Display Indexed Images

To display an indexed image using either imshow function or the Image Viewer app, specify both the image matrix and the colormap. This sample code uses the variable name X to represent an indexed image in the workspace, and map to represent the colormap.

imshow(X,map)

or

imageViewer(X,Colormap=map)

For each pixel in X, these functions display the color stored in the corresponding row of map. If the image matrix data is of data type double, the value 1 points to the first row in the colormap, the value 2 points to the second row, and so on. However, if the image matrix data is of data type uint8 or uint16, the value 0 (zero) points to the first row in the colormap, the value 1 points to the second row, and so on. This offset is handled automatically by the Image Viewer app and the imshow function.

If the colormap contains a greater number of colors than the image, the functions ignore the extra colors in the colormap. If the colormap contains fewer colors than the image requires, the functions set all image pixels over the limits of the colormap's capacity to the last color in the colormap. For example, if an image of data type uint8 contains 256 colors, and you display it with a colormap that contains only 16 colors, all pixels with a value of 15 or higher are displayed with the last color in the colormap.

Display Grayscale Images

To display a grayscale image, call the imshow function or open the Image Viewer app. This documentation uses the variable name I to represent a grayscale image in the workspace.

Both functions display the image by scaling the intensity values to serve as indices into a grayscale colormap.

If I is double, a pixel value of 0.0 is displayed as black, a pixel value of 1.0 is displayed as white, and pixel values in between are displayed as shades of gray. If I is uint8, then a pixel value of 255 is displayed as white. If I is uint16, then a pixel value of 65535 is displayed as white.

Grayscale images are similar to indexed images in that each uses an m-by-3 RGB colormap, but you normally do not specify a colormap for a grayscale image. MATLAB^® displays grayscale images by using a grayscale system colormap (where R=G=B). By default, the number of levels of gray in the colormap is 256 on systems with 24-bit color, and 64 or 32 on other systems. (See Display Colors for a detailed explanation.)

imshow(I,DisplayRange=[low high])

imtool(I,DisplayRange=[low high])

I = imread("testpat1.png");
J = filter2([1 2;-1 -2],I);
imtool(J,DisplayRange=[]);

Display Binary Images

In MATLAB, a binary image is of data type logical. Binary images contain only 0's and 1's. Pixels with the value 0 are displayed as black; pixels with the value 1 are displayed as white.

To display a binary image, call the imshow function or open the Image Viewer app. For example, this code reads a binary image into the MATLAB workspace and then displays the image. The sample code uses the variable name BW to represent a binary image in the workspace.

BW = imread("circles.png");
imshow(BW)

imshow(~BW)

imshow(BW,[1 0 0; 0 0 1])

Display Truecolor Images

Truecolor images, also called RGB images, represent color values directly, rather than through a colormap. A truecolor image is an m-by-n-by-3 array. For each pixel (r,c) in the image, the color is represented by the triplet (r,c,1:3).

To display a truecolor image, call the imshow function or open the Image Viewer app. For example, this code reads a truecolor image into the MATLAB workspace and then displays the image. This sample code uses the variable name RGB to represent a truecolor image in the workspace.

RGB = imread("peppers.png");
imshow(RGB)

Systems that use 24 bits per screen pixel can display truecolor images directly, because they allocate 8 bits (256 levels) each to the red, green, and blue color planes. On systems with fewer colors, imshow displays the image using a combination of color approximation and dithering. See Display Colors for more information.