How to convert a grayscale matrix to an RGB matrix in MATLAB?

Question

rgbImage = grayImage / max(max(grayImage));

or

rgbImage = grayImage / 255;

Which of the above is right,and reason

Answer 1

By definition, an RGB image has 3 channels, which implies you need a three-dimensional matrix to represent the image. So, the right answer is:

rgbImage = repmat(255*grayImage/max(grayImage(:)),[1 1 3]);

Be careful when normalizing grayImage. If grayImage is uint8 then you will lose some precision in the 255*grayImage/max(grayImage(:)) operation.

Also, normalizing grayImage depends on the data. In your question, you used two methods:

rgbImage = grayImage / max(max(grayImage));

which normalizes the grayscale image such that the maximum value in the image is 1 and

rgbImage = grayImage / 255;

which only makes sense if the values in grayImage lie in the 0-255 range.

So it really depends on what you want to do. But, if you want an RGB image you need to convert your single-channel matrix to a 3-channel matrix.