Separating RGB components from two different .png files

I used a file 'facebook.png' and separated the RGB components. The following code worked fine.

if true
red=f(:,:,1);   %Sieved R component
green=f(:,:,2);   %Sieved G component
blue=f(:,:,3);     %Sieved B component
end

But I used the same code for 'android.png' and I got an error 'Index exceeds matrix dimensions.', 'green=f(:,:,2); %Sieved G component'.

Why are the two files being treated differently?

Answer 1

[KSSV]

If you read RGB image file it will be 3D matrix and hence you can separate R,G and B as you did. If the file is binary, on reading it you will get 2D matrix. Check the size of matrix obtained on reading android.png, it would be 2D. So you cannot extract it like 3D matrix and error popped.

[Guillaume]

"if the file is binary". That should be if the file is greyscale.

[KSSV]

Yes...True...grayscaleimage..

[fiona rozario]

How is an indexed image 2D?

[KSSV]

It's size will be of length 2. :)

[Image Analyst]

It has a width and a height, and only one color plane. So it's 2-D. Do you have a definition of 2-D different than that?

[fiona rozario]

No...I thought it would have just two colour channels and was wondering how that could be possible. Thanks, for the code below...

Answer 2

[Image Analyst]

See this code demo:

fontSize = 20;

filename = 'android.png';
% Read in indexed image.
[img, storedColorMap] = imread(filename);
[rows, columns, numberOfColorChannels] = size(img)

% Display indexed image without colormap being applied.
subplot(3, 2, 1);
imshow(img, []);
caption = sprintf('Indexed image without\ncolormap being applied');
title(caption, 'FontSize', fontSize);

% Display indexed image with colormap being applied.
subplot(3, 2, 2);
imshow(img, storedColorMap);
caption = sprintf('Indexed image with\ncolormap being applied');
title(caption, 'FontSize', fontSize);

% Convert to RGB image by appying the colormap
rgbImage = ind2rgb(img, storedColorMap);
% Display RGB image.
subplot(3, 2, 3);
imshow(rgbImage);
title('RGB Image', 'FontSize', fontSize);

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);

% Display the three color channels separately
subplot(3, 2, 4);
imshow(redChannel);
title('Red Channel Image', 'FontSize', fontSize);
subplot(3, 2, 5);
imshow(greenChannel);
title('Green Channel Image', 'FontSize', fontSize);
subplot(3, 2, 6);
imshow(blueChannel);
title('Blue Channel Image', 'FontSize', fontSize);