MATLAB: When I make the image grayscale the image shows up red, why is this

grayscaleimageImage Processing Toolboxrgb

I use the following code to turn my image into grayscale but then most of the image turns out red for some reason. flower_gray_scale = 0.2989*double(flowerarrangement(:,:,1)) + 0.587*double(flowerarrangement(:,:,2)) + 0.114*double(flowerarrangement(:,:,3)); image(flower_gray_scale)

Best Answer

It's because image() clips anything after 64 gray levels to the color of 64 gray levels. I just answered this today http://www.mathworks.com/matlabcentral/answers/153539#answer_151050. Run the demo there (also attached below this next demo). Also see this demo, and try using imshow instead of image(). Or try using jet(256) instead of the default jet(64) that image uses.

flowerarrangement = imread('peppers.png'); % Demo image.
flower_gray_scale = 0.2989*double(flowerarrangement(:,:,1)) + 0.587*double(flowerarrangement(:,:,2)) + 0.114*double(flowerarrangement(:,:,3));
image(flower_gray_scale);
% imshow(flower_gray_scale, []);
colormap(jet(256)); % colormap() uses jet(64) which is not good with image().
colorbar;
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')

Bottom line, either

Use imshow() instead of image(), or
Use a 256 colormap if you're going to use image(), or
Use colormap(gray(256)) if you want gray scale, not pseudocolored image.

Related Solutions

MATLAB: How to display the intensity of 2D image as a colormap

You need to pass in a colormap, N-by-3 array with values between 0 and 1, to to the colormap() function, NOT an image. And you should probably also pass in a handles to the axes you want to apply it to, otherwise it will apply that colormap to every axes in the figure window, which is probably not what you want. So in short, you want:

myColorMap = hsv(256);      % Create a colormap.


colormap(gca, myColorMap);  % Now apply the colormap.

You can also pass the colormap directly into imshow() and skip calling the colormap() function if you want.

See this full demo and study it:

grayImage = imread('cameraman.tif');
subplot(3, 1, 1);
imshow(grayImage);
axis('on', 'image');
title('Original Image');
subplot(3, 1, 2);
% Create a colormap.
myColorMap = hsv(256);
% Display image with that colormap.
imshow(grayImage, 'Colormap', myColorMap);
colorbar;
axis('on', 'image');
title('Overlay Image');
% Or equivalently, using the colormap() function:
subplot(3, 1, 3);
% Display image with no colormap.
imshow(grayImage);
% Create a colormap.
myColorMap = hsv(256);
% Now apply the colormap.  Pass in the axes or else it will apply the colormap
% to ALL images on the figure, which is probably not what you want.
colormap(gca, myColorMap);
colorbar;
axis('on', 'image');
title('Overlay Image');

Now, if you already have a colormap, and you want to produce an RGB image, like to write to a file on disk rather than just pseudocoloring a displayed image, you want to call ind2rgb()

rgbImage = ind2rgb(grayImage, myColorMap);

MATLAB: 8 bit of size 256*256 grey image for testing

imread() the color image, rgb2gray() it, imresize() it to 256 x 256 or imresize() it to 256 on the shorter side and imcrop() the rest of the longer side away if the image is not originally square.

Best Answer

Related Solutions

MATLAB: How to display the intensity of 2D image as a colormap

MATLAB: 8 bit of size 256*256 grey image for testing

Related Question