The Quick Start Guide to the GIMP, Part 3

This month we learn how to use the Image Window and layers in building our images with the GIMP, a Linux power tool for the graphics artist.

Patience and persistence has paid off. After two articles you sat and wondered “When do we get to the meat?” In the first article of this series I introduced the GIMP and explained some basics about retrieving the program and getting it running. Last month I took a look at some of the GIMP's windows and and described the image file formats it supports. We're now ready to start looking at how you can use the GIMP. I'll start by looking at where images are displayed and manipulated: the Image Window.

The Image Window

Figure 1 shows an Image Window along with the Layers and Channels dialog. The visible parts of the Image Window consist of the Image display area, the rulers across the top and left sides and the scrollbars across the bottom and right sides. Pop down menus and the Ruler Guides are not visible by default but are easy to access.

Figure 1. Image Window with Dialog Box

By default, if the Image Window's dimensions fit into the displayed screen area, the rulers are divided by units of 10 pixels with major tick marks at intervals of 100 pixels. There are two other sizes of tick marks, those for units of 20 pixels and those for units of 10 pixels. If the Image Window's dimensions are larger than the displayable screen area, the GIMP will provide a window that uses major tick marks at units of 250 pixels. However, since the image would actually be larger than the screen, the Image Window does not display the image pixels one per display pixel. Don't let this confuse you. You can zoom in on your image to get pixel-level details. The GIMP is just smart enough to know how to handle images that are too big for the display. This capability is useful for design work destined for print media.

The units represented by the tick marks in the rulers can be specified in the gimprc file (which was covered in the first article in this series). By default the units are in pixels, but this can be changed to inches or centimeters at the user's discretion.

Guides are straight dashed lines alternating red and black. These extend from the left to right edges of the Image Window's display area or from the top to the bottom of the display area. To create a new Guide, move the cursor over a ruler (top or left side), hold down the left mouse button and drag into the display area. A new Guide, parallel to the ruler from which you dragged the cursor, is created. When the cursor is over the Guide, it changes to a left-hand pointer (a hand with the index finger extended). Hold down the left mouse button to grab the Guide and move it to a new location. To remove the Guide, grab it and drag it back over the ruler and release the mouse button.

Figure 2. Two Guides

Figure 2 shows two Guides, one horizontal and one vertical. Guides are useful for aligning objects or for snapping selections to specific areas. You can toggle selections to snap to the Guides from the Image Window's pop-down menu (“View->Snap” To Guides). Guides, although visible in the Image Window, are not saved to files of any image type except XCF. If you save a file to GIF or JPEG or TIFF for example, the Guides will not show up in the image file.

The scrollbars perform just as scrollbars in any other windowing application. Grab the slider with the left mouse button held down to slide the image up/down or left/right. Click in the scrollbar outside of the slider (if the slider does not extend the length of the scrollbar) to page through the display area. Click on the arrow buttons on the ends of the scrollbar to increment or decrement your way through the display area.

The most important features of the Image Window are the pop-down menus. From these you will have access to all of the GIMP's image processing capabilities. To post (i.e., display and leave open) the menus, place the cursor over the display area and click the right mouse button once. The top level menu will open and stay open until you either click outside the menu windows or select a menu option. Some of the options open submenus, which will also stay open if you simply click on those options. Clicking/dragging over the menus also works, although the menus will close if you drag the mouse off the menus and release the mouse button.

The Image Window's pop-down menus are divided into the following categories:

  • File Menu: new, open, close (window), quit (completely), etc.

  • Edit Menu: copy, cut, paste, clear, stroke, etc.

  • Select Menu: all selection methods, including growing, feathering and shrinking selections

  • View Menu: zoom in/out, toggle rulers/guides, etc.

  • Image Menu: invert, color balance/contrast/adjust, convert to/from RBG/Grayscale/Indexed, etc.

  • Layers Menu: functions applicable to the currently active layer.

  • Tools Menu: access to all the tools available from the Toolbox.

  • Filters Menu: the large set of image filters such as blurring, embossing, bump mapping and many more.

  • Dialogs Menu: provides a convenient way to open the various dialogs such as the Gradient Editor, the Layers and Channels dialog, the color palette and the brushes dialog.

Descriptions of all of these would be far more than I can fit into one series of articles. I will cover some of the more interesting filters in next month's article and will provide a detailed description of the Toolbox. Most icons accessible from the Toolbox are also accessible from Image Window menus, and those that aren't are modifiable through Image Window menu options.

The rest of the menu options each have fairly obvious meanings. The File Menu options are nothing special, although you may want to take a look at the Preferences... option. Selecting this option opens a window which provides the user with access to configuring the way transparency is represented in layers and in the gradient editor. These changes are for the current session only and are not saved across invocations of the GIMP. You must manually edit the gimprc file to make permanent changes to these preferences.

An interesting option under the Edit Menu is the Stroke option, available after a selection is made. I'll discuss selections more in a moment, but for now, note that there are many ways to select portions of your images in the GIMP, and each method is a form of a selection. A selection is denoted by a dashed line around the edge of the selection that appears to move around that edge and is known as “marching ants”. The selection outline can be used to draw a line along that edge by using the Stroke option. The color of the line must be chosen after the selection is made, and you should also select the appropriate brush. Once that is done, just select “Edit->Stroke” and a line in the selected color will be drawn using the selected brush. I know, I know—“How do you choose the color?”, “How do you choose the brush?” That's next month—there's just so much to cover.

You will find that one of the most important operations you will use with the GIMP is that of selecting portions of the image for modification. There are rectangular, ellipse, free-hand, fuzzy and bezier selection tools available from the Toolbox. The Image Window's Select Menu offers other methods such as all, none and select by color. This last method is one I use quite often to accurately select regions of irregularly shaped (but uniformly colored) areas of an image. I can then keep that selection, add a new transparent layer and fill the selection with a gradient in the new layer. Selections can be made in any layer. The selection doesn't actually affect particular pixels. They just mark a region of the image. Because of this, you can change which layer you are working on after you've made a selection and then work with the same selected region in another layer. This will allow changes to the new layer within the selected region and leave the old layer, where the selection was originally made, unchanged.

The GIMP is an image processing tool. It works by manipulating images based on the color, distribution, frequency and other aspects of the pixels which make up those images. I haven't talked before this about the sorts of manipulations that can be done. A great majority of them are done through the use of Filters, also known as Plug-Ins. This is a large area of discussion and better suited to the last article in this series, however not all image manipulation is done through Filters. The Image Menu in the Image Window's menu allows users to do a number of image processing tasks on an image and/or its layers. Most of the image processing tasks here relate to the general color, brightness or tint to an image as a whole and worry less about how individual pixels relate to their neighbors.

The Image Menu also allows transforming the image type from RGB (generally known as high color, 24-bit images) to Indexed (256-color images) to Grayscale (black and white) images. If you find you want to change a color image to black and white, but leave it as an RGB image (so you can add color to it later), you can use the “Image->Adjust->Desaturate” option. You can also adjust the amount of Red, Green or Blue in an image using the “Image->Adjust->Curves” option. I've often used the Curves and “Image->Adjust->Color Balance” option in conjunction with the Chrome Script-Fu script to add or enhance color to the edges of text logos. The latest version of the Linux Gazette logo was created in part using this technique.