A digital image is a representation of a two-dimensional image using ones and zeros (binary). Depending on whether or not the image resolution is fixed, it may be of vector or raster type. Without qualifications, the term "digital image" usually refers to raster images.

Raster

Raster images have a finite set of digital values, called picture elements or pixels. The digital image contains a fixed number of rows and columns of pixels. Pixels are the smallest individual element in an image, holding quantized values that represent the brightness of a given color at any specific point.

Typically, the pixels are stored in computer memory as a raster image or raster map, a two-dimensional array of small integers. These values are often transmitted or stored in a compressed form.

Raster images can be created by a variety of input devices and techniques, such as digital cameras, scanners, coordinate-measuring machines, seismographic profiling, airborne radar, and more. They can also be synthesized from arbitrary non-image data, such as mathematical functions or three-dimensional geometric models; the latter being a major sub-area of computer graphics. The field of digital image processing is the study of algorithms for their transformation.

Raster image types

Each pixel of a raster image is typically associated to a specific 'position' in some 2D region, and has a value consisting of one or more quantities (samples) related to that position. Digital images can be classified according to the number and nature of those samples:

• binary
• grayscale
• color
• false-color
• multi-spectral
• thematic
• picture function

The term digital image is also applied to data associated to points scattered over a three-dimensional region, such as produced by tomographic equipment. In that case, each datum is called a voxel.

Raster file formats

Most users come into contact with raster images through digital cameras. Some digital cameras give access to almost all the data captured by the camera, using the RAW file format. The Universal Photographic Imaging Guidelines (UPDIG) suggests this format be used when possible since RAW files produce the best quality images. This file format allows the photographer and the processing agent the greatest level of control and accuracy for output. Unfortunately, there is an issue of proprietary information secrets for some camera makers, but organizations are attempting to influence the manufacturers of them to avail these records publicly. An alternative may be a Digital Negative (DNG) an Adobe product described as “the public, archival format for digital camera raw data”. Although this format is not yet universally accepted, support for the product is growing and archival confidence is building.