MATLAB: Need a grayscale non-indexed image

grayscaleImage Processing Toolboxind2graynon-indexedrgb2gray

Hi all!

I'm trying to convert some images to gray scale.

I first used rgb2gray(); But while checking results using info=iminfo(image_name) I found that info.ColorType='indexed'

So I tried I=ind2gray(image_name,map) where map was obtained from info.Colormap but I got I=0

My guess is that I put the wrong colormap. What is the right one to put???

In general, how do I get a grayscale non-indexed image????

Thanks!!!! Ilaria

Best Answer

rgb2gray and ind2gray work on an image in memory. They will both convert an image to greyscale exactly as you want.

imfinfo (I assume that's what you meant, there's no iminfo) works on an image on disk. You haven't shown us the step where you saved the image. This is probably where you've gone wrong. Most likely you've chosen an image format that do not support greyscale images. BMP for example can only store colour or indexed images so a greyscale image would have to be converted to one of these. Similarly, GIF only store images as indexed.

A format that can store greyscale (and coloured and indexed) images is PNG.

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MATLAB: Image save with colormap

When you said "I made gray image to color image with jet colormap" it's not clear if you used

rgbImage = ind2rgb(grayImage, jet(256));
imshow(rgbImage);

Or if you did

imshow(grayImage, []);
colormap(jet(256));

The first snippet creates a new full color RGB image, while the second snippet just displays the gray scale image with a pseudocolor lookup table. Which did you do?

Then you say " imshow function does not support the full scale". Not sure if you mean the size scale or the intensity scale. Anyway, imshow() supports both. The image will be scaled to fit so that the whole image will fit into whatever size the axes control is.

Then you say "the imshow function does not support the full ... size saving". You don't use imshow() to save the image, you use imwrite(), and imwrite() will save the full sized image.

If you want to display the full size image, you'll need to use a imscrollpanel. See attached demo from the Mathworks.

MATLAB: How to change colortype truecolor to grayscale

You have to display the gray scale image with imshow() after you do the conversion.

grayImage = rgb2gray(rgbImage);
imshow(grayImage, []);  % You probably forgot to do this step.

Best Answer

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