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Chapter 2: GIMP Basics
Drawing, Painting, Modeling and Rendering
The GIMP was not originally designed to do line drawing or painting specifically. There are drawing tools and a collection of brush types that can be used, but their use is not as encompassing as, say, MetaCreation's ArtDabbler or Adobe's Illustrator. In spite of this, you can use the GIMP to do certain amounts of drawing and painting.
Drawing in the GIW is the ability to make freehand lines or to trace paths with a computer pencil of some kind. Painting can encompass filling in a region with a specified color or pattern, or it can be a freehand design using a simulated paint or airbrush. The GIMP provides a pencil, a paintbrush, an airbrush and a paint bucket (for doing bucket fills of a specified color or pattern). Each of these has various options such as antialiasing or rate of flow of the "paint". All but the paintbucket can use a variety of brush types which are selectable via a special brush dialog window. It is also possible to design your own brush types-using the GIW's built-in brush file format, using nothing more than a simple File->Save operation. Drawing and painting tools are discussed in depth in Chapter 8 - Drawing and Painting.
Tracing a path is also possible and actually rather simple to do. Just create a selection using one of the various selection tools, choose a brush type, and select File- >Edit->Stroke from the Image menus and a line will be painted following the edge of the selection. The colors used can also be set prior to setting the stroked path. Stroke paths are discussed in Chapter 5 - Selections.
One of the most important sets of tools you will want to use with the GIMP is the selection tools. These tools allow the user to select regions of an image using rectangular, round, and freehand styles, and even by colors. Once a region is selected, it can be manipulated in a way that does not affect areas outside of the selected region. Selections can be cut, copied, pasted, rotated and resized no matter what shape they may take. Selections are the basis for doing stroke paths, where the edge of the selection is used as a path to draw and paint lines.
Selections can also be feathered to varying degrees, allowing a region to have a fading effect into the region outside of the selection. Selections can be enlarged, reduced and even inverted. Inverting a selection allows you to select a region that normally might be difficult to select on its own. The area that is to be outside the chosen region is selected first, then the selection is inverted and you have the region that would have been harder to select.
Chapter 5 - Selections gives more complete descriptions of all the selection tools available in the GIMP.
Layers and Compositing
Images in the GIMP consist of one or more layers. A layer is like a clear acetate sheet-those transparent sheets often used with overhead projectors. The layer is opaque4 where there is image information for that layer and transparent elsewhere. The layers are stacked and the image window displays what the currently visible layers will look like when combined in the final image. Each layer can be turned on or off, that is, you can make them visible and part of the overall image or turn them off so they are not used in calculating pixel values for the displayed image.
Each layer can be composited, that is, combined, with lower layers using a variety of modes. The default mode for a layer is Normal meaning that the pixels in the current layer are not combined with lower layer pixels that reside in the same location in the image. Layers can be added, subtracted, multiplied or have any one of more than a dozen operations applied. The layer modes for compositing are separate the overall opacity of the layer, which can also be set from 0 (fully transparent) to 100 (fully opaque).
One of the less obvious uses of layers is creating animations. A single initial layer can be created and duplicated. The duplicated layer can then be modified slightly and duplicated again. This process can continue or virtually indefinitely. The final set of layers can then be saved as a series of individual frames in an animated GIF.
Layers are discussed in detail in Chapter 6 ayers and Channels.
The GIMP displays images on your monitor in a best-fit manner. This means that the image is scaled to fit on your display, so that images you create which would normally be too large for the display can be viewed in their entirety. The image windows which display the images can be zoomed to view regions that may have been scaled too small to view important details. Startup configuration options in the gimprc file allow the user to have the GIMP resize windows during zoom operations as well.
Images are normally displayed based on a pixel resolution basis. For example, the default width of a new image window is 256 pixels wide by 256 pixels high. The size units can be changed from pixels to inches to centimeters in the gimprc start-up file.
An image's resolution, that is, the number of pixels in width by height, determines an image's file size. Since the GIMP uses layers, there is no direct correspondence between number of pixels and the file size for the native file format, .xcf. However, it is generally obvious that larger resolutions create larger files.
Often you will find that stock images that you read from a file or CD are not the resolution you require for the images you are creating. In this case, you will need to resize diem. There are a number of methods for dealing with this issue. One is to select a sub-region of the image and then crop the image down to the selected area. This process doesn't change the resolution but does reduce the size of the file with which you are dealing. An alternative to this method is to scale the image. Scaling changes the dimensions of the image-scaling up will increase the width and height of the image. Scaling down reduces the width and height. Scaling changes the total number of pixels being used in the image. Finally, an image can be resized. This reduces the size of the viewable region of the image in the same manner that cropping the image works. Resizing affects the entire image or layer, whereas cropping generally is applied to a region of an image or layer.
The GIMP supports all three types of resolution and sizing options. The dialog window which provides for resizing also allows an interactive placement of the box which defines the cropped region. Chapter 3 GIMP Windows provides a detailed look at using these features.
The GIMP and Desktop Publishing
Much of the material in this section is general information applicable to desktop publishing. You need to be acquainted with these topics to best utilize the GIMP in your art.
Computer displays are often discussed in terms of pixels to describe image resolution. Printers are often discussed in the more confusing terms of dots per inch (dpi), lines per inch (lpi) and halftones. Each of these is related to determining the mapping of a raster image to a printed one in a fairly straightforward manner. Chapter 11 - Scanning, Printing, and Print Media discusses these at length, along with issues related to creating graphics specifically for web pages. It also discusses the specific printers supported by the GIMP through the Print Plug-In....