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Displaying Graphics Images

Image Types and Display Methods

To display a graphics file image, use either image or imagesc. For example, read the image ngc6543a.jpg to a variable RGB and display the image using the image function. Change the axes aspect ratio to the true ratio using axis command.

RGB = imread('ngc6543a.jpg');
image(RGB);
axis image;

This table summarizes display methods for the three types of images.

Image Type

Display Commands

Uses Colormap Colors

Indexed

image(X); colormap(map)

Yes

Intensity

imagesc(I,[0 1]); colormap(gray)

Yes

RGB (truecolor)

image(RGB)

No

Controlling Aspect Ratio and Display Size

The image function displays the image in a default-sized figure and axes. The image stretches or shrinks to fit the display area. Sometimes you want the aspect ratio of the display to match the aspect ratio of the image data matrix. The easiest way to do this is with the axis image command.

For example, these commands display the earth image using the default figure and axes positions:

load earth
image(X); colormap(map)

The elongated globe results from stretching the image display to fit the axes position. Use the axis image command to force the aspect ratio to be one-to-one.

axis image

The axis image command works by setting the DataAspectRatio property of the axes object to [1 1 1]. See axis and axes for more information on how to control the appearance of axes objects.

Sometimes you want to display an image so that each element in the data matrix corresponds to a single screen pixel. To display an image with this one-to-one matrix-element-to-screen-pixel mapping, you need to resize the figure and axes. For example, these commands display the earth image so that one data element corresponds to one screen pixel:

[m,n] = size(X);
figure('Units','pixels','Position',[100 100 n m])
image(X); colormap(map)
set(gca,'Position',[0 0 1 1])

The figure's Position property is a four-element vector that specifies the figure's location on the screen as well as its size. The figure command positions the figure so that its lower left corner is at position (100,100) on the screen and so that its width and height match the image width and height. Setting the axes position to [0 0 1 1] in normalized units creates an axes that fills the figure. The resulting picture is shown.

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