tileable Blur is a script, so it can't be reversed with Undo.

## Variable Blur

| Variable Blur is much like ordinary blur, but it allows you to regulate the amount of blur by adjusting the Mask Size. A large Mask Size will result in a high amount of blur, and vice versa.

## Antialias...

This Antialias... script allows you to fine-tune the contrast in the softened (antialiased) border. The antialiasing border is always 2 pixels wide, but you can set a preference for what the contrast should be between those pixels. The default value (0.33333) produces a normal antialias border, with an even distribution of 1/3 of the new color in the first pixel and 2/3 in the second pixel.

A lower value results in low contrast between the two pixels because they'll both get a similar intermediate color, while high values will increase the contrast between the pixels. This script uses the Antialias filter described in the beginning of this chapter.

Figure 0.1 The upper gear has been blurred with Radial motion blur, which makes the gear appear to be in movement, while the chain and lower gear remain still. This is naturally impossible in real life, but Gimp makes the illusion seem real enough.

Figure 0.1 We have blurred the surroundings of the running man (inverted selection of the man) with the Zoom blur, making him seem to be flying toward us at the speed of light

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