java.awt.image.BufferedImage 24-bit RGB to 8-bit Grayscale conversion using custom ColorSpace

Question

I want to do a simple color to grayscale conversion using java.awt.image.BufferedImage . I'm a beginner in the field of image processing, so please forgive if I confused something.

My input image is an RGB 24-bit image (no alpha), I'd like to obtain a 8-bit grayscale BufferedImage on the output, which means I have a class like this (details omitted for clarity):

public class GrayscaleFilter {
    private BufferedImage colorFrame;
    private BufferedImage grayFrame = 
        new BufferedImage(width, height, BufferedImage.TYPE_BYTE_GRAY);

I've succesfully tried out 2 conversion methods until now, first being:

    private BufferedImageOp grayscaleConv = 
        new ColorConvertOp(ColorSpace.getInstance(ColorSpace.CS_GRAY), null);

    protected void filter() {
        grayscaleConv.filter(colorFrame, grayFrame);
    }

And the second being:

    protected void filter() {       
        WritableRaster raster = grayFrame.getRaster();

        for(int x = 0; x < raster.getWidth(); x++) {
            for(int y = 0; y < raster.getHeight(); y++){
                int argb = colorFrame.getRGB(x,y);
                int r = (argb >> 16) & 0xff;
                int g = (argb >>  8) & 0xff;
                int b = (argb      ) & 0xff;

                int l = (int) (.299 * r + .587 * g + .114 * b);
                raster.setSample(x, y, 0, l);
            }
        }
    }

The first method works much faster but the image produced is very dark, which means I'm losing bandwidth which is unacceptable (there is some color conversion mapping used between grayscale and sRGB ColorModel called tosRGB8LUT which doesn't work well for me, as far as I can tell but I'm not sure, I just suppose those values are used). The second method works slower, but the effect is very nice.

Is there a method of combining those two, eg. using a custom indexed ColorSpace for ColorConvertOp ? If yes, could you please give me an example?

Thanks in advance.

Answer 1

public BufferedImage getGrayScale(BufferedImage inputImage){
    BufferedImage img = new BufferedImage(inputImage.getWidth(), inputImage.getHeight(), BufferedImage.TYPE_BYTE_GRAY);
    Graphics g = img.getGraphics();
    g.drawImage(inputImage, 0, 0, null);
    g.dispose();
    return img;
}

Answer 2

There's an example here which differs from your first example in one small aspect, the parameters to ColorConvertOp . Try this:

protected void filter() {
   BufferedImageOp grayscaleConv = 
      new ColorConvertOp(colorFrame.getColorModel().getColorSpace(), 
                         grayFrame.getColorModel().getColorSpace(), null);
   grayscaleConv.filter(colorFrame, grayFrame);
}

Answer 3

Try modifying your second approach. Instead of working on a single pixel, retrieve an array of argb int values, convert that and set it back.

Answer 4

The second method is based on pixel's luminance therefore it obtains more favorable visual results. It could be sped a little bit by optimizing the expensive floating point arithmetic operation when calculate l using lookup array or hash table.

Answer 5

Here is a solution that has worked for me in some situations.

Take image height y, image width x, the image color depth m, and the integer bit size n. Only works if (2^m)/(x*y*2^n) >= 1. Keep an bit integer total for each color channel as you process the initial gray scale values. Divide each total by the (x*y) for the average value avr[channel] of each channel. Add (192 - avr[channel]) to each pixel for each channel.

Keep in mind that this approach probably won't have the same level of quality as standard luminance approaches, but if you're looking for a compromise between speed and quality, and don't want to deal with expensive floating point operations, it may work for you.