How to use function 'ind2gray'

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I'm trying to convert uint8 image to grayscale image using the following command
a = imread('image.png'); % a dimension 18*24*3
b = ind2gray(a, map);
But I'm getting an error message saying
Undefined function or variable 'map'

Accepted Answer

Image Analyst
Image Analyst on 10 Jul 2016
You need to have some way to tell the function what color should get mapped to what gray level. The colors are a 2D array with columns for red green, and blue in the range 0-1, and one row for each gray level or range of gray levels. There are several built-in color maps to select from - see the help for colormap. For example you could do
rgbImage = ind2rgb(grayImage, jet(256));
colorbar;
jet() is a function that builds a colormap of a certain pattern with the number of gray levels you specify. There are others too. Or you could make up your own colormap completely customized.
  2 Comments
Image Analyst
Image Analyst on 10 Jul 2016
Then image.png is an RGB image, not an indexed image. You have to convert it to a grayscale image somehow. There are a variaety of ways to do this and I don't know what method you want to use. For example, one way is to use rgb2gray():
indexedImage = rgb2gray(rgbImage);
Or you could take just one color channel, such as the green channel, which tends to be the least noisy:
indexedImage = rgbImage(:, :, 2);

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