how to encode for 256 color pixmap?

[shigang <shigang chpc ict ac cn>]

Hi every:
	
	I have a pixmap file which have 256 grey colors. Because of using ASCII code, each pixel is 

descibed by two characters. This make xpm file as twice larger as raw data file.

	Can someone tell me how to encode xpm file so each byte can instead a pixel?   

thanks a lot!
shigang