MATLAB: What is an indexed image

Question

What is indexed image and what does map do with indexed image?

Answer 1

There are very good and thorough explanations online:

http://www.mathworks.com/help/matlab/creating_plots/image-types.html

https://en.wikipedia.org/wiki/Indexed_color

In a nutshell: imagine that you have an image with just two colors, blue and red. Encoding this simple image in a way that can represent every possible RGB color would be totally unnecessary and a waste of memory... so instead you can simply index the colors by defining a map of colors (which defines as many colors as the image has):

map = [
 0,0,1; % blue;
 1,0,0; % red
 ]

And then the image matrix consists simply of indices into this map:

img = 
 1,2,1,2
 1,2,2,1
 1,1,2,2

So every 2 in the image matrix uses the color specified by the second row of the map, etc.. So you can recreate the the pixel colors of the image:

 blue,red,blue,red
 blue,red,red,blue
 blue,blue,red,red

For a limited set of colors this method can take up a lot less memory than specifying the complete RGB for every pixel. Usually the order of the index is arbitrary, but specific to that image. Read the documentation for a more detailed explanation with lots of examples!