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30.3 Representing Images

In general Octave supports four different kinds of images, gray-scale images, RGB images, binary images, and indexed images. A gray-scale image is represented with an M-by-N matrix in which each element corresponds to the intensity of a pixel. An RGB image is represented with an M-by-N-by-3 array where each 3-vector corresponds to the red, green, and blue intensities of each pixel.

The actual meaning of the value of a pixel in a gray-scale or RGB image depends on the class of the matrix. If the matrix is of class double pixel intensities are between 0 and 1, if it is of class uint8 intensities are between 0 and 255, and if it is of class uint16 intensities are between 0 and 65535.

A binary image is an M-by-N matrix of class logical. A pixel in a binary image is black if it is false and white if it is true.

An indexed image consists of an M-by-N matrix of integers and a C-by-3 color map. Each integer corresponds to an index in the color map, and each row in the color map corresponds to an RGB color. The color map must be of class double with values between 0 and 1.

Function File: [img, map] = gray2ind ()

Convert a gray scale intensity image to an Octave indexed image.

Function File: ind2gray (x, map)

Convert an Octave indexed image to a gray scale intensity image. If map is omitted, the current colormap is used to determine the intensities.

See also gray2ind, rgb2ntsc, image, colormap

Function File: [x, map] = rgb2ind (rgb)

Function File: [x, map] = rgb2ind (r, g, b)

Convert an RGB image to an Octave indexed image.

See also ind2rgb, rgb2ntsc

Function File: rgb = ind2rgb (x, map)

Function File: [r, g, b] = ind2rgb (x, map)

Convert an indexed image to red, green, and blue color components. If the colormap doesn't contain enough colors, pad it with the last color in the map. If map is omitted, the current colormap is used for the conversion.

See also rgb2ind, image, imshow, ind2gray, gray2ind

Function File: colormap (map)

Function File: colormap ("default")

Set the current colormap.

colormap (map) sets the current colormap to map. The color map should be an n row by 3 column matrix. The columns contain red, green, and blue intensities respectively. All entries should be between 0 and 1 inclusive. The new colormap is returned.

colormap ("default") restores the default colormap (the jet map with 64 entries). The default colormap is returned.

With no arguments, colormap returns the current color map.

See also jet

Function File: autumn (n)

Create color colormap. This colormap is red through orange to yellow. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: bone (n)

Create color colormap. This colormap is a gray colormap with a light blue tone. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: cool (n)

Create color colormap. The colormap is cyan to magenta. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: copper (n)

Create color colormap. This colormap is black to a light copper tone. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: gray (n)

Return a gray colormap with n entries corresponding to values from 0 to n-1. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

Function File: hot (n)

Create color colormap. This colormap is black through dark red, red, orange, yellow to white. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: hsv (n)

Create color colormap. This colormap is red through yellow, green, cyan, blue, magenta to red. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: jet (n)

Create color colormap. This colormap is dark blue through blue, cyan, green, yellow, red to dark red. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: ocean (n)

Create color colormap. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

Function File: pink (n)

Create color colormap. This colormap gives a sephia tone on black and white images. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: prism (n)

Create color colormap. This colormap cycles trough red, orange, yellow, green, blue and violet. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: rainbow (n)

Create color colormap. This colormap is red through orange, yellow, green, blue to violet. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: spring (n)

Create color colormap. This colormap is magenta to yellow. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: summer (n)

Create color colormap. This colormap is green to yellow. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: white (n)

Create color colormap. This colormap is completely white. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

Function File: winter (n)

Create color colormap. This colormap is blue to green. The argument n should be a scalar. If it is omitted, the length of the current colormap or 64 is assumed.

See also colormap

An additional colormap is gmap40. This code map contains only colors with integer values of the red, green and blue components. This is a workaround for a limitation of gnuplot 4.0, that does not allow the color of line or patch objects to be set, and so gmap40 is useful for gnuplot 4.0 users, and in particular in conjunction with the bar, barh or contour functions.

Function File: gmap40 (n)

Create a color colormap. The colormap is red, green, blue, yellow, magenta and cyan. These are the colors that are allowed with patch objects using gnuplot 4.0, and so this colormap function is specially designed for users of gnuplot 4.0. The argument n should be a scalar. If it is omitted, a length of 6 is assumed. Larger values of n result in a repetition of the above colors

See also colormap

ISBN 095461206X

GNU Octave Manual Version 3

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