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Introduction

What is the Gimp? Gimp stands for the GNU Image Manipulation Program. Gimp is a freely available, GPL licensed software for doing image touchups and creation. It understands a variety of input formats, and can acquire images from digital cameras, scanners, and from screenshots. It is a bitmap based editor. I.e., it operates on individual pixels of an image rather than with graphical object.

Gimp is often compared to Adobe Photoshop. This comparison is somewhat valid, but Gimp can do quite a few things that Photoshop cannot, and vice versa. Most folks will find that Gimp can satisfy all their image creation and editing needs.

How can I install it? All the major distributions now ship Gimp on their installation CDs. If you decide to build Gimp from scratch, make sure that you have the full set of development libraries installed, including libpng, libgd, etc.. The Gimp website at www.gimp.org has the newest sources available.

What are the system requirements? Like anything, hardware requirements depend heavily on what sort of images you intend to edit. You could run Gimp with 16 colors and 32M RAM. I don't. A more realistic setup would be 128M RAM and about a Pentium 233/MMX or equivalent. Keep in mind scanned TIFF images are huge, on the order of several megabytes per image.

User Interface

Keyboard Controls To make efficient use of the Gimp, try to take a few minutes to learn the extensive keyboard controls. Almost every option has a keyboard equivalent and learning them will speed up your editing sessions dramatically.

Tool Name	Keystroke
Airbrush	a
Bezier Select	b
Blend	l
Bucket Fill	S-b
Clone	c
Color Picker	o
Picker Convolve	v
Crop & Resize	S-c
Default Colors	d
Dodge & Burn	S-d
Ellipse Select	e
Select Eraser	S-e
Flip	S-f
Free Select (Lasso)	f
Fuzzy Select (Magic Wand)	z
Ink	k
Intelligent Scissors	i
Magnify	S-m
Move	m
Paintbrush	p
Pencil	S-p
Rectangle Select	r
Select Smudge	S-s
Swap Colors	x
Text	t
Transform	S-t
XinputAirbrush	S-a

Figure: Main Gimp Toolbar

Tear Off Menus Each Gimp menu can be ``torn off'' by clicking on the dashed line on the menu. This allows the most frequently used menu options to be readily available on the workspace.

Image Acquisition

SANE/TWain Scanners supported by the SANE project can be directly accessed from within the Gimp application. These include most SCSI scanners, a large number of parallel port models, and increasingly, lower end USB models. If the correct plugins are installed, scanners will appear under the "Acquire" option of Gimp.

Using the Twain interface, compliant digital cameras can also upload directly into the Gimp. ¹ There are even v4l (Video For Linux) drivers that allow television/composite images to be loaded directly into Gimp.

Tablets Several tablet devices are supported by Gimp, allowing a more natural way to draw or paint images. You will likely need to recompile Gimp to support the extended features such as pressure senstivity, brush direction, etc.. Otherwise, tablets will appear to Gimp as just another mouse device. Consult the Wacom Tablet HOWTO for more information.

Image imports, screenshots Gimp supports image imports from a variety of formats. Its native format is XCF, and this is the best format for saving in-progress works and image masters since all layer information is retained. Among the supported formats are JPG, GIF, PNG, TIFF, PSD (PhotoShop), even old Atari ST and Amiga images. Screen captures are also available.

Basic Photo Editing

If Gimp is launched interactively, the first screen that appears is the tools window.

Tool Options

Double-clicking on any of the tool icons will bring up tool options specific to the selected tool. Use the options window to change the behaviour of each tool. E.g., double clicking on the Brush icon allows the brush size and mode to be changed.

Selection Tools

The first group of tools are for selecting sections of an image for editing. Besides facilitating the cut-and-paste operation, they allow tools and filters to work only on a specific area of an image. Once a region is selected, you can invert the selection. This is useful when eliminating a background or when the region to be selected is complex and difficult to define otherwise.

Rectangle This is the most straightforward selection tool. Click and drag on the image to define a rectangular selection region. Hold down the SHIFT key while dragging to define an equilateral region (square).

Figure: Rectangular Selection

Ellipse Use this selection tool to define elliptical regions. Hold down the SHIFT key while dragging to define a circular region.

Figure: Elliptical Selection

Free Hand (Lasso) Use this tool to draw a selection region. To make selection easier, try increasing the maginification of the image.

Magic Wand Use this selection to to select contiguous regions of a color or color range. Click and drag the mouse pointer horizontally to change the color range to be selected. This tool is effective for removing non-uniform color areas such as a color gradient.

Bezier Curve This tool defines regions based on Bezier curves. Bezier curves are defined using two or more control points. Gimp will then calculate a smooth curve based on the control points. Once a region is defined by closing the curve (i.e., clicking on the first control point), you can use the control points to deform the curve. Use the CONTROL key to move a control point. Use the SHIFT key to grab only one handle. Click once inside the selection region to commit changes to the curve.

Figure: Bezier Selection

Polygon This tool is useful for selecting shapes where the Magic Wand cannot. Click around the border of a region and Gimp will automatically fit a curve to the border of the region, based on the contrast between regions.

Transform Tools

Transform tools operate on a selection or the entire image. Use these tools to manipulate the shape, orientation, or size of an object.

Move Layer or Selection The Move Layer or Selection tool will translate a selection elsewhere on the screen.

Crop or Resize Use the Crop and Resize tools to change the size of your image. Cropping an image will delete everything but the rectangular selection region. Resizing will scale the image to a new size. Please note that if an image is reduced by resizing there will be a loss of detail.

Figure: Crop and Resize

Transform Tool Use these tools to change the orientation or size of an image or selection. Figure  shows common options.

The Rotation tool can be used to correct upside-down scans, images taken in portrait mode (i.e., the camera turned to the side), or landscapes where the horizon is not level.

The Scaling tool can resize an image or selection.

The Shearing Tool will, umm, shear an image selection. The selection can be non-rectangular, but a second shearing map will appear over the selection region.

The Perspective Tool deforms a selection to give the appearance of depth. This tool is useful for creating ``3D'' images.

Figure: Transform Tool

Flip Tool The Flip Tool flips an image horizontally or vertically. Note that this transformation is different than a $180^o$ rotation.

Figure: Flip Tool

Text

Use the Text tool to add text to an image. Remember that once text is added there is no easy way to remove it except by reverting to an earlier version on disk.

Image Modification

These tools operate directly on the image. I made this distinction with the transform tools mainly because the modification tools work on a per-pixel basis, rather than on a selection.

Color Picker Use the ``Eye Dropper'' to choose the active color from one found in the image. This is useful for fixing scratches and drop-outs in an image.

Bucket Fill The Bucket Fill will fill a selection or image with a color or pattern. The fill will limit itself to areas of contiguous color. Use the Threshold slider to change how much ``bleeding'' will occur across color borders.

Blend Tool The Blend Tool creates gradient fills from one color to another. The fills can be linear (moving evenly from the foreground to background colors across a range), or more complex such as starburst or radial patterns. Using this can help to create a 3D effect for simple graphics.

Figure: Blend Tool

Pencil Use the Pencil for freehand drawing. Pressing the CONTROL key will bring up the Color Picker tool. Pressing SHIFT will enter a polygon mode and allow you to create straight lines.

Paintbrush The Paintbrush is similar to the Pencil, except that the ``paint'' will fade at the end of the brushstroke. If the Opacity level is reduced, you can create interesting water color effects.

Eraser The eraser is similar to the Pencil, but ``paints'' with the background color. Selecting the Hard Edge option will turn off anti-aliasing.

Airbrush The Airbrush creates an Airbrush (!!) effect by slowly ``spraying'' the foreground color onto the image. I.e., the opacity of the effect will increase with each pass.

Clone Tool The Clone Tool is one of the more interesting image tools. With it, sections of an image can be replaced with another section of the same image. This allows you to remove undesirable elements of an image. E.g., use the clone to delete rust stains from a picture of a house or remove telephone wires from a scenic landscape. To use, hold down the CONTROL key to select the image source. Select a brush size from the tools dialog. Finally, paint over the unwanted sections of the image. The crosshairs will show which portion of the image is being cloned. Figure .

Figure: Clone Tool

Blur or Sharpen The Blur tool will change the apparent contrast between neighboring regions of high contrast. This effect is useful for blending pasted regions with the background, or even to minimize defects in an image. For example, images taken in low light conditions with a digital camera may appear splotchy. Use the blur tool to minimize this effect.

The Sharpen tool does the opposite, and increases the contrast between regions of lower contrast. This has the effect sharpening details. ²

Ink Tool The Ink Tool mimics a calligraphic pen. I.e., diagonal and horizontal strokes are thinner than vertical strokes. This creates a ``Black Letter'' effect. Adjust the sliders to vary aspects of the pen, such as tilt or width. Figure  shows the options for the Ink Tool.

Figure: Ink Tool

Dodge or Burn Use this tool to selectively darken or lighten portions of an image. It can be configured to modify the darkest, lightest, or middle tones of an image to allow selective highlighting. Unlike the Airbrush, the Dodge/Burn tool does not have a cumulative effect.

Smudge The Smudge tools will create a smearing effect by shifting and strething pixels in the direction of mouse movement. This is useful for blurring lines and softening edges.

Measure Tool

Use this to measure distances or angles.

Advanced Editing

Layers

TBA

Web Graphics

As mentioned, Gimp is highly suitable for creating and editing graphics destined for the Web. Besides the usual functions such as the ability to convert from one format to another, Gimp has a rich suite of scripting tools to ease the creation of buttons, logos, backgrounds, and image maps.

Animated GIFs

Many people see animated GIFs as the bane of the web, equating them with BLINK tags and scrolling marquees. However, they are useful in many cases when an animation can help convey information. There are two main ways to create animations in the Gimp, either using files or using layers to save the animation frames.

Files

Create your intermediate frames then save them to a new directory. Image names should be in the form filename####.ext. That is, use a four digit sequence number appended to the end of the filename.

Load the first frame of the animation into Gimp.

Right click the new image window and select ``VCR Navigator'' from the ``Video'' menu.

Layers

Transparent GIFs

Image Maps

References

http://www.silab.dsi.unimi.it/ ss546470/gimping/gimp_keyb_shorts.html

http://gimp-savvy.com

Bibliography

Knuth86

Donald E Knuth, The TEXbook, Addison-Wesley, 1986, ISBN 0-201-13447-0

Lamport86

Leslie Lamport, L^ATEX A Document Preparation System, Addison-Wesley, 1986, ISBN 0-201-15790-X

Rubinstein88

Richard Rubinstein, Digital Typography: An Introduction to Type and Composition for Computer System Design, Addison-Wesley, 1988, ISBN 0-201-17633-5

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