Max - 9 months ago 43

Java Question

I found some bitshift code in the Java implementation of Hough Transform on Rosetta code, I understand in general what the code does, except this part:

`rgbValue = (int)(((rgbValue & 0xFF0000) >>> 16) * 0.30 + ((rgbValue & 0xFF00) >>> 8) * 0.59 + (rgbValue & 0xFF) * 0.11);`

I think it takes the average of all 3 pixels, that is at least what it looks like when I output the result. But how does this work? What are these magic numbers?

Method in which this function is used, pasted for convenience:

`public static ArrayData getArrayDataFromImage(String filename) throws IOException`

{

BufferedImage inputImage = ImageIO.read(new File(filename));

int width = inputImage.getWidth();

int height = inputImage.getHeight();

int[] rgbData = inputImage.getRGB(0, 0, width, height, null, 0, width);

ArrayData arrayData = new ArrayData(width, height);

// Flip y axis when reading image

for (int y = 0; y < height; y++)

{

for (int x = 0; x < width; x++)

{

int rgbValue = rgbData[y * width + x];

// What does this do?

rgbValue = (int)(((rgbValue & 0xFF0000) >>> 16) * 0.30 + ((rgbValue & 0xFF00) >>> 8) * 0.59 + (rgbValue & 0xFF) * 0.11);

arrayData.set(x, height - 1 - y, rgbValue);

}

}

return arrayData;

}

Answer Source

This is the trick that converts a 24-bit RGB value to a grayscale value using coefficients `0.3`

, `0.59`

, and `0.11`

(note that these values add up to `1`

).

This operation `(rgbValue & 0xFF0000) >>> 16`

cuts out bits 17..24, and shifts them right to position 0..7, producing a value between 0 and 255, inclusive. Similarly, `(rgbValue & 0xFF00) >>> 8`

cuts out bits 8..16, and shifts them to position 0..7.

This Q&A talks about the coefficients, and discusses other alternatives.