Introduction into Digital Images
The first digital image was made on a computer in 1957 and showed researcher Russell Kirsch's baby son. Since then the era of imaging technologies has begun - satellite imaging, CAT scans, bar codes on packaging, desktop publishing, digital photography etc. sprang from this grainy image.
A digital image is a representation of a two-dimensional image using ones and zeros. Depending on whether or not the image resolution is fixed, it may be of vector or raster type:
| A vector graphics image is an image created using geometrical primitives such as points, lines, curves and polygons, which are all based on mathematical equations. |
A raster graphics image or bitmap is a data structure representing a generally rectangular grid of pixels, or points of color. Raster graphics is typically used for the representation of photographic images. |
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 map, a two-dimensional array of small integers. These values are often transmitted or stored in a compressed form.
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 related to that position. Digital images can be classified according to number and nature of those samples:
- A binary image has only two possible values for each pixel. Typically the two colors used for a binary image are black and white though any two colors can be used.
- In a grayscale image the value of each pixel is a single sample, it carries only intensity information. Images of this sort are composed exclusively of shades of gray, varying from black at the weakest intensity to white at the strongest.
- A color image includes color information for each pixel. Each pixel has three samples, which are interpreted as coordinates in some color space. For more information about color space see the Glossary.
- In a false-color image the correspondence between subject color and image color is altered. The opposite of it is a true-color image that appears to the human eye just like the original subject would.
- A multi-spectral image captures image data at specific frequencies across the electromagnetic spectrum. It was originally developed for space-based imaging.
- etc.
Raster images can also be classified by the number of bits used to represent the color of a single pixel, in other words, by color depth:
- 1-bit monochrome - typically the two colors used for a binary image are black and white, though any two colors can be used.
- 8-bit greyscale - greyscale images have many shades of grey in between in comparison with one-bit black-and-white images.
- 8-bit color - the maximum number of colors that can be displayed at any one time is 256.
- 15/16-bit color (highcolor) - it allows 32,768/ 65,536 possible colors for each pixel.
- 24-bit color (truecolor) - it means at least 256 shades of red, green, and blue, for a total of at least 16,777,216 color variations.
- 30/36/48-bit color (deepcolor) - anything superior to truecolor, typically over a billion colors; the xvYCC, sRGB, and YCbCr color spaces can be used with deepcolor systems.