| Image Processing Toolbox | |
Displaying Indexed Images
To display an indexed image with imshow, specify both the image matrix and the colormap.
For each pixel in X, imshow displays the color stored in the corresponding row of map. The relationship between the values in the image matrix and the colormap depends on whether the image matrix is of class double, uint16, or uint8. If the image matrix is of class double, the value 1 points to the first row in the colormap, the value 2 points to the second row, and so on. If the image matrix is of class uint8 or uint16, there is an offset; the value 0 points to the first row in the colormap, the value 1 points to the second row, and so on. (The offset is handled automatically by the image object, and is not controlled through a Handle Graphics property.)
imshow directly maps each pixel in an indexed image to its corresponding colormap entry. If the colormap contains a greater number of colors than the image, imshow ignores the extra colors in the colormap. If the colormap contains fewer colors than the image requires, imshow sets all image pixels over the limits of the colormap's capacity to the last color in the colormap. For example, if an image of class 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.
The Image and Axes Properties of an Indexed Image
When you display an indexed image, imshow sets the Handle Graphics properties that control how colors display, as follows:
CData property is set to the data in X.
CDataMapping property is set to direct.
CLim property does not apply, because CDataMapping is set to direct.
Colormap property is set to the data in map.
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