how to convert tiff image into gray image?

how to convert tiff image into gray image? I tried rgb2gray but it show me an error about "RGB must be a m x n x 3 array" my tiff image is (m x n x 4)

[Svetlana Zeveleva]

I'm having the same issue. But how did you find out the array size of your image?

Answer 1

[Guillaume]

Your image has a transparency channel in addition to the three colour channels. If you don't want to do anything with the transparency information, just discard it:

 img(:,:,4) = [];
 %proceed with rgb2gray

[Mohammad]

Suppose I did my processes on resulted gray image and i want to convert it again to tiff image and write it again, so what I can do?

[John]

Do you want to convert it back to RGB? And save it as an RGB tiff? Or just a grayscale tiff?

[Guillaume]

You can just save the grayscale image as tiff. If you wanted to convert it to rgb, it simply would be:

 rgbimg = repmat(img, [1 1 3]);

And if you wanted a non-transparent transparency channel

 rgbaimg = [repamt(img, [1 1 3]) zeros(size(img))];

There wouldn't be much point in either though.

[Image Analyst]

What do you mean "again"? Once it's in memory, it's just a 2D array of numbers. You can save the gray image out to a grayscale tiff image with imwrite:

filename = fullfile(folder, 'mygrayScaleTiff.tif');
imwrite(grayImage, filename);

If you want to save your RGB image, without the 4th channel which you removed and before you converted to grayscale and did other stuff, as an RGB image, you can (using Guillaume's code) save it with imwrite() like this:

filename = fullfile(folder, 'myRGBTiff.tif');
imwrite(img, filename);