converting True color RGB image into grayscale Image

In matlab there is a function rgb2gray() to convert rgb image into gray scale image.
we can do this manually also.

1. rgb = imread('Sun.jpg');
2. figure('name','Original Image')
3. imshow(rgb);
4. 
   
5. gray_manual = 0.2989 * rgb(:,:,1) + 0.5870 * rgb(:,:,2) + 0.1140 * rgb(:,:,3);
6. figure
7. imshow(gray_manual);

Output

Original Image

Converted GrayScale Image

Formula to do this is

Intensity = 0.2980 * red + 0.5870 * green + 0.1140 * blue