31.2 Displaying Images

A natural part of image processing is visualization of an image. The most basic function for this is the imshow function that shows the image given in the first input argument. This function uses an external program to show the image. If gnuplot 4.2 or later is available it will be used to display the image, otherwise the display, xv, or xloadimage program is used. The actual program can be selected with the image_viewer function.

Function File: imshow (im)

Function File: imshow (im, limits)

Function File: imshow (im, map)

Function File: imshow (rgb, …)

Function File: imshow (filename)

Function File: imshow (…, string_param1, value1, …)

Display the image im, where im can be a 2-dimensional (gray-scale image) or a 3-dimensional (RGB image) matrix.

If limits is a 2-element vector [low, high], the image is shown using a display range between low and high. If an empty matrix is passed for limits, the display range is computed as the range between the minimal and the maximal value in the image.

If map is a valid color map, the image will be shown as an indexed image using the supplied color map.

If a file name is given instead of an image, the file will be read and shown.

If given, the parameter string_param1 has value value1. string_param1 can be any of the following:

‘"displayrange"’

value1 is the display range as described above.

See also: image, imagesc, colormap, gray2ind, rgb2ind.

Function File: image (img)

Function File: image (x, y, img)

Display a matrix as a color image. The elements of x are indices into the current colormap, and the colormap will be scaled so that the extremes of x are mapped to the extremes of the colormap.

It first tries to use gnuplot, then display from ImageMagick, then xv, and then xloadimage. The actual program used can be changed using the image_viewer function.

The axis values corresponding to the matrix elements are specified in x and y. If you're not using gnuplot 4.2 or later, these variables are ignored.

See also: imshow, imagesc, colormap, image_viewer.

Function File: imagesc (a)

Function File: imagesc (x, y, a)

Function File: imagesc (…, limits)

Function File: imagesc (h, …)

Function File: h = imagesc (…)

Display a scaled version of the matrix a as a color image. The colormap is scaled so that the entries of the matrix occupy the entire colormap. If limits = [lo, hi] are given, then that range is set to the 'clim' of the current axes.

The axis values corresponding to the matrix elements are specified in x and y, either as pairs giving the minimum and maximum values for the respective axes, or as values for each row and column of the matrix a.

See also: image, imshow, caxis.

Function File: [fcn, default_zoom] = image_viewer (fcn, default_zoom)

Change the program or function used for viewing images and return the previous values.

When the image or imshow function is called it will launch an external program to display the image. The default behavior is to use gnuplot if the installed version supports image viewing, and otherwise try the programs display, xv, and xloadimage. Using this function it is possible to change that behavior.

When called with one input argument images will be displayed by saving the image to a file and the system command command will be called to view the image. The command must be a string containing %s and possibly %f. The %s will be replaced by the filename of the image, and the %f will (if present) be replaced by the zoom factor given to the image function. For example,

image_viewer ("eog %s");

changes the image viewer to the eog program.

With two input arguments, images will be displayed by calling the function function_handle. For example,

image_viewer (data, @my_image_viewer);

sets the image viewer function to my_image_viewer. The image viewer function is called with

my_image_viewer (x, y, im, zoom, data)

where x and y are the axis of the image, im is the image variable, and data is extra user-supplied data to be passed to the viewer function.

With three input arguments it is possible to change the zooming. Some programs (like xloadimage) require the zoom factor to be between 0 and 100, and not 0 and 1 like Octave assumes. This is solved by setting the third argument to 100.

See also: image, imshow.