Images appear black and white instead of coloured

I plot a montage of images onto a UIPanel and it worked fine when the images were tiffs. My images are now jpgs and I notice that the coloured images now appear black and white. I can't see why this would be the case. Here is my code

This is in a loop for all images.

       hp = handles.uipanelM;
       h=subplot('Position',positionVector,'Parent',hp); %to enable deleting use handles
       colormap (jet);
       % update handle array
       hSubplots = [hSubplots ; h];   %Concatenate
       axis off;
       hold off;
       %Create thumbnail sized image
       IM1=imresize(IM, 0.05);
       allImgs{y1,x1} = IM1 ;             

       imagesc(Image,[mlo mhi])
       colormap (jet);
       axis off;
       set(gca,'ytick',[]);
       set(gca,'xtick',[]);

       drawnow;

[Adam]

You are plotting a variable called 'Image' but I don't see any such variable in your code.

Using a jet colourmap an image should never appear in greyscale.

I would always recommend using the explicit version of any plotting-related function though - i.e.

colormap( hFig, 'jet' )

or better (if using R2014b)

colormap( hAxes, 'jet' )

since R2014b now has a colourmap per axes rather than just a global one per figure.

[Jason]

My image is from a populated listbox, I loop over the index variable.

     name=file_list{index};
         f = fullfile(folder,name);
         IM = imread(f);

I've also tried your suggestion:

   colormap(hp, 'jet' )

But get an error

Error using colormap (line 57)
First argument must be a scalar axes or figure handle.

Error in Montage>pButtonGo_Callback (line 521)
       colormap(hp, 'jet' )

[Adam]

hp is a panel handle, it needs to be either a figure handle or an axes handle.

[Jason]

OK, but I am plotting onto the panel handle. The only other handle I use is:

h=subplot('Position',positionVector,'Parent',hp)

Im not convinced this is the issue, as it works with 12 bit tiffs, but not with 8 bit jpgs.

Answer 1

[Image Analyst]

JPG images are usually color, even if they appear as gray scale - it's just that all the color channels are the same. Try:

imshow(IM1, []);  % Display color image

Or extract one of the color channels:

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
imshow(redChannel, []);  % Display one monochrome color channel
colormap(jet(256));

Or convert to grayscale if the color channels are NOT all identical:

grayImage = rgb2gray(IM1);
imshow(grayImage); % Display converted image.
colormap(jet(256));
colorbar;

[Jason]

Unfortunately its still black and white.

[Image Analyst]

Please attach your jpg image so I can try it. Maybe it's grayscale but 3D RGB (all three color channels are identical) so a colormap will not apply. Colormaps do not apply to color images.

[Image Analyst]

Jason's image moved from an "Answer" to here since it's a direct response to my request:

[Image Analyst]

Adam's comment to Jason's Answer moved here:

That is a true RGB greyscale image even in the jpg. Just take one channel from it and use that if you want to apply a colourmap.

By the way, please add things like this as a comment to the relevant answer rather than your own answer.

[Image Analyst]

Jason's "Answer" moved here:

Perfect, that did it, thanks to both of you. I now use

Image(:,:,1)

Jason

[Image Analyst]

Jason: That won't do it. First of all, Image was not even defined in your code. You should use the red channel of IM or IM1. Second, Image is a bad name since image() is the name of a built in function. I know MATLAB is case sensitive, but nonetheless it's not a wise idea to use variables with the same names as the built in functions, even if the case of some of the letters is different.

[Jason]

Hi IA. I actually put Image(:,:,1) in a function that I pass IM1 to.

function Im_Scale(handles,Image,mlo, mhi,ext)

    size(Image, 3)   %If 3 then RGB, if not then grayscale
    if ext=='.jpg'
       imagesc(Image(:,:,1),[mlo mhi])

    else

        imagesc(Image,[mlo mhi])
    end    
    colormap (jet);
    axis off;
    set(gca,'ytick',[]);
    set(gca,'xtick',[]);
    drawnow;
end

I will take your advice regarding the name Image. As I correct though to use the index 1 as representing the red channel?

Thanks