画像は GIMP が扱う根源的な要素です。 誤解を恐れずに言えば、 ひとつの「画像」が TIFF や JPEG といった種類のファイルひとつに対応しています。 画像ひとつにつき画像ウィンドウひとつが対応するという見方もできます。 (ただし実際はひとつの画像ファイルを複数のウィンドウで同時に開くことも可能です。) もちろんひとつのウィンドウのなかでふたつ以上の画像は表示できませんが、 一方でウィンドウ表示をしないまま開いている画像がありえます。

GIMP 形式の画像はいくぶん複雑なものです。 画像は一枚の紙面に見立てるのではなく、 そのようなシートをいくつも重ね合わせたものととらえてください。 その一層一層を「レイヤー」と呼びます。 GIMP 形式ではさらに選択マスクや一揃いのチャネル、 数面のパスが画像に含められます。 じっさい GIMP は画像に対するいろんな加工をひとつひとつ情報の断片としてとらえる機構があり、 それらを画像の「parasites (寄生体)」と呼んでいるのです。

GIMP は複数の画像を同時に開いておくことが可能です。 大きな画像だと数メガバイトものメモリーを占めるはずですが、 GIMP では精巧にタイル分割してメモリー管理を行うしくみがあり、 たいへん大きな画像でも流麗に扱えるようになっています。 無論これにも限界がありますが、 もっと使えるメモリーを増やせばシステムの処理能力は高まります。

If a simple image can be compared to a single sheet of paper, an image with layers is likened to a sheaf of transparent papers stacked one on top of the other. You can draw on each paper, but still see the content of the other sheets through the transparent areas. You can also move one sheet in relation to the others. 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. For more info see Introduction to Layers.

Digital images consist of a grid of square pixels. Each image has a size measured in two dimensions, such as 900 pixels wide by 600 pixels high. But pixels don't have a set size in physical space. To set up an image for printing, we use a value called resolution, defined as the ratio between an image's size in pixels and its physical size (usually in inches) when it is printed on paper. Most file formats (but not all) can save this value, which is expressed as ppi—pixels per inch.

When printing a file, the resolution determines the size the image will have on paper, and as a result, the physical size of the pixels. The same 900×600 pixel image may be printed as a small 3×2" card with barely noticeable pixels—or as a large poster with large, chunky pixels.

Images imported from cameras and mobile devices tend to have a resolution attached to the file. The resolution is usually 72 or 96ppi. It is important to realize that this resolution is arbitrary and was chosen for historic reasons. You can always change the resolution with GIMP—this has no effect on the actual image pixels. Furthermore, for uses such as displaying images online, on mobile devices, television or video games—in short, any use that is not print—the resolution value is meaningless and is ignored. Instead, the image is usually displayed so that each image pixel conforms to one screen pixel.

チャンネルとは画素の色成分ひとつを示すことばです。 GIMP で扱う色つき画素は、 通常は赤・緑・青の色成分と、 必須ではありませんが透過効果 (アルファ) 成分によって構成されます。 グレースケール 画像は灰色陰影のチャンネルとアルファチャンネルで、 インデックス化 カラー画像はインデックス化カラーマップとアルファチャンネルで構成されています。

The entire rectangular array of any one of the color components for all of the pixels in an image is also referred to as a Channel. You can see these color channels with the Channels Dialog.

画像を表示するとき、 GIMP は色成分を画面やプリンターをはじめとする出力デバイスに合わせて再編成します。 一部の出力デバイスは赤・緑・青とは異なる色チャンネル構成を使用しています。 このようなデバイスで画像が表示されるとき、 GIMP のチャンネルはそのデバイスに最適なものへ変換されます。

ある特定の画像の色を調節したいとき、 チャンネルを利用すると便利です。 たとえば写真から「赤目」を除去したいときは、 赤チャンネルで作業するとよいでしょう。

You can look at channels as masks which allow or restrict the output of the color that the channel represents. By using Filters on the channel information, you can create many varied and subtle effects on an image. A simple example of using a Filter on the color channels is the Channel Mixer filter.

3 つの色チャンネルに加え、 GIMP では他にもチャンネル (正確にはチャンネルマスク) を作成できます。 これらはチャンネルダイアログの下側に表示されます。 新しい チャンネルを追加 したり選択範囲を チャンネルに保存 することもできます。 この他のチャンネルについての情報は用語集の マスク の項でご覧ください。

Often when modifying an image, you only want a part of the image to be affected. The 「selection」 mechanism makes this possible. Each image has its own selection, which you normally see as a 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 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 Quick Mask 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 provides many 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」. For more information, see also The Selection.

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. For more information, see also Undoing.

A lot of the things that you do to an image in GIMP are done by GIMP 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 are bundled with GIMP, but there are also many available by other means. In fact, writing plug-ins (and scripts) is the easiest way for people not on the GIMP development team to add new capabilities to GIMP.

実際フィルターメニューの全コマンドと、 その他のメニューのかなりの数のコマンドが、 プラグインで実装されています。

In addition to plug-ins, GIMP can also make use of scripts. Scripts are written in a language called Script-Fu, which is unique to GIMP (for those who care, it is a dialect of the Lisp-like language called Scheme). In the past there was a clear distinction between scripts and plug-ins, but that is disappearing. Depending on which Script-Fu interpreter you use, Scheme scripts can also be installed as plug-ins.