Getting Greyscale pixel value from RGB colourspace in Java using BufferedImage

Question

Anyone know of a simple way of converting the RGBint value returned from getRGB(i,j) into a greyscale value?

I was going to simply

Answer 1

This tutorial shows 3 ways to do it:

By changing ColorSpace

ColorSpace cs = ColorSpace.getInstance(ColorSpace.CS_GRAY);
ColorConvertOp op = new ColorConvertOp(cs, null);
BufferedImage image = op.filter(bufferedImage, null);

By drawing to a grayscale BufferedImage

BufferedImage image = new BufferedImage(width, height,
    BufferedImage.TYPE_BYTE_GRAY);
Graphics g = image.getGraphics();
g.drawImage(colorImage, 0, 0, null);
g.dispose();

By using GrayFilter

ImageFilter filter = new GrayFilter(true, 50);
ImageProducer producer = new FilteredImageSource(colorImage.getSource(), filter);
Image image = this.createImage(producer);

Answer 2

Averaging sounds good, although Matlab rgb2gray uses weighted sum.

Check Matlab rgb2gray

UPDATE
I tried implementing Matlab method in Java, maybe i did it wrong, but the averaging gave better results.

Answer 3

this isn't as simple as it sounds because there is no 100% correct answer for how to map a colour to greyscale.

the method i'd use is to convert RGB to HSL then 0 the S portion (and optionally convert back to RGB) but this might not be exactly what you want. (it is equivalent to the average of the highest and lowest rgb value so a little different to the average of all 3)