How to improve invert + grayscale operator in Java?

Question

I want to invert and gray scale an image. Here is my original image:

Here is the final result I want to achieve (produced with Paint.NET):

However using some (basic?) Java code found on Internet, I only get the picture below:

Here is the code I used:

private static final byte[] invertTable;

static {
    invertTable = new byte[256];
    for (int i = 0; i < 256; i++) {
        invertTable[i] = (byte) (255 - i);
    }
}

private static BufferedImage grayScale(BufferedImage source) {
    ColorConvertOp grayScale = new ColorConvertOp(ColorSpace.getInstance(ColorSpace.CS_GRAY), null);

    return grayScale.filter(source,null);
}

private static BufferedImage invertImage(final BufferedImage src) {
    final int w = src.getWidth();

    final int h = src.getHeight();

    final BufferedImage dst = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);

    final BufferedImageOp invertOp = new LookupOp(new ByteLookupTable(0, invertTable), null);

    return invertOp.filter(src, dst);
}

// ...
BufferedImage sourceImage = ...
BufferedImage convertedImage = grayScale(invertImage(sourceImage));

How can I improve the above code?

Answer 1

Using Imgproc and Core, I did

Mat src = new Mat();
Mat gray = new Mat();
src = Highgui.imread("...");
Imgproc.cvtColor(src, gray, Imgproc.COLOR_BGR2GRAY);
Core.bitwise_not(gray, gray);
Highgui.imwrite("...", gray);

and got something closer, though not identical:

Answer 2

In the title it says OpenCV but there's a tag for BoofCV. So here's how to do it in BoofCV:

BufferedImage orig = UtilImageIO.loadImage("filename");
ImageUInt8 gray = ConvertBufferedImage.convertFrom(orig, (ImageUInt8) null);
ImageUInt8 inverted = new ImageUInt8(gray.width,gray.height);
GrayImageOps.invert(gray,255,inverted);

Answer 3

While I'm not familiar with ColorConvertOp, I would suspect that it is converting the color image to greyscale in a different way then Paint.NET is. There is no correct way.

Take a look at the yellow. Should yellow be a dark or light grey? It looks like Paint.NET thinks it is light and the ColorConvertOp thinks it is dark.

Try doing the conversion yourself by just averaging the red/green/blue.

(red+green+blue)/3