col2gray - convert an image to a grayscale image

Calling Sequence:

	img = col2gray(im,my_select)
	
Parmeters:
	
	im: matrix of an input image
	my_selcet: select a mothod for converting
	img: output image,it's a grayscale image
	
Description:
	
	 this function provides three methods for converting an image to a grayscale image:
	 (1)average.	rgb=R/3+G/3+B/3
	 (2)max.			rgb=max(R,G,B)
	 (3)weighted average.		rgb=0.11*R+0.59*G+0.30*B
	 if my_select=1,then we use the first method to convert the image to a grayscale image
	
Example:

	im = imread("lena.png");
	img = col2gray(im,2);