convert a RGB image to grayscale Image reducing the memory in java

Question

I have an RGB bufferedImage bImg .
I want to convert bImg to gray image.

BufferedImage grayIm=new BufferedImage(bImg.getWidth(null), bImg.getHeight(null), BufferedImage.TYPE_BYTE_GRAY); 

I have tried this grayIm but i can not set the grayscale values for this grayIm.

Answer 1

One way could be to convert the color space (poor performance):

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

Another would be to use a BufferedImage, as you do (better performance):

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

Last but not least, the best performance is using a GrayFilter:

ImageFilter filter = new GrayFilter(true, 50);  
ImageProducer producer = new FilteredImageSource(colorImage.getSource(), filter);  
Image mage = Toolkit.getDefaultToolkit().createImage(producer);  

source: http://www.codebeach.com/2008/03/convert-color-image-to-gray-scale-image.html

edit: per Mark's comment.