A raster graphics image, or bitmap, is a data file or structure that consists of a generally rectangular array of pixels, or points of color, on a computer monitor, paper, or other display device. Each pixel has a corresponding red, green, and blue value that combine to determine the colour displayed by that pixel. In this sense, typical raster graphics are said to operate in the RGB color space. This is both the raw format that computer graphics hardware uses to project an image on your monitor, and the basis for many graphics file formats.
A bitmap corresponds bit for bit with an image displayed on a screen, probably in the same format 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 colored raster 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, 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 that the other two to cater for the human eye's greater discrimination in this component.
The quality of a raster image is determined by the total number of pixels (called its resolution), and the amount of information in each pixel (often called colour depth). For example, an image that stores 24 bits of colour information per pixel (the standard for most high-quality displays in 2001) can represent smoother degrees of shading than one that only stores 15 bits per pixel, but not as smooth as one that stores 48 bits. Likewise, an image sampled at 640 x 480 pixels (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, data compression techniques are often used to reduce this size for images stored on disk. Some of these techniques actually lose information, and therefore image quality, in order to achieve a smaller file size. Compression techniques that lose information are referred to as "lossy" compression.
Raster graphics cannot be scaled (resized) up without loss of apparent quality (or more accurately, once an image is rasterized, its quality is fixed and cannot improve even on better display devices). This is in contrast to vector graphics, which easily scale to the quality of the device on which they are rendered. Raster graphics work much better than vector graphics, though, for photographs and photo-realistic images. Late 20th century computer monitors typically display about 72 to 96 dots per inch (dpi), while modern printers can resolve 600 dpi or more, so working with images destined for print can be difficult or require large monitors and powerful computers. Monitors with resolutions of 200 dpi are available in late 2001 and higher resolutions are to be expected in future.
|Suppose the smiley face in the top left corner is an RGB bitmap image. When zoomed in, it might look like the big smiley face to the right. Every square represents a pixel. Zoomed in further we see three pixels whose colors are constructed by adding the values for red, green and blue.|
To illustrate the matter further - here's the letter "J":
Look closely at it... Take a magnifying glass to it if you like -- you won't get fried, although you may see some chromatic aberration at the edges of the magnifier. You see a "J", the computer sees something more like this, where '.' represents a zero and 'X' represents a one:
....X ....X ....X ....X X...X .XXX.
Where you see a zero, the computer instructs its video hardware to paint the current background colour. A one calls for the current foreground colour. Yes, it is actually a bit more complicated, but it all basically boils down to one bit or the other making a distinction between the colours of adjacent pixels, which together form an image. This is the basic principle behind drawing on a computer.
In 3D computer graphics, the concept of a flat raster of pixels is sometimes extended to a three dimensional volume of voxels. In this case, there is a regular grid in three dimensional space with a sample containing color information at each point in the grid. Although voxels are powerful abstractions for dealing with complex 3D shapes, they do have large memory requirements for storing a sizable array. Consequently, vector graphics are used more frequently than voxels for producing 3D imagery.
Raster graphics was first patented by Texas Instruments in the 1970s, and is now ubiquitous.