Manipulating Images

Part IV
Exploring Filters
and Effects
IN THIS PART
Chapter 13
Implementing Blur,
Enhancement, and Distortion
Filters
Chapter 14
Using Image Creation Filters
Chapter 15
Using Compositing Filters
Chapter 16
Enhancing Images with Artistic
Filters
Chapter 17
Working with Specialized
Filters
Chapter 18
Batch Processing with
Automating Filters
Chapter 19
Using GIMP Animation
Package
Chapter 20
Working with Video-Specific
Functions in GIMP

Implementing Blur,
Enhancement,
and Distortion Filters
IN THIS CHAPTER
Introducing GIMP’s filters
Increasing depth and focus
by – ironically – using blur
Using automated ways of
cleaning up images
Deforming your images and
actually making them look
better
T
his chapter marks the beginning of an entire section of this book
devoted to GIMP’s extensive and impressive list of filters. Simply put,
a filter is a small program that takes image data as input, performs
one or more processing steps on that image data, and returns a modified
version to you. The filter may require additional input from you in the form
of settings and parameters to help customize and control those process-
ing steps. In the case of most GIMP filters, the active layer (or your cur-
rent selected pixels in that layer) serves as the image data that’s fed to the
filter.
This chapter covers some of the most commonly used filters in image edit-
ing. The Blur filters are designed to obscure details, but in using them you
can focus the viewer’s attention on more important parts of your images. In
GIMP’s array of Enhance filters, you’re given tools that allow you to accen-
tuate specific details or all details in your images. The filters in the Dis-
torts submenu can take your image data and dramatically transform it into
something that only hints at what your original image was. It’s really quite
exciting.
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Part IV: Exploring Filters and Effects
Filters Anonymous: A Serious Note on Filter Abuse
W
ith this chapter and the seven others that belong in Part IV, you get to explore the incredible
world of GIMP filters. A word of warning, especially if you’re new to computer graphics: filters
are incredibly powerful tools that can produce some seriously cool effects on your images. Why is this
a cause for warning? Well, just because you
can
do something, that doesn’t mean it’s always a good
idea. It’s a common thing that you see among people who are just starting out with digital imagery.
They have hard drives just chock full of over-processed images where they’ve taken a perfectly passable
photograph that may need just a little bit of work and run nearly every one of the available filters
and plug-ins at their disposal. The final result may look cool or interesting because of how drastically
different it is from its original state, but that doesn’t necessarily mean that it’s a good image.
Now, that’s not to say that there isn’t a time and a place for heavy processing every now and again.
I’m sure on some occasions even using a lens flare makes sense and doesn’t look too cheesy. But it’s
really worth it to sit down and try to understand what an image
needs
rather than what’s going to
make it look ‘‘cool.’’ Furthermore,
anybody
can go to the Filters menu and create one of these special
effects with the click of a few buttons. It’s a ‘‘canned effect’’ and by itself, it usually lacks the punch
that you’d want to give an image. A good digital artist knows how to make the most of a filter by
using it minimally where it’s needed and combining that with skillful use of more manual tools to get a
powerful and unique final image.
Filters are a little bit like alcohol. When used with moderation, they can be a lot of fun and do a lot
to enhance the look of your images. However, when used in excess, there’s a good chance that you’re
going to puke all over your image and make it a muddy mess. Use filters responsibly.
Common Features among Filters
Most of the filters in GIMP share some common features. The most obvious of these features
is the fact that they’re all accessible from the Filters menu in the image window, shown in
Figure 13-1.
In this menu, the first four items are incredibly useful:

Repeat Last (Ctrl+F) — Click this menu item to launch the last filter you ran, using all
of the same settings from the last time you used it. Using this function’s keyboard shortcut,
you can quickly apply the same filter to multiple selections, layers, or even images. An
important thing to note about this feature is that it remembers only the last filter you used
in your current GIMP session. If you close and restart GIMP, this item is grayed out and
inaccessible.

Re-show Last (Shift+Ctrl+F) — This option works just like the Repeat Last feature,
but instead of blindly reusing the same filter settings, it shows you that filter’s dialog — if
it has one — and allows you to make changes before re-running it. This is a great feature
when you’re trying various settings on a filter on a particular image. You can preview the
changes it makes within the filter’s Preview pane. You can run the filter, Undo (Ctrl+Z)
the processed effect(s) if you don’t like the results, and then re-show that filter’s dialog
(Shift+Ctrl+F) to try out new settings. Some filter effects do not have a Preview pane, so
the effect is previewed within the image window. You can view the effect, Undo (Ctrl+Z)
and then try something new (Shift+Ctrl+F).
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Chapter 13: Implementing Blur, Enhancement, and Distortion Filters

Recently Used — The submenu that this option reveals shows a list of all the filters and
plug-ins you’ve used in your current GIMP session. Click any one of them and that filter’s
dialog window appears with its last-used settings. This menu item and its associated sub-
menu appears only if you’ve run plug-ins or filters in this session. Otherwise it’s hidden.

Reset All Filters — Typically when you re-show a filter, whether by using the Re-show
Last feature or just by choosing it from its window a second time, that filter is shown with
its last-used settings. In most cases, this is the desired behavior and it’s great for keeping a
speedy workflow. However, on occasion, it’s more useful to start fresh with a filter using its
default values. This is especially true on some of the more complex filters. Click this menu
item to return all filters in your GIMP session to their default values.
Note
The Repeat Last, Re-show Last, and Recently Used menu items actually work on more than just the oper-
ations that are accessible from the Filters menu. Any installed plug-in that registers within GIMP can be
called from these functions. For example, the Autocrop Layer feature found in Layer

Autocrop Layer is
actually implemented as a plug-in. When you run it, it’s added to the Recently Used submenu and is repeat-
able using the Repeat Last and Re-show Last functions. This is convenient if you want to autocrop a lot of
layers without navigating through the menu because you can just use the Ctrl+F shortcut. However, it can
be disorienting to see operations that aren’t really filters listed at the top of the Filters menu. Fortunately,
now you know why that happens and it should be less troublesome for you. 
FIGURE 13-1
All of GIMP’s bundled filters and many of the plug-ins you can download are included in the Filters
menu of the image window.
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Part IV: Exploring Filters and Effects
Another common feature that many of GIMP’s filters have is a small preview window.
Figure 13-2 shows three typical versions of the preview windows you may run into.
FIGURE 13-2
From left to right, a basic preview window, one with panning ability, and one with zooming ability
As the figure shows, you can have a basic preview window that shows only a thumbnail view
of your image, a slightly more advanced version with panning navigation, or a deluxe preview
window that allows both panning and zooming. Most preview windows are of the middle vari-
ety; they show a 100% zoom version of your active layer or selection and you’re allowed to pan
around that space by clicking and dragging your mouse within the window’s space. You can also
click the Navigation crosshairs in the bottom right of the preview window to move the preview
area interactively around a thumbnail version of your image. This works just as the same button
in the image window works.
Tip
The common panning-only preview window can be pretty frustrating to work with on larger images because
it shows only a small square of the available pixels in the image. Though there’s no easy way to zoom this
preview, you can increase its viewable size. Simply resize your filter’s dialog window and make it bigger
by clicking and dragging one of its borders. The preview window should adjust to accommodate for the
newly available space. You’re still not zoomed out on your image, but this does provide you with greater
context. 
The panning-only and panning/zooming varieties of preview windows have one additional con-
trol on them. Right-click in the preview window and a small context menu appears. If your
active layer or selection has an alpha channel, you can dictate how the preview window displays
transparent regions using these two menu items:

Check Style — This controls the checkerboard style that’s used to represent the trans-
parent regions of your image in the preview window. You can choose light, medium, or
dark checks, or you could forgo the checkerboard altogether and set transparent areas to
be solid white, gray, or black.

Check Size — If you do choose to show the checkerboard pattern in the preview win-
dow, this second menu item controls whether those checks are small, medium, or large.
On images with a lot of detail, it’s often beneficial to have larger checks so the view isn’t
cluttered and noisy.
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Chapter 13: Implementing Blur, Enhancement, and Distortion Filters
Using the Blur Filters
It may seem a bit ironic that in a medium that has an implied goal of communicating visual
information some of the most commonly used tools are filters designed to make details more
difficult to see. Strange as that may be, it’s definitely true. Filters in the blur category are use-
ful for everything from creating glows and drop shadows to implying motion and even assisting
with image restoration. As a matter of fact, quite a few other filters in GIMP, such as some edge
detection filters, call filters from Filters

Blur as a step in their processing. From an artistic per-
spective, blurring part of an image is an excellent way to guide viewers to focus their attention
on specific parts of your composition.
Blur
This first item on the Blur submenu is the simplest of the set. Run it by going to Filters

Blur

Blur. When you call this filter, it runs immediately; there is no dialog window for it. Each pixel
in your active layer or selection has its color values mixed with the color values of the pixels
adjacent to it. It runs very fast, even on larger images. However, if you are using this filter on a
very large image, it’s likely that the results will be too subtle for you to notice. It’s not uncom-
mon to see people running this filter multiple times on larger images. In contrast, when you run
this filter on smaller images, the results are much more pronounced. The best applications of
this filter are for slightly softening an image or a quick-and-dirty means of antialiasing an image
with extremely harsh edges or color transitions. However, if you find yourself running the Blur
filter and then pressing Ctrl+F another dozen times, you may want to consider using one of the
other filters in the blur category. Figure 13-3 compares the results of running the Blur filter on
an image once versus calling the Blur filter 20 times in a row.
FIGURE 13-3
From top to bottom, the original image, blurred once, and blurred 20 times
Original
Blur 1x
Blur 20x
Gaussian Blur
The Gaussian Blur algorithm is probably the most commonly used blurring algorithm in com-
puter graphics, and for good reason. It works quickly, produces consistent results, and is easy
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Part IV: Exploring Filters and Effects
to control. It’s the go-to filter that digital artists rely on when they need to blur any part of an
image. When you run the Gaussian Blur filter in GIMP (Filters

Blur

Gaussian Blur), you
should get a dialog window like the one shown in Figure 13-4.
FIGURE 13-4
The Gaussian Blur dialog gives you refined control over blurring your image, including the amount
of blur in horizontal and vertical directions and the blurring method used.
Below the preview window in the Gaussian Blur’s dialog are two primary controls for the nature
of your blur:

Blur Radius — These two numeric entry fields control the strength of your blur. By
default, they’re chained together so changing one value changes both. However, you
can unchain the fields and blur your image more in one direction than in the other. The
default unit for the Blur Radius is pixels, but you can use the units drop-down next to the
Vertical text field and use any of the other units that GIMP supports.

Blur Method — The Gaussian Blur filter offers two different methods for implementing
the blur. They produce identical results, but depending on the content of your image, one
may run faster than the other:

IIR — An abbreviation for infinite impulse response, this Gaussian Blur method is most
effective on images with a lot of variety in them, such as photographic content. It’s also
a good option to choose if your Blur Radius values are set high.

RLE — The run-length encoding (RLE) method of Gaussian Blur works best on images
with large sections of flat (or near-flat) color.
Figure 13-5 shows the results of using the Gaussian Blur filter on an example image. You
can also look back to Chapter 6 to see how the Gaussian Blur can be used to create a drop
shadow effect.
Motion Blur
By unchaining and isolating the Horizontal or Vertical Blur Radius values in the Gaussian Blur
filter, you can create a rudimentary motion blur effect in either the horizontal or vertical direc-
tions. However, what if you want to have motion blur at an arbitrary angle? Or what if you
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Chapter 13: Implementing Blur, Enhancement, and Distortion Filters
FIGURE 13-5
The Gaussian Blur filter can very easily blur an image into obscurity.
Original 5px blur 25px blur 100px blur
want to get a blur effect that looks like the camera is quickly zooming in on your subject? These
effects are difficult or impossible using just the Gaussian Blur filter. Fortunately, GIMP includes
the Motion Blur filter and it can easily create these effects as well as a couple more. Activate this
filter by going to Filters

Blur

Motion Blur and you get a dialog like the one in Figure 13-6.
FIGURE 13-6
The Motion Blur filter’s dialog
Beneath the preview window in this dialog are three primary controls for dictating the nature of
the blur that the filter produces: Blur Type, Blur Center, and Blur Parameters. The following is a
description of each control and the options available within each one:

Blur Type — The Motion Blur filter gives you multiple varieties of blurred motion:

Linear — This is similar to using the Gaussian Blur filter. The difference, however,
with this blur is that you can blur in any arbitrary direction, not just horizontally or
vertically.
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