MATLAB: How to convert red channel of a rgb image to grayscale image

grayscale convertionimage processingImage Processing Toolboxrgb image

I have some rgb images. I want to know is there any way to only convert the red channel of them to grayscale?

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Yes. Very easy:

redChannel = rgbImage(:, :, 1); % Extract the red channel only into a gray scale image.

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MATLAB: How to convert RGB image to grayscale image

Sounds like homework. You can get the color channels:

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

and then do a weighted average of them.

grayImage = a1 * redChannel + a2 * greenChannel + a3 * blueChannel;

Look up standard weights, or pick your own.

MATLAB: CieLaB grayscal, Red and Green Channels

I think you're confused. But here is your first paragraph:

% gets an RGB image, 
rgbImage = imread('peppers.png'); % Read in sample demo image.
% transforms it to grayscale 
grayImage = rgb2gray(rgbImage);
% then extracts its red and green channel. 
redChannel = rgbImage(:, :, 1); % Extract red channel.
greenChannel = rgbImage(:, :, 2); % Extract green channel.
% I need to get the same image, convert it to Cielab color space 
labImage = rgb2lab(rgbImage);
% and then transform it to grayscale and then extract the red and green channels too.
% We already did that -- just use grayImage, redChannel, and greenChannel.

Now, for your second paragraph:

1) Is the L "channel" the CieLab image already in grayscale or should I perform something else to get it?

The L channel is a monochrome, single valued image that represents the lightness of the image. You can consider it gray scale, just like you can with redChannel or greenChannel or grayImage.

2) Is it possible to get separated red and green channels from A "channel" as I got from the image in RGB color space?

No. The A channel is also a monochrome channel and it has no red and green channels. The A channel represents the redness to greenness of the color. But the A channel is not a color image. Neither is the L channel or B channel. They're monochrome. And the labImage is not a "true color" image, as in an RGB image, so if you display it with imshow, which expects an RGB image, it will look weird. It will display the L image as red, the A image as green, and the B image as blue and the whole image will look bizarre. The lab image is not meant to be displayed. It's meant to allow for color segmentation or to measure color differences.

And to make it more confusing, the lab image you get from rgb2lab() or applycform() does not give the lab colors you'd get from your colorimeter or spectrophotometer, but that's a whole other discussion.

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MATLAB: How to convert RGB image to grayscale image

MATLAB: CieLaB grayscal, Red and Green Channels

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