Glossary

A

Alpha

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

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.

B

BMP

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.

Bitmap

" 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. "

C

Channels

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 usedwhen 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, theremoval 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

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.

Color modes
RGB : Red Green Blue

This model is used to represent colors on computers and television monitors. These colors are emitted by screen phosphors and not reflected as they are on paints. The resulting color is a combination of the three primary RGB colors, with different degrees of lightness. If you look at your television screen closely, whose pitch is less than that of a computer screen, you can see the red, green and blue phosphors differently enlighted. It is said that this color model is additive

GIMP uses an eight bits (8-bit) channel for each primary color and so 256 intensities (Values) are available resulting in 256x256x256 = 16,777,216 colors (called True Color).

It is not evident why these combinations produce rather unexpected colors. Why, for instance, 229R+205G+229B gives a kind of pink? This depends on our eye and our brain. There is no color in Nature, only a continuous variation of the light wavelength. In retina are three kinds of cones. The same wavelength acting on the three types of cones stimulates them differently and mind has learned after several millions of years of Evolution how to recognize a color in these differences.

You can easily understand that no light (0R+0G+0B) gives complete darkness, black, and full light (255R+255G+255B) gives white. Equal intensity in all channels gives a gray level. So you can have only 256 gray levels.

Mixing two primary colors in RGB mode gives a Secondary color that is a color of the CMY mode. So combining Red and Green gives Yellow, Green and Blue give Cyan, Blue and Red give Magenta. Don't mistake secondary colors for Complementary colors which are diametrically opposed to a primary color in the chromatic circle:

Mixing a primary color with its complementary color gives gray (neutral color).

It is important to know what happens when you handle colors. The rule to remember is that decreasing a primary color results in increasing the saturation of the complementary color (and conversely). Here is the explanation: When you decrease a channel value, for instance the Green one, you increase the relative importance of other both, here Red and Blue. Now combination of these two channels gives the secondary color, Magenta, that is quite the complementary color of Green.

Exercise: You can check this. Create a new image with only a white background (255R+255G+255B). Open the Tools/Color Tools/Levels dialog and select the Red channel. If necessary, check the preview box. Move the white slider to the left to decrease the Red value. You will notice that the background of you image turns more and more to Cyan. Now, decrease the Blue channel: only the Green will persist. As a training go backwards, add color and try to guess what hue will appear.

The Color Picker tool allows you to know the RGB values of a pixel and the HTML hextriplet for the color.

HSV: Hue Saturation Value

The RGB mode is well adapted to computer screens but it doesn't allow to describe what we see in every day life: a light green, a pale pink, a dazzling red... The HSV mode accounts for these features. HSV and RGB are not independant. You can see that with the color-picker: when you change one the other is also modified. Brave can read Grokking the Gimp which explains their relations.

  • Hue: It's the color itself, resulting from the combination of primary colors. All color shades (except greylevels) are represented in a chromatic circle: Yellow, Blue, and also purple, orange... It goes from 0° to 360°. ("Color" term is often used instead of "Hue". RGB colors are "Primary colors").

  • Saturation: This parameter describes how pale the color is, as when you add white in a can of paint: a completely saturated hue will be pure. If less saturated, it will be pastel. Very saturated is almost white. Saturation ranges from 0 to 100, from white to the purest color.

  • Value: It is merely Luminosity, the luminous intensity. It's the amount of light emitted by a color. You notice this change of luminosity when a color goes from shadow to sun or when you increase the luminosity of your screen. It ranges from 0 to 100. Pixels values in the three channels are also luminosities: "Value" is the vectorial sum of these elementary values in the RGB space.

CMYK: Cyan Magenta Yellow black

Let us say first that Gimp doesn't support the CMYK mode. (An experimental plugin providing rudimentary CMYK support can be found at www.blackfiveservices.co.uk/separate.shtml .)

This mode is that of printing, that of your printer whose cartridges contain these colors. It's the mode of painting and of all the objects around us, where light is not emitted but reflected. Objects absorb apart of the light wave and we see only the reflected part. Notice that our eye with its cones sees this reflected light in RGB mode. An object is red because Green and Blue have been absorbed. Now, Green and Blue combination is Cyan. So, Cyan is absorbed when you add Red. Conversely, if you add Cyan the complementary Red is absorbed: this system is substractive. If you add Yellow, you decrease Blue and if you add Magenta, you decrease Green.

You could logically think that by mixing Cyan, Magenta and Yellow you substract Red, Green and Blue, and so that the eye sees no light at all, that's to say Black. The question is more complex. In fact you will see a dark brown. That's why this mode has also the Black color and why your printer has a Black cartridge. That's finally cheaper: the printer has not to mix the three other colors to create an imperfect black. It has only to add some Black.

Graylevel

When you create a new image you can choose the graylevel mode (that you can colorize later by transforming it to the RGB mode). You can also transform an existing image to graylevel (but all formats do not accept this transformation) thanks to the command Graylevel.

As we have explained in RGB mode, Gimp 24-bit images cann't have more than 256 gray levels, coded on 8-bit. If you switch from graylevel to RGB mode you give your image a RGB structure with three color channels but of course your image remains gray.

Graylevel image files (8-bit) are smaller than RGB files.

You can also transform a RGB image to graylevel by desaturating it thanks to the HSV option of the Decompose filter, the Colorize tool, or the Hue-Saturation filter.

Indexed colors

The indexed mode has been invented by Compuserve at the beginning of the Web to create small color image files, that could be easily transmitted. GIF (Graphic Interchange Format) was the first indexed format.

The principle is to code each pixel color in a table attached to the image, all pixels with the same color have the same code.

You can see this palette when you have opened a GIF image thanks to the Dialogs / Indexed Palette. It allows you to edit and modify each color.

See also Indexed Palette.

In a GIF image, transparency is coded on one bit: transparent or not.

D

Dithering

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.

F

File Format

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

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

FLoating Selection

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.

Floyd-Steinberg Dithering

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

G

GIF

Trademarked by CompuServe, with LZW compression patented by Unisys. GIF images are in 8 bit indexed color and support 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

GNU's Not Unix, an organization devoted to the creation and support of Open Source software. GIMP is an official GNU application.

Guides

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

H

Hextriplet

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

HSV

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.

I

Image Hose

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.

Incremental, paint mode

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.

Interpolation

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.

J

JPEG

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.

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] 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] 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.

L

Layer

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:

M

Marching Ants

The name for the dotted line which delineates a selection.

Layer Modes

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.

Normal

This is the default layer mode. The layer will be viewed normally.

Dissolve

The Dissolve layer mode dissolves the layer into the layer beneath it. It does so by dispersing pixels. This can best be seen ina close-up screenshot.

The image on the left illustrates a normal layer mode and the image on the right shows the same two layers in dissolve mode.

Multiply

This mode multiplies the pixel values of the layer with those that are visible beneath it.

Divide

This mode divides the pixel values of the layer by the values of the visible pixels beneath it.

Screen

The values of the visible pixels in the two layers are inverted, multiplied, and the product inverted again. The result is usually a brighter picture.

Overlay

Overlay is a combination of Multiplication and Screen modes.

Dodge

The values of the two layers are inverted, divided, and the result inverted again. This lightens the upper layer.

Burn

Burn mode inverts the layers, multiplies, and inverts again. This darkens the upper layer.

Hard Light

This mode is a combination of Screen and Multiplication modes.

Soft Light

This mode gives a "soft" effect to the otherwise sharp edges of the image, and lightens the colors.

Grain Extract

Extracts the "film grain" from a layer into a new layer that is pure grain.

Grain Merge

Merge a grain layer (possibly created from the Grain Extract operation into the current layer, leaving a grainy version of the original layer.

Difference

Addition

A simple operation, adding the pixel values at each location.

Substract

A simple operation, subtracting the pixel values at each location.

Darken Only

Replace the current layer's pixels with the minimum of the pixel values of the two layers at each location.

Lighten Only

Replace the current layer's pixels with the maximum of the pixel values of the two layers at each location.

Hue

Saturation

Color

Value

The image on the left illustrates a normal layer mode and the image on the right shows thesame two layers in value mode.

P

Path

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.

PDF

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.

Postscript

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:

  • Go to the Ghostscript project page on Sourceforge.

  • Look for package gnu-gs or ghostscript (non-commercial use only) and go to the download section.

  • Download a prepared Windows distribution like gs650w32.exe or gs700w32.exe.

  • Start the executable and follow the instructions of the installation procedure.

  • Copy the executable gswin32c.exe from the directory bin of the ghostscript installation to the Windows directory (or any other directory that is contained in the PATH). As an alternative, advanced users can set an environment variable GS_PROG to point to gswin32c.exe (e.g. c:\gs\gsX.YY\bin\gswin32c.exe)

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.)

PNG

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

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.

S

Sample Merge

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.

Supersampling

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

T

TGA

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

TIFF

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.

U

URL

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

X

XCF

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 sophisticatedway 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.