Raster graphics

Imagine the smiley face in the top left corner as an RGB bitmap image. When zoomed in, it might look like the big smiley face to the right. Every square represents a pixel. Zooming in further, the individual pixels can be analyzed, with their colors constructed by adding the values for red, green and blue.

In computer graphics, a raster graphics image or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium. Raster images are stored in image files with varying formats (see Comparison of graphics file formats).

A bitmap corresponds bit-for-bit with an image displayed on a screen, generally in the same format used for storage in the display's video memory, or maybe as a device-independent bitmap. Bitmap is technically characterized by the width and height of the image in pixels and by the number of bits per pixel (a color depth, which determines the number of colors it can represent).

The printing and prepress industries know raster graphics as contones (from "continuous tones") and refer to vector graphics as "line work".

The word "raster" has its origins in the Latin rastrum (a rake), which is derived from radere (to scrape), and recalls metaphorically the systematic sampling of a grid-pattern of individual pixel-spaces with a view to representing an overall image.

Colors

Each pixel has an individually defined color; images in the RGB color space, for instance, often consist of colored pixels defined by three bytes — one byte each for red, green and blue. Less colorful images require less information per pixel; for example, an image with only black and white pixels requires only a single bit for each pixel. One can distinguish raster graphics from vector graphics in that vector graphics represent an image through the use of geometric objects such as curves and polygons.

A colored raster image (or pixmap) will usually have pixels with between one and eight bits for each of the red, green, and blue components, though other color-encodings also occur, such as four- or eight-bit indexed representations that use vector quantization on the (R, G, B) vectors. The green component sometimes has more bits than the other two to allow for the human eye's greater discrimination of this component.

Encoding

The total number of pixels (resolution), and the amount of information in each pixel (often called color depth) determine the quality of a raster image. For example, an image that stores 24 bits of color-information per pixel (the standard for displays since around 1995) can represent smoother degrees of shading than one that only stores 16 bits per pixel, but not as smooth as one that stores 48 bits. Likewise, an image sampled at 640 x 480 pixels (and therefore containing 307,200 pixels) will look rough and blocky compared to one sampled at 1280 x 1024 (1,310,720 pixels). Because it takes a large amount of data to store a high-quality image, computer software often uses data compression techniques to reduce this size for images stored on disk. Some techniques sacrifice information, and therefore image quality, in order to achieve a smaller file-size. Computer scientists refer to compression techniques that lose information as lossy compression.

Resolution

Raster graphics are resolution dependent. They cannot scale to an arbitrary resolution without loss of apparent quality. This deficiency contrasts with the capabilities of vector graphics, which easily scale to the quality of the device rendering them. Raster graphics deal more practically than vector graphics with photographs and photo-realistic images, while vector graphics often serve better for typesetting or for graphic design. Modern computer-monitors typically display about 72 to 130 pixels per inch (PPI), and some modern consumer printers can resolve 2400 dots per inch (DPI) or more; determining the most appropriate image resolution for a given printer-resolution can pose difficulties, since printed output may have a greater level of detail than a viewer can discern on a monitor. Typically, a resolution of 150 to 300 pixel per inch works well for 4-color process (CMYK) printing.

See also

• Dither
• Halftone
• GIS file formats
• Posterization
• Raster image processor
• Rasterisation

References

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.