Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!asuvax!enuxha!rao
From: rao@enuxha.eas.asu.edu (Arun Rao)
Newsgroups: comp.windows.x
Subject: Converting color images to monochrome - can I do it any faster ?
Message-ID: <324@enuxha.eas.asu.edu>
Date: 20 Oct 89 23:17:51 GMT
Organization: Arizona State Univ, Tempe
Lines: 65


	My image processing application requires the user to select
	an arbitrary rectangular section from the screen to be
	processed further. Since the processing depends only upon
	brightness levels, I would like to display the selected portion
	as a gray-scale image regardless of what it looks like on
	the screen.

	Here's the code fragment I use to convert color pixel values
	to equivalent gray-scale values:

  	width = image->width;
        height = image->height;


        /* Get default Colormap */
        cmap = DefaultColormap(dpy, screen);


        for (i = 0; i < height; i++)
                for (j = 0; j < width; j++)
                {
                        /* This is the most portable way of doing it,
                           but slow as hell */
                        /* Get pixel values from the image and fill XColor's */
                        xcolor.pixel = XGetPixel(image, j, i);

			/* Get RGB values corresponding to pixel value */
                        XQueryColor(dpy, cmap, &xcolor);

                        /* RGB to gray-scale formula (from Adrian Nye's book)*/
                        intensity = (int)((0.30 * xcolor.red)
                                   +(0.59 * xcolor.green)
                                   +(0.11 * xcolor.blue));

			/* Put (equal) gray level values in RGB */
                        xcolor.red = (unsigned short)intensity;
                        xcolor.green = (unsigned short)intensity;
                        xcolor.blue = (unsigned short)intensity;

			/* Get pixel values corresponding to
			   gray-level */
                        XAllocColor(dpy, cmap, &xcolor);

			/* Put pixel back in image */
                        XPutPixel(image, j, i, xcolor.pixel);
                }


	The above is very portable, but is horrendously slow, even
	for small images. Is there a better way of doing this without
	resorting to machine dependent bit-fiddling ?
	
	The machine is an Ardent Titan running SysV and X11R3 and I've
	tested the program in addition on an HP 9000 server.

	Thanks in advance.


	-Arun
-- 
Arun Rao
ARPANET: rao@enuxha.eas.asu.edu BITNET: agaxr@asuacvax, agazr@asuacad
950 S. Terrace Road, #B324, Tempe, AZ 85281
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