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### 3.5. Basic Gimp Concepts

This section is intended to give you a brief introduction to the basic concepts and terminology you will need to understand in order to make sense to the rest of the documentation. Everything here is explained in much greater depth elsewhere. With a few exceptions, we have avoided cluttering this section with a lot of links and cross-references: everything mentioned here is so high-level that you should easily be able to locate it in the index.

Overview

  The Gimp is an image manipulation program. At the most sweeping level, using Gimp involves three basic steps: (1) opening images or creating new ones; (2) altering those images; (3) saving the results.

  Opening Images

    Depending on how Gimp was started, there may already be one or more images open when you begin. You can open new images from files using the Open command from the File menu. Gimp is capable of opening a large variety of graphics file formats; see Files for more information. Depending on how your system is set up, you may also be able to open images by clicking on icons in a file manager, or by drag-and-drop from other programs. If you aren't sure whether you can do this, just try it. The worst thing that can happen is that your computer could explode.

  Altering Images

    Gimp provides you with an enormous number of ways of acting on images: painting tools, color manipulation tools, transformation tools, filters, etc. The bulk of this manual is devoted to describing these tools and how to work with them.

  Saving Images

    When you are finished working with an image, you will want to save the results. (In fact, it is often a good idea to save at intermediate stages too: Gimp is a pretty robust program, but we have heard rumors, possibly apocryphal, that it may have been known on rare and mysterious occasions to crash.) Most of the file formats that Gimp can open, can also be used for saving. There is one file format that is special, though: XCF is Gimp's native format, and is useful because it stores everything about an image (well, almost everything; it does not store "undo" information). Thus, the XCF format is especially suitable for saving intermediate results, and for saving images to be re-opened later in Gimp. XCF files are not readable by most other programs that display images, so once you have finished, you will probably also want to save the image in a more widely used format, such as JPEG, PNG, TIFF, etc.

Images

  Images are the basic entities that Gimp works with. Roughly speaking, an "image" corresponds to a single file, such as a TIFF or JPEG file. You can also think of an image as corresponding to a single display window, but this is not quite correct: it is possible to have multiple windows all displaying the same image. It is not possible to have a single window display more than one image, though, or for an image to have no window displaying it.

  A Gimp image may be quite a complicated thing. Instead of thinking of it as something like a sheet of paper with a picture on it, you should think of it as more like a book, whose pages are called "layers". In addition to a stack of layers, a Gimp image may contain a selection mask, a set of channels, and a set of paths. In fact, Gimp provides a mechanism for attaching arbitrary pieces of data to an image, as what are called "parasites".

  In Gimp, it is possible to have many images open at the same time. If they are large, each image may use many megabytes of memory, but Gimp uses a sophisticated tile-based memory management system that allows it to handle even very large images gracefully. There are, however, limits, and it is usually beneficial when working with images to put as much memory into your system as possible.

Layers

  If an image is like a book, then a layer is like a page within the book. The simplest images only contain a single layer, and can be treated like single sheets of paper, but sophisticated Gimp users often deal with images containing many layers, even dozens of them. Layers need not be opaque, and they need not cover the entire extent of an image, so when you look at an image's display, you may see more than just the top layer: you may see elements of many layers.

Channels

  TO BE WRITTEN

Selections

  Often when you do something to an image, you only want a part of it to be affected. The "selection" mechanism makes this possible. Each image has its own selection, which you normally see as an moving dashed line separating the selected parts from the unselected parts (the so-called "marching ants"). Actually this is a bit misleading: selection in Gimp is really graded, not all-or-nothing, and really the selection is represented by a full-fledged grayscale channel. The dashed line that you normally see is simply a contour line at the 50%-selected level. At any time, though, you can visualize the selection channel in all its glorious detail by toggling the QuickMask button.

  A large component of learning how to use Gimp effectively is acquiring the art of making good selections---selections that contain exactly what you need and nothing more. Because selection-handling is so centrally important, Gimp gives you a large number of tools for doing it: an assortment of selection-making tools, a menu of selection operations, and the ability to switch to Quick Mask mode, in which you can treat the selection channel as though it were a color channel, thereby "painting the selection".

Undoing

  When you make mistakes, you can undo them. Nearly everything you can do to an image is undoable. In fact, you can usually undo a substantial number of the most recent things you did, if you decide that they were misguided. Gimp makes this possible by keeping a history of your actions. This history consumes memory, though, so undoability is not infinite. Some actions use very little undo memory, so that you can do dozens of them before the earliest ones are deleted from this history; other types of actions require massive amounts of undo memory. You can configure the amount of memory Gimp allows for the undo history of each image, but in any situation, you should always be able to undo at least your 2-3 most recent actions. (The most important action that is not undoable is closing an image. For this reason, Gimp asks you to confirm that you really want to close the image if you have made any changes to it.)

Plug-ins

  Many, probably most, of the things you do to an image in Gimp are done by the Gimp application itself. However, Gimp also makes extensive use of "plug-ins", which are external programs that interact very closely with Gimp, and are capable of manipulating images and other Gimp objects in very sophisticated ways. Many important plug-ins come packaged together with Gimp, but there are also many available by other means. In fact, the ability to write plug-ins (and scripts) is the easiest way for people not on the Gimp development team to add new capabilities to Gimp.

  All of the commands in the Filters menu, and a substantial number of commands in other menus, are actually implemented by plug-ins.

Scripts

  In addition to plug-ins, which are programs written in the C language, Gimp can also make use of scripts. The largest number of existing scripts are written in a language called Script-Fu, which is special to Gimp (for those who care, it is a dialect of the Lisp-like language called Scheme). It is also possible to write Gimp scripts in Python or Perl. These languages are more flexible and powerful than Script-Fu; their disadvantage is that they depend on software that does not automatically come packaged with Gimp, so they are not guaranteed to work correctly in every Gimp installation.

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