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

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 differnt 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 model

TODO

D

Dithering

Dithering refers to the math and voodoo involved in renderingan 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 tomix 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

GNU

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

Guides

Guides are a convenient way to help you align layers and tool functions on your drawable image, layer, or selection. Guides are created manually by clicking and dragging from either the topor left rulers in to the image area. They appear as blue dashed lines. They do not print. There are some useful options available to the user to help use guides efficiently, such as Snapto Guides, Toggle Guides, and Center Guide.

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 exmaple 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

Interpolation is the means by which the pixels of an image are rendered to represent changes in an image size or angle. Interpolation methods in The GIMPare labelled with a speed marker. Faster methods lead to lowerquality transformations whereas slower methods lead to higher quality transformations.

M

Marching Ants

The name for the dotted line which delineates a selection.

Layer Modes

There are fifteen 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
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

PDB

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.

U

URL

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