How to use function 'ind2gray'

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'

Answer 1

[Image Analyst]

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.

[Atinesh Singh]

When I'm reading an image using this command

a = imread('image.png');

I'm getting three dimensions

18*24*3

I just need an array of 2 dimensions.

[Image Analyst]

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);