From separate channels to RGB

Hello everyone,

I have images in single bands ( Red, Blue and Green) and I need to combine them to obtain an RGB image.

R = imread('red.png');
G = imread('green.png');
B = imread('blue.png');
RGB = cat(3,R,G,B);

I wrote this code: how can I write and save the so obtained RGB image?

Thanks for your kind help.

Answer 1

[Stephen23]

I am guessing that the image files are actually Truecolor RGB (with all channels identical) rather than true Grayscale (a sadly all-too-common mixup made by many apps and users), in which case this should work:

RGB = cat(3,R(:,:,1),G(:,:,1),B(:,:,1));
imwrite(RGB, 'rgb.png');

Note that this does not scale/weight the channels!

See also: https://www.mathworks.com/help/matlab/creating_plots/image-types.html

[Claudio Esposito]

Thank you both!

[Claudio Esposito]

Excuse me sir, I have another issue, which is the following.

I have obtained the following image:

The fact is that if I write : RGB = cat(3,G(:,:,1),R(:,:,1),B(:,:,1)), I obtain this image instead:

How can I fix this problem?

[Walter Roberson]

Why are you writing green then red then blue instead of red then green then blue?

[Claudio Esposito]

I was just trying what would have happen because R-G-B seems too reddish purple in colour.

[Stephen23]

"I was just trying what would have happen because R-G-B seems too reddish purple in colour."

You might need to consider this:

https://en.wikipedia.org/wiki/Grayscale#Colorimetric_(perceptual_luminance-preserving)_conversion_to_grayscale

Answer 2

[Walter Roberson]

imwrite(RGB, 'rgb.png');

[Claudio Esposito]

Thank you for your replying.

I already tried that, but Matlab displays this error:

Error using writepng (line 23)

PNG image data must be either MxN or MxNx3.

Error in imwrite (line 566)

feval(fmt_s.write, data, map, filename, paramPairs{:});

Error in RGB_conversion (line 14)

imwrite(RGB, 'rgb.png')