How to convert RGB image to grayscale image

Please help me to convert RGB image to grayscale without using rgb2gray().

Answer 1

[Image Analyst]

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.

[Soomin Lee]

How do we get standard weights?

[Image Analyst]

@Soomin Lee try the Math section on these web sites:

http://www.brucelindbloom.com/

https://www.easyrgb.com/en/math.php

[DGM]

What's wrong with just giving the luma constants?

[0.299 0.587 0.114] for Rec 601 (what rgb2gray() uses)

or

[0.2126 0.7152 0.0722] for Rec 709