Refers to transparency. An Alpha Channel allows transparency control. Certain image formats may only contain a single Alpha Channel allowing a transparency of on or off. Other formats allow a variable level of transparency.

Antialiasing is the process of reversing an alias. Antialiasing produces smoother curves by adjusting the boundary betweenthe background and the pixel region that is being antialiased. Generally, pixel intensities or opacities are changed so a smoother merge with the background is achieved. Withselections, the selection edge is affected so that the selection edge opacity is lowered.

An uncompressed bitmap format used by Microsoft Windows for displaying graphics. Color depth is typically 1, 4 or 8 bits, although the format does support more.

"From The Free On-line Dictionary of Computing (13 Mar 01) : bitmap A data file or structure which corresponds bit for bit with an image displayed on a screen, probably in the sameformat as it would be stored in the display's video memory or maybe as a device independent bitmap. A bitmap is characterised by the width and height of the image in pixels and the number of bits per pixel which determines the number of shades of grey or colours it can represent. A bitmap representing a coloured image (a "pixmap") will usually have pixels with between one and eight bits for each of the red, green, and blue components, though other colour encodings are also used. The green component sometimes has more bits that the other two to cater for the human eye's greater discrimination in this component. "

Each image is divided up into separate channels and then recombined before being sent to the output device. An output device is most usually a screen. The channels that are used when rendering images to a screen are Red, Green, and Blue. Other output devices may use different channels.

Channels can be useful when working on images that need adjustment to one particular color. If, for example, the removal of "red-eye" is the goal, work on the Red channel is most obviously a ready solution. Channels can be seen as masks that allow or restrict the output of the color that the channel represents. By running filters against this channel information, many varied and subtle effects can be put in to play by the experienced GIMP user.

Clipboard is the term used to describe a temporary area of memory that is used to transfer data between applications or documents. The GIMP uses slightly different clipboard approaches when used under different operating systems. Under Linux/XFree, the XFree clipboard is used for text and The GIMP internal image clipboard is used for images that are being transferred between image documents. When The GIMP is used with other operating systems, differences may be apparent. Any differences should be outlined in the operating system specific documentation for the individual GIMP package.

The most fundamental operations provided by a clipboard interface allow for Cut, Copy, and Paste. Cut is used to denote the removal of an item that is sent to the clipboard. Copy leaves the item in the document and copies it to the clipboard. Paste copies to the document whatever happens to be in the clipboard. The GIMP will make an intelligent decision about what to paste depending upon the target. If the target is a canvas, then paste operation will use the image clipboard. If the target is a text entry box, then the paste operation will use the text clipboard.

Dithering refers to the math and voodoo involved in rendering an image that has few colors seem like it has many. Dithering is accomplished in different ways depending on the output device and the program. One particularly effective method is clustering pixels of color together in an attempt to simulate another color. This is achieved by the human eye and the tendency for it to mix colors while viewing complex color patterns. A common dithering effect is seen on television screens or in newspaper print.From a distance the images seem to be constructed of many varied colors or shades, but upon closer inspection this is certainly not the case. A color television uses only three colors clustered together in various states of on or off. A black and white newspaper uses only black ink, yet pictures in newspapersappear to be constructed of grey tones. Furthermore, there are techniques used to achieve greater success in dithering.

The GIMP can use the Floyd-Steinberg dithering technique, for example. This dithering method is simply put, a mathematical way of clustering the pixels to accomplish better results thanother dithering methods. Of course, there are always exceptions and there are many different dithering models that are in usetoday.

A way that an image is written. You should select a file format which is suitable for your situation. JPEG and PostScript are examples of file formats.

Feathering is a process by which the edges of a region are softly blended with the background.

Floating selections are similar in function to layers except that floating selections must be anchored before work can resume on any other layers in the image. While a selection is floating, any number of functions can be used to alter the image data contained within the float.

There are two methods available for anchoring a float. The first, and most useful, is to change the float into a new layer. This is achieved by creating a new layer while the float is active. The second method involves anchoring the float to an already existing layer. This is done by clicking anywhere on the image except on the float. Doing so will merge the float with the background layer.

Any pasted selection will be first rendered as a floating selection.

This method of dithering looks at the current pixel color and retrieving the closest values from the palette. These colors are then distributed to the pixel areas below and to the right of the original pixel.

Trademarked by CompuServe, with LZW compression patented by Unisys. GIF images are in 8 bit indexed color and support binary transparency (but not semi-transparency). They can also be loaded in interlaced form by some programs. The GIF format also supports animations and comments. Use GIF for transparent Web graphics and GIF animations. For most purposes, though, PNG can be used in place of GIF and is a better choice.

GNU's Not Unix, is a project of a free (Unix like) operative system. The project was started back in 1983 thanks to the effort of the Free Software Fundation, an organization devoted to the creation and support of free software. GIMP is an official GNU application. You can find a better definition on the wikipedia sitehttp://en.wikipedia.org/wiki/GNU.

To place a guide, left-click-and-hold on a ruler and drag the mouse pointer in to the image. A guide appears and follows the pointer. You can so place two guides, a horizontal one and a vertical one. They appear as blue dashed lines. They do not print.

Guides are a convenient way to position a selection or a layer. As soon as a guide is created, the Move tool is selected and the mouse pointer turns to a move icon.

Guide behaviour depends on the Affect mode selected in the Move tool. When the Transform Layer mode is selected, the mouse pointer turns to a small hand when it reaches a guide that becomes red and active. You can then move it by a click and drag. When the Transform Selection mode is selected you can place a guide, but you can no longer move it after quitting it.

To make positioning easier you can 'magnetize' guides with the option Snap to Guides

You can abort displaying guides without removing them by the option Show Guides

A way of representing color in the form #rrggbb where "rr" represents red, "gg" green, and "bb" blue. Commonly used in web design.

Hue Saturation Value, a way of representing color. The Hue is the color like red or blue, the Saturation is how strong the color is and the Value is the brightness. This is sometimes called HSB or Hue Saturation Brightness.

Image Hoses are special brushes that contain many different frames. An example of this might be a footstep brush that contains two images. One of a left footprint and one of a right footprint. During the application of this hypothetical hose brush, one would see the left footprint followed by that of the right in a continuous fashion. This method of animation for brushes is very powerful.

This paint mode renders each brush stroke directly onto the active layer. If incremental mode is not set, there is a canvas buffer that is composited with the active layer.

The two images above were created using a brush with spacing set to sixty. The image on the left shows non-incremental painting and the image on the right shows the difference that incremental painting can produce. Incremental paint mode results in each brush application, through the duration of a stroke, being rendered in addition to any previous brush renderings.

When you enlarge an image pixels become apart if image resolution is not enough. These missing pixels are replaced by pixels that are calculated from surrounding pixels, by interpolation. Interpolation methods in The GIMP are labelled with a speed marker. Faster methods lead to lower quality transformations whereas slower methods lead to higher quality transformations.

This format supports compression and works at all color depths. The image compression is adjustable, but beware: Too high a compression could severely reduce image quality, since JPEG compression is lossy. Use JPEG to create TrueColor Web graphics, or if you don't want your image to take up a lot of space. JPEG is a good format for photographs.

JPEG files usually have an extension .jpg, .JPG, or .jpeg. It is a very widely used format, because it compresses images very efficiently, while minimizing the loss of image quality. No other format comes close to achieving the same level of compression. It does not, however, support transparency, or multiple layers. For this reason, saving images as JPEG often requires them to be exported.

Figure 460.  The JPEG Save dialog

When you save a file in JPEG format, you get a dialog that allows you to set the Quality level, which ranges from 1 to 100. Values above 95 are generally not useful, though. The default quality of 85 usually produces excellent results, but in many cases it is possible to set the quality substantially lower without noticably degrading the image. You can test the effect of different quality settings by checking "Show Preview in image window" in the JPEG dialog. Checking this causes each change in quality (or any other JPEG parameter) to be visualized in the image display. (This does not alter the image, though: it reverts back to its original state when the JPEG dialog is closed.)

The JPEG algorithm is quite complex, and involves a bewildering number of options, whose meaning is beyond the scope of this documentation. Unless you are a JPEG expert, the Quality parameter is probably the only one you will benefit from adjusting.

Caution

After you save an image as a JPEG file, the image will no longer be considered "dirty" by Gimp, so unless you make further changes to it, you will not receive any warning if you close it. Because JPEG is lossy and does not support transparency or multiple layers, some of the information in the image mightthen be lost. If you want to save all of the information in an image, use Gimp's native XCF format.

JPEG files from many digital cameras contain extra information called EXIF data, specifying camera settings and other information concerning the circumstances under which the image was created. Gimp's ability to handle EXIF data depends on whether the library "libexif" is available on your system; it is not automatically packaged with Gimp. If Gimp has been built with libexif support, then EXIF data is preserved if you open an JPEG file, work with the resulting image, and then save as JPEG. The EXIF data is not altered in any way when you do this (which means that certain fields within it are no longer valid). If Gimp is not built with EXIF support, this does not prevent files with EXIF data from being opened, but it means that the EXIF data will not be present when the resulting image is later saved.

Note

Some information about the advanced settings: DCT Method.  DCT is "discrete cosine transform", and is the first step in the JPEG algorithm going from spatial to frequency domain. The choices are "float", "integer" (the default), and "fast integer". The float method is very slightly more accurate than the int method, but is much slower unless your machine has very fast floating-point hardware. Also note that results of the floating-point method may vary slightly across machines, while the integer methods should give the same results everywhere. The fast integer method is much less accurate than the other two.

You can think of layers as a stack of slides or clothes on your body. Each part of clothes you're wearing is a layer in the layers dialog. Layers are stacked on top of each other. The bottom layer is the background of the image and the components in the foreground of the image come above it.

Representation of an image with layers:

The final image:

The name for the dotted line which delineates a selection.

There are twenty-one available layer modes. Selecting a layer mode changes the way that layer or paint application is viewed based on the layer or layers beneath it.

Masks are special elements associated with a layer or a selection. They modify the transparency of this layer or selection.

They belong to two types:

A path is a one-dimensional tracing, either polygonal, or curved, or a mixture of segments of both types. In GIMP, the main uses of paths are either to form the boundaries of selections, or to be stroked to create visible curves on an image. See the Paths section for basic information on paths, and the Path Tool section for information on how paths can be created and edited.

The PDF format was developed by Adobe to address some of the deficiencies of Postscript: most importantly, PDF files tend to be much smaller than equivalent Postscript files. As with Postscript, Gimp's support of the PDF format is via Ghostscript.

Created by Adobe, PostScript is a page description language mainly used by printers and other output devices. It's also an excellent way to distribute documents. Gimp does not support Postscript directly: it depends on a powerful free software program called Ghostscript.

The great power of Postscript is its ability to represent vector graphics–lines, curves, text, paths, etc–in a resolution-independent way. Postscript is not very efficient, though, when it comes to representing pixel-based raster graphics. For this reason, postscript is not a good format to use for saving images that are later going to be edited using Gimp or another graphics program.

Linux distributions almost always come with Ghostscript already installed (not necessarily the most recent version). For other operating systems, you may have to install it yourself. Here are instructions for installing it on Windows:

Now you should be able to read PostScript files with GIMP. Please note that you must not move the directories of ghostscript once the installation has finished. Registry entries have been created that allow ghostscript to find libraries. (These instructions courtesy of http://www.kirchgessner.net.)

The format that is supposed to replace the GIF format and thus provide a solution to GIF's trademark and patent issues. Indexed color, grayscale, and truecolor images are supported, plus an optional alpha channel. PNG also uses compression, but unlike JPEG it doesn't lose image information.

PSD is Adobe Photoshop's native file format, and thus is comparable to XCFin complexity. Gimp's ability to handle PSD files is sophisticated but limited: some features of PSD files are not loaded, and only PSD versions XX or less are supported. Unfortunately, Adobe has now made the Photoshop Software Development Kit ­ which includes their file format specifications ­ proprietary, and only available to a limited set of developers blessed by Adobe. This does not include the Gimp development team; and the lack of information makes it very difficult to maintain up-to-date support for PSD files.

Sample Merge is a technique useful when working with more than one layer where operations that affect one layer may take advantage of color or pixel information on all visible layers. Consider selection by color as an example of a time that this function may be useful.

With this technique Gimp takes more pixels around to calculate a transitional color for instance for interpolation. Render is better but treating time longer.

The Targa file format supports compression to 8, 16, 24 or 32 bits per pixel.

Designed to be a standard, TIFF (Tagged Image File Format) files come in many different flavors. Six different encoding routines are supported, each with one of three different image modes: black and white, grayscale and color. Uncompressed TIFF images may be 1, 4, 8 or 24 bits per pixel. TIFF images compressed using the LZW algorithm may be 6, 8 or 24 bits per pixel. This is a high quality file format, perfect for images you want to import to other programs like FrameMaker or CorelDRAW.

Uniform Resource Locator: the "address" format for the World Wide Web.

The XCF file type is special because it is Gimp's native file type: that is, it was designed specifically to store all of the data that goes to make up a Gimp image. Because of this, XCF files may be quite complicated, and there are few programs other than Gimp that can read them.

When an image is stored as an XCF file, the file encodes nearly everything there is to know about the image: the pixel data for each of the layers, the current selection, additional channels if there are any, paths if there are any, and guides. The most important thing that is not saved in an XCF file is the undo history.

The pixel data in an XCF file is represented in a raw, uncompressed form: each byte of image data equals one byte in the XCF file. Thus, XCF files for even modestly sized images can be quite large: for example, a 1000x1000 RGB image with 3 layers will produce an XCF file of over 10 megabytes. It is not at all difficult to get XCF files of over 100 megabytes. Fortunately, although XCF files do not intrinsically compress their data, Gimp allows you to compress the files themselves, using either the gzip or bzip2 compression methods, both of which are fast, efficient, and freely available. Compressingan XCF file will often shrink it by a factor of 10 or more.

The Gimp developers have made a great effort to keep the XCF file format compatible across versions. If you create a file using Gimp 2.0, it ought to be possible to open the file in Gimp 1.2. However, some of the information in the file may not be usable: for example, Gimp 2.0 has a much more sophisticated way of handling text than Gimp 1.2, so a text layer from a Gimp 2.0 XCF file will appear as an ordinary image layer if the file is opened in Gimp 1.2.