Image Representation

with , and typically . Each element of the array is called a pel (picture element), or more commonly a pixel. Typical image dimensions are M x N = 256 x 256 and 512 x 512.

This loads the package.

A number of example images are included with the Digital Image Processing package. They are located in the Data directory of the root ImageProcessing directory. The location on any system is given by the system variable $ImageDataDirectory. In a typical Windows installation, the path may be as follows.

When executing ImageRead, Mathematica tries to find the file in these directory paths. ImageRead also accepts absolute or relative file paths.

This loads an example image.

This extracts a small 8 x 8 image segment.

Small values represent dark areas of an image, while large values represent bright pixels.

A color digital image is typically represented by a triplet of values, one for each of the color channels, as in the frequently used RGB color scheme. The individual color values are almost universally 8-bit values, resulting in a total of 3 bytes (or 24 bits) per pixel. This yields a threefold increase in the storage requirements for color versus monochrome images. Naturally, there are a number of alternative methods of storing the image data. Most widely used are the so-called pixel-interleaved (or meshed) and color-interleaved (or planar) formats. Less frequent are row-wise or column-wise interleaving methods. In a pixel-interleaved format, every image pixel is represented by a list of three values.

In the color-interleaved format, the color information is separated into three matrices, one for each of the three color channels.

Here we read an example color image.

Here we extract a 4 x 4 region and display it in a meshed format.

Here the same region of the color image is displayed in a planar format.

The RGB color scheme is just one of many color representation methods used in practice. The letters R, G, and B stand for red, green, and blue, the three primary colors used to synthesize any one of or approximately 16 million colors. Equal quantities of the three color values result in shades of gray in the range {0, 255}. Other supported color models include monochrome; HSV, which stands for hue, saturation, value; and CMYK, which stands for cyan, magenta, yellow, black. The latter has found application primarily in the printing and graphics markets. HSV is useful in color image processing because it separates the color information from brightness.