Introduction to GIMP Image Editing Software
GIMP is a free open source image editing program. It has a comprehensive set of tools comparable to those available in commercial software such as Adobe Photoshop. GIMP can help you with simple and advanced image editing tasks. This document will look at the GIMP features that are of most use to those wanting to edit images for use in teaching and research.
- What is GIMP?
- The GIMP user interface
- What features of GIMP are useful to teaching and research staff?
- A selection of useful filters and actions
- Other useful features
- Limitations of the software
- Further resources
GIMP is a free open source image editing program. GIMP stands for the 'GNU Image Manipulation Program' (GNU is itself a recursive acronym for 'GNU's Not Unix'). It is also referred to as 'The GIMP'. It has a comprehensive set of tools comparable to those available in commercial software such as Adobe Photoshop.
GIMP can help you with simple and advanced image editing tasks, such as photo retouching, image composition, image optimisation and image creation.
This document will look at the GIMP features that are of most use to those wanting to use images in teaching and research. It will not describe each and every tool, but it should serve as a useful starting point for those new to GIMP.
Note: at the time of writing (July 2010) we used the most recently available version: GIMP 2.6.10 for Windows. Because GIMP is open source software with a wide developer community, new releases and updated versions are made available more frequently than with commercial software. It's also available for Mac OS X and Linux.
You may also be interested in our advice on Image Editing Software. Our guide to Free Online Image Editors is also worth looking at if you need to undertake simple image editing tasks without having to install software.
The user interface on the Windows version of GIMP can take a little time getting used to, but the tools on offer make it worth investigating, especially if cost is a major consideration.
The first thing you will notice is how there is no main 'container' window for the individual tools, palette and image windows. On launching GIMP for the first time you have three windows - the central one is where your image will sit along with the menu bar, while the other two contain the tools, properties, features and palettes. You can dock or undock each of these (i.e. move them away from their original positions to new areas of the screen).
A useful tip for new users: Once you are happy with the layout of GIMP's various dialogue windows in relation to each other, go to the Edit > Preferences menu and under Window Management select 'Save Window Positions Now'. Next time you open GIMP, the windows will be positioned as you saved them. You can also choose to 'Save window positions on exit' so that it will open with however the windows were positioned last time you closed down. To restore the original settings, select 'Reset Saved Window Positions to Default Values'.
Getting to know how the interface best works for you is key to using GIMP effectively. However, this document is not going to take you through the details of how to customise the interface (docking/undocking windows, adding/moving tools in the toolbox, etc). All this and more is already well documented in the user manual. For a good introduction to finding your way around the workspace, see the manual's third chapter.
GIMP offers a full range of tools, many of which may be beyond the needs of those looking for a basic image editor. If you're after something more basic, see our guide to Free Online Image Editors. However, don't be put off by its advanced features - GIMP is also well suited to the more basic tasks you may need to make. Here we look at some of the more commonly used tools and features:
Opening images in various file formats
File > Open
GIMP can open many common image file formats (including TIFF, JPEG, GIF, PNG) as well as EPS, PDF, SVG, Photoshop PSD files and Paint Shop Pro PSP files. This is particularly useful if you have been given an image saved, for example, in the Photoshop file format (PSD), but don't have a copy of Photoshop. It can also open and read many Raw file formats from digital cameras.
Saving images into different file formats
File > Save as
GIMP can save images in many different formats. GIMP's native file format is XCF, and this should be used for any files you intend to edit again with GIMP. Images destined for the web, PowerPoint or VLEs should be saved as JPEG, GIF or PNG. You can also use these formats for images you intend to print on a desktop printer, but you need to be aware that the quality may not be as good if a JPEG has been compressed, and that GIF images only contain 256 colours. Certainly if you are having something professionally printed, you will need to think carefully about the format and the effect compression will have on the quality (for further details see JISC Digital Media's advice on File Formats and Compression). You can also save images in the Photoshop format (PSD) if anyone needs to work on them in Photoshop (but see Limitations of the software below).
The Crop tool is one you will use all the time - for trimming an image to the area of interest, or removing unnecessary edges and distracting backgrounds.
Image > Scale Image
You will often need to scale an image in order to make sure it is the right size for its intended use. For example, you may need to create smaller versions of images that are too large for screen display. Making images larger than their original size is not usually recommended, as it will often have a detrimental effect on the visual quality.
Flipping and rotating images
Image > Transform > . . .
This allows you to rotate an image in 90° increments, and flip it along its horizontal or vertical axis. You can also 'free rotate' to any angle and flip layers or selections using these buttons in the Main Toolbox:
Colours > . . .
It's easy to make adjustments to brightness and contrast, colour balance and saturation, or to convert images to greyscale (black and white) in just a few steps. More precise adjustments can be made using Levels and Curves.
Cleaning, healing and 'dust busting'
The Clone tool (left) is used to sample and paste 'clean' parts of an image over dust marks, scratches and other areas you want to remove. The Healing tool (right) can be used in a similar way to retouch blemishes and other marks.
Acquiring images directly from a scanner or camera
File > Create > Scanner/Camera
If you have a camera or scanner connected to your computer, you can import images directly from those devices into GIMP.
Creating/editing images using drawing, painting and text tools
The Pen and Paintbrush tools can be used to create images from scratch or to add elements to existing images. The brush size, hardness, opacity and colour can all be edited easily. You can use the Text tool to add text using any of your system's fonts, and edit font size, colour, line spacing, justification, and various other parameters.
Creating simple animations
Filters > Animation > Optimise (for GIF)
Very basic animated GIFs can be created by placing each frame of your animation on a separate layer, then going to Filters > Animation > Optimise (for GIF). You don't get many options (e.g. you can't set the time between each frame, or stop the animation from looping). To playback the animation, go to Filters > Animation > Playback. The optional GIMP Animation Package plug-in has more features.
You can use GIMP to apply different effects to an image using filters and actions. Some of these are gimmicky or artistic (e.g. adding coffee stains to an image, making a photo look like a cubist painting) while the ones we look at below are of more practical use.
Most of them can be found under the Filters menu - in some cases they are applied as a series of actions (or what GIMP calls 'Script-Fu's' - small scripts that run a set of commands in sequence.) After a Filter has been applied, it is possible to see each step that has been taken by looking at the Undo History (File > Dialogues > Undo History). You can then step back and forth through each step and make adjustments if necessary. See below for more on the Undo History.
Adjusting depth of field
Filters > Blur > Gaussian Blur
The Gaussian Blur filter can be used to simulate narrow depth of field i.e. where one area of a photo is in sharper focus than the rest. Normally you would do this when taking the photo by adjusting your camera settings, but if your camera doesn't have aperture control or you have an image that needs adjusting, you can do so easily in GIMP. This can be useful if you want to focus attention on a particular area of a photograph:
Before (left) and after Gaussian Blur is applied to a selection (selection shown by dotted lines in left image made using the Lasso tool)
Note: the Preview checkbox available with many of GIMP's filters provides a useful way of seeing how an action will affect the original image - simply tick it on and off to switch between the before and after states.
Filters > Blur > Pixelise
The Pixelise filter can be used to distort part of an image, e.g. to anonymise a person.
Sharpening 'soft' images
Filters > Enhance > Unsharp Mask
When used with care, the Unsharp Mask can help sharpen images that are slightly 'soft'. It is particularly helpful for enhancing smaller thumbnail images:
Before (left) and after appplying the Unsharp Mask to sharpen a thumbnail size image
Filters > Light and Shadow > Drop-Shadow
Drop shadows can be used to 'raise' an image from the page. It is possible to adjust various parameters for the shadow, such as its opacity, blur radius, colour and how far it is offset from the image.
Drop shadow applied to an image
Filters > Decor > Border
A quick way of adding a border around an image - you can adjust the size and colour of the border.
As you become more familiar with GIMP, you will learn which features are best suited to the work you need to do. Listed here are some of the features that new users may not be immediately aware of, but which will help you get the most from the program and save time.
Select the Layers tab on the righthand window - if the window is not there select Windows > Layers, Channels, Paths, Undo...
Layers are common to most image editing programs and help you organise the different parts of an image. As soon as you need to add a new element to an image, it's a good idea to add it to a new layer.
Layers allow you to work on the different parts of an image individually. So, if you want to move part of the image, you can do so by selecting the layer it's on rather than the entire image. Layers are arranged in a stack, with the top most layer appearing at the front of the image:
An image made up of four layers. The Layers window on the right shows the stacking order
You can move layers up and down the stack either by dragging them or using the green arrows at the bottom of the Layers window. You can also link layers to other layers (so they move as one), merge them together (into one layer - they will no longer be individually editable), hide them, delete them, duplicate them and rename them. In the above screenshot, the selected text layer is highlighted in the Layers window and also has a dotted line around it in the image window. Clicking the eye symbol next to a layer hides/unhides the layer. The Opacity slider (seen above set to 100%) can be used to fade a layer into the one beneath it.
Select the Undo History tab on the righthand window - if the window is not there select Windows > Layers, Channels, Paths, Undo...
The Undo History keeps a record of all the actions you have made since opening the image, enabling you to undo and redo previous steps. You can set the number of undo levels, and the amount of system memory your computer uses to record them, in the Preferences menu (Edit > Preferences > Environment).
These four tools (Rectangle, Ellipse, Lasso/Free Selection, Magic Wand/Fuzzy Selection) let you select rectangular, elliptical, hand-drawn or colour-similar areas of your image.
Once selected you can move, duplicate, delete or apply filters and other actions to the selected area.
You can 'feather' selections (Select > Feather) to make them 'softer' at the edges. In our earlier example on using the Gaussian Blur filter to simulate narrow depth of field, we used the Lasso tool to hand-draw around the background part of the image. That selection was feathered by 12 pixels in order to make a more natural transition between the background and foreground. If the feathering were not applied, the image would appear more like it was made from two parts stuck together:
The different effects not feathering (top) and feathering (bottom) a selection can have on an image
Compare the two images where the blurred background meets the top of the hand and the edge of the monitor. In the top image you can clearly see how the hard edge of a non-feathered selection made using the Lasso tool has left the image looking messy. The bottom image shows how feathering the exact same selection by 12 pixels can help blend the two areas.
Feathering also allows you to be a little less precise when drawing around an object: here we made a couple of deliberate small triangular 'dents' into the hand, which are visible in the top image, but unnoticeable on the feathered version.
Here's an example of how the Lasso selection tool can be used in combination with the rectangular selection tool to create a torn image effect - useful if you want to suggest that you are not showing an image in its entirety:
Image of an incomplete table with a torn effect at the bottom. We used the rectangular selection and Lasso tools to draw the jagged edge then used the drop shadow filter to lift it slightly from the page
Rulers, guides and grids
View > Show Rulers; View > Show Guides; View > Show Grid
The rulers on the top and left edges of the image window show the image dimensions (in pixels by default, other units can be chosen using the drop-down menu in the bottom left-hand corner of the image).
By clicking and dragging either ruler, you can bring horizontal and vertical guidelines onto the image. These are particularly useful if you need to precisely align two or more elements within an image.
For even greater accuracy when positioning parts of an image, you can place a grid on top of the image. You can set up the spacing, line style and colour of the grid by going to Image > Configure Grid.
Neither the grid nor the guides are included when printing the image.
Open recently opened files
File > Open Recent
The Open Recent command lists the images you last opened, and very usefully places a small preview image to the left of each filename. The preview images are pretty small, but just about big enough to distinguish between different images. As you move your mouse over each filename, a larger preview appears:
The Open Recent list with preview images to the left and a larger preview of each image as your move your mouse over each filename
If you select Document History at the bottom of the list, you get a more complete list of previously opened images than the Open Recent list. It also displays the image dimensions alongside the filename and you can adjust the preview size (from 'Tiny' through to 'Gigantic').
You can remove items from the Document History, which are then removed from the Open Recent list - this does not affect the actual images or their location, but simply removes them from the list of recently opened files.
Making screenshots of your computer's desktop
a) File > Create > From Clipboard
This is incredibly useful if you ever need to create materials that include screenshots taken from your computer's desktop. This is how the images in this document were created. Simply press your keyboard's Print Screen button (usually to the top right of the keyboard. There's no such button on a Mac keyboard, use Command-Shift-3 instead), then in GIMP go to File > Create > From Clipboard and your entire desktop will be displayed as an image. You can then crop to the relevant area and insert the image into a Word document, a PowerPoint presentation, VLE or other application.
You can also use this method to 'acquire' any other image you have copied to the computer's clipboard.
b) File > Create > Screen Shot. . .
This is very similar to the Print Screen method described above, but as well as allowing you to capture the entire screen, gives you the option of limiting the screenshot to a specific window.
Navigating around large images
When an image is too large to fit the window (or has been zoomed beyond the window size), you can navigate around the image by clicking and dragging the icon in the bottom right hand corner. On clicking the icon, a small preview of the entire image will appear. As you drag the icon around the preview image, a rectangle marking your position will move in relation to the main image window:
Navigation control - moving around a large image with the navigation preview
The size of the preview image can be adjusted (from 'Tiny' to 'Gigantic') in the Preferences menu under Edit > Preferences > Interface > Navigation preview size.
If you open the Navigation window (View > Navigation Window), you get a dedicated (and larger) navigation preview window with a slider for zooming and further options.
Despite its many useful features, there are a few areas where GIMP does not yet compete with software such as Photoshop. The user interface has already been mentioned, but it would also be good to see future versions address the following limitations (note that as an application in constant development it may not be too long before some of these are taken up):
- 'Save for web' visual previews - although you get a preview for saving as a JPEG, GIMP can only show the effect of one setting at a time (where Photoshop can preview four different settings/file types). There are no visual previews when saving as a GIF or PNG.
- Ability to record actions - although GIMP has its own pre-recorded actions ('Script-Fu's, see A selection of useful filters and actions above), it does not allow you to record your own actions, so you cannot for example batch process the resizing of a number of images
- Support for 16-bit images - GIMP cannot handle images that are greater than 8-bits per channel. On opening a 16-bit image GIMP gives you a warning that the image will be converted to 8-bits per channel. This will not be a problem if you are working with JPEGs and GIFs (as JPEGs are a maximum of 8-bits per channel and GIFs are 8-bits for the entire image), but many other file types (such as TIFF, PNG, PSD) can be 16-bit, and conversion to 8-bit will lose information.
- Greater support for PSD files - you can save GIMP images as Photoshop PSD files - any layers you added in GIMP are recognised by Photoshop, but if any are text layers, you will not be able to edit the text in Photoshop.
- Integrated Raw format support - by default GIMP is unable to open Raw camera files. If you try to open a Raw file you may get a small JPG preview image. However, GIMP can open Raw files if you install the UFRaw plugin.
- The GIMP User Manual
- GIMP Tutorials
- GIMP Plug-In Registry
- JISC Digital Media: Free Online Image Editors
- JISC Digital Media: Finding and Using Images
- JISC Digital Media: Using Images in Education
- JISC Digital Media: Image Editing Software