[csjava-developer] performance puzzle

[Toby D. <tj...@sf...>]

Hi,

Can anyone help me to figure out why the code below is so slow? It  
converts and image to grayscale pixel by pixel, but when I've run it  
(on two different computers --- Windows and Mac OS X), it takes 20 or  
more seconds to process a medium sized image. I would have expected  
it take less than a second or so.

The bit-twiddling code should be quite efficient, since such  
operations are (hopefully) converted to single assembly-language  
instructions.

I've been using JDK 5.0 (aka 1.5), so maybe that's causing the problem?

Toby
--
Dr. Toby Donaldson
School of Computing Science
Simon Fraser University (Surrey)



public static void dprint(String s) {
         System.out.println(s + " " + (new Date()));
     }

     public static void test4(String fname) {
         try {
             dprint("Creating buffered image:");
             BufferedImage img = ImageIO.read(new File(fname));
             dprint(".. done");
             final int width = img.getWidth();
             final int height = img.getHeight();
             dprint("Starting grayscale conversion:");
             for (int i = 0; i < width; i++) {
                 for (int j = 0; j < height; j++) {
                     int rgb = img.getRGB(i, j);
                     int b = (rgb & 0x000000FF);
                     int g = (rgb & 0x0000FF00) >> 8;
                     int r = (rgb & 0x00FF0000) >> 16;

                     int lum = (int) ((r + g + b) / 3.0);
                     int alpha = rgb & 0xFF000000;
                     int red = lum << 16;
                     int green = lum << 8;
                     int blue = lum;
                     int newARGB = alpha | red | green | blue;

                     img.setRGB(i, j, newARGB);
                 }
             }
             dprint("... done");
         } catch (IOException e) {
             e.printStackTrace();
         }
     }