CS 424, Fall 2015, Lab 1

CS 424, Fall 2015:
Gimp 1: Tools and Filters

GIMP, the Gnu Image Manipulation Program, is a free program that has many of the capabilities of the better-known commercial program, Photoshop. Gimp can be used both for creating images from scratch and for modifying existing images. This is the first of two Monday lab sessions that introduce Gimp as an example of a 2D graphics application. The labs cover only a very limited subset of Gimp's features. It's easy to find documentation and tutorials on Gimp, starting with its "Help" menu.

We will be using Gimp 2.8, which is installed on our Linux computers and is also the latest version available for download at gimp.org. Gimp is available for Windows and Mac OS as well as for Linux.

When you first start Gimp, it will probably be in multi-window mode. I strongly suggest that you switch to single-window mode: Just check the option "Single-Window Mode" in the "Windows" menu. You will then see a window with a central area where you can work on images with dialogs along the left and right edges. The central editing area uses tabs when multiple images are open. There are also tabs on the dialog window to the right that allow you to access several different dialogs.

If you ever mess up the window layout, it can be difficult to figure out how to get it back. To do that: Go to the "Preferences" in the "Edit" menu (in Linux). Go to the "Window Management" section of the preferences. Click "Reset Saved Window Positions to Default Values." Restart Gimp for the changes to take effect. And finally, switch back to Single-Window Mode.

Assignment

The two Gimp labs will ask you to create or edit several images. The images that you create should be submitted in a folder named gimp in your homework folder, which is in /classes/cs424/homework. The file names should clearly identify the exercises for which the files are being submitted. For example, you could give the files names such as ex1.jpg or exercise7.png. I will not look for your work until one week after the second Gimp lab. The grading will be along the lines of: "Not done" for an F; "Minimal effort and incomplete" for a D; "Minimal effort" for a C; "Acceptable" for a B; or "Excellent" for an A.

New, Open, Save

Gimp's "File" menu has a "New" command that lets you create a new image from scratch. You will be able to set the size of the image and other properties, such as background color. And there is an "Open" command that lets you open an existing image for editing.

Saving is a little more problematic. The "Save" command will save an ".xcf" file, which is Gimp's own format. An xcf file is not an image, and it can only be opened with Gimp. It saves the full Gimp editing environment, which you would need for more complex projects if you want to be able to return to editing them later.

To save an image, instead of using "Save," you should use the "Export" or "Export As" command in the "File" menu. If you opened an image file for editing, the "Export" command becomes an "Overwrite" command that is used to replace the original image with the edited version. These commands let you save images in a wide variety of formats. For the lab, you should save your images in PNG or JPEG format. In particular, do not turn in xcf files!

Painting Tools

Exercises 1 and 2. For the first two exercises, you should use Gimp to draw two images from scratch, using Gimp's painting tools. Use several different tools. Try to make something that looks attractive and/or try to draw a (vaguely) representational image. Don't just turn in your scribblings!

Gimp has a wide variety of drawing tools, which you can find in the "Toolbox" in the upper left corner of the window. Point to a tool button to find out what the tool is for. Here is the Toolbox with a few annotations:

Below the Toolbox is the "Tool Options" dialog, which contains options for the drawing tool that is currently selected. The contents of the dialog change when you select a new tool. Here are the options for the Brush tool, which is probably the most basic and useful tool:

Use the "New" command in the file menu to create a new, empty image. Don't make it too big—maybe 640-by-480 or 500-by-400. Try out the Brush tool, and try changing some of its options. Try changing the foreground color (which is used for drawing) by clicking on the forgeground color patch in the Toolbox. Remember that CONTROL-Z can be used to undo any change you make. Also, of course, you can always start over. You will want to spend some time experimenting with various tools before setting out to create your final images.

You'll notice that Gimp does not have tools for drawing shapes such as rectangles and circles. However, it is possible to draw such shapes using selections. The selection tools—at the top of the Toolbox—can be used to select regions in the image. For example, click the Rectangle tool, and drag the mouse on the image to select a rectangular region. Note that one of the options for the Rectangle select allows you to round off the corners of the rectangle. The Ellipse Select tool can be used to select oval-shaped regions. The Free Select (or Lasso) tool, which is next to the Ellipse, can be used to select polygonal regions: Just click a sequence of points to select the vertices of the polygon, and click back on the initial point to close the polygon. You can also drag the Lasso tool to draw the outline of a region freehand. Once you have a selection, there are many things that you can do with it.

One important fact is that when there is a selection, you can only draw inside the selection—the area outside the selection is unaffected by painting tools, or by anything else that you try to do the image!

The Bucket Fill Tool, which looks like a spilling paint bucket, is especially useful with selections. Choose the bucket tool, and set its option to "Fill whole selection". Then click inside the selected area to fill that area with color. Also try setting the "Pattern fill" option for the Bucket tool, which allows you to fill the selected area with a pattern. To change the pattern that is used, click on the image of the pattern, just below the "Pattern fill" option.

Drawing straight lines in Gimp is a little strange. To draw a line, click the image and release the mouse button. Then immediately press the shift key. Move the mouse while holding down the shift key (without holding down any button on the mouse). Then click the mouse again. A line is drawn from the original click to the final click.

You will certainly want to try the Gradient Tool. A gradient is a sequence of colors, arranged in some pattern. Many different gradients are available in the Gradient tool options; click the image of the gradient in the Tool Options, to choose a different gradient. Apply a gradient by dragging the mouse to specify the points where the color sequence begins and ends. The colors can be applied in various shapes; try changing the "Shape" option to see how this works. Remember that you can limit the area affected by the gradient by making a selection. Also note that some of the more interesting gradients include transparent colors, which create regions where the gradient is transparent or translucent.

Important Note about the gradient tool: For smoother gradients, turn on "Adaptive supersampling" in the gradient tool options.

The picture on the right, for example, was made entirely with the Gradient tool, using, for example, the "Square Wood Frame" gradient with the shape option set to "Square" and the "Radial Eyeball" and "Radial Rainbow Hoop" gradients with the shape set to "Radial." (A selection was used to limit the size of the rainbow, which would otherwise have been a full circle.)

The Gradient tool can be used for some neat effects. The background image for this page was prepared from the original with one application of the Gradient tool, with the foreground color set to white and the gradient set to the "FG to Transparent" gradient. To make a similar background image:

Start with an existing image. Cut a vertical strip from the image to use as the basis for the background. To do that, use the Rectangle Select tool to select the region that you want to use, then use the "Crop to Selection" command from the "Image" menu.
Choose the Gradient tool, and select the gradient named "FG to Transparent".
Change the Foreground Color to white. You can do this by clicking the little Swap Colors arrow next to the color patches in the Toolbox.
Now, drag on the image from right to left. The point where you start the drag will be white (the foreground color), while the point where you end will be unaffected. The white color will fade in over the length of the drag. Use CONTROL-Z to undo, and try again, until you get exactly what you want.

You will want to try some of the other tools as well, such as the Smudge tool, the Eraser, and maybe the Clone tool. For help on using any tool, look at the message in the bottom of the image window while using the tool.

Color Tools

Exercise 3. The third exercise is to use one of Gimp's color tools to change the color of the purple flowers in the following image (which you can find in the file purple-flowers.jpg in the directory /classes/cs424/gimp-pics).

You should change the color of the flowers without changing the color of the background. Here is how to do it:

Make a copy of the original image file, and open it in Gimp.
Locate the "Select by Color Tool" in the toolbox. (It is the fifth tool.) When you click with this tool, you will select all the pixels that have a color similar to the color of the pixel that you clicked.
You will use the Select by Color Tool to select just the purple flowers in the image. This will be easiest if you change the setting of "Threshold" in the Tool Options from 15 to 30. The Threshold says how similar a color has to be to be selected.
Holding the shift key down, click pixels in the purple flowers. By holding the shift key down, the new selection will be added to the old selection instead of replacing it. Your goal is to select all of the purple pixels, and only the purple pixels. Remember that at any point, you can use CONTROL-Z to undo the most recent action. The quality of the final image will depend on how good your selection is.
Now, you are ready to apply a color tool. Select the "Hue-Saturation" command in the "Color" menu. You can adjust the color by changing the "Hue" slider. Remember that only selected pixels can be affected by the change. You might also want to change the "Saturation" and "Lightness" to get the color that you want.
When you like the color of the flowers, make the change permanent by clicking "OK" in the Hue-Saturation dialog. (Note: You can use the command "Select" / "None" to get rid of the selection so that you can see the image without the selection border.)
Use the "Export As" command in the "File" menu to save your modified image with a name such as exercise3.jpg.

This exercise just gives you a small taste of what you can do with colors in Gimp. The "Color" menu in Gimp contains several tools for adjusting the colors of an image. You might want to try some of the other tools!

Filters

Exercises 4 and 5. For the two final exercises for this week, you will apply "filters" to existing images. You can use your own images, or you can use images from the folder /classes/cs424/gimp-pics. Another source of images is the Wikimedia Commons. To find images in the commons that you can use freely, try adding "public domiain" to a search. For example, search for "flower public domain" to search for images of flowers in the public domain.

A "filter" is something that can be applied to an image, usually to modify the image in some way. They might better be called "effects." For example, there is a filter for blurring the image, one for making the image look like an old photograph, and one to make it look like it's made out of cloth. Some filters in Gimp generate images from nothing, and some do even more complicated things. You will find Gimp's filters in the "Filter" menu.

You should make two attractive or interesting images by applying one or more filters to existing images.

As you experiment, use the "Undo" command, CONTROL-Z, to undo any changes that you don't like!

Some suggested filters to try: Distorts/Emboss, Distorts/Mosaic, Distorts/Ripple, Edge-Detect, Artistic/Apply-Canvas, Artistic/Cubism, Decor/Old-Photo, Map/Warp.