What result would I get for the following?

Question

Assume we have Image object, width and height already.
public int[][] getRedArray()
int values[] = new int[imageWidth * imageHeight];
PixelGrabber grabber = new PixelGrabber(imageobj.getSource(), 0, 0, imageWidth, imageHeight, values, 0, imageWidth);
try
if(grabber.grabPixels() != true)
try
throw new AWTException("Grabber returned false: " + grabber.status());
catch (Exception e) {};
catch (InterruptedException e) {};
int r[][] = new int[imageWidth][imageHeight];
int index = 0;
for (int y = 0; y < imageHeight; ++y)
for (int x = 0; x < imageWidth; ++x)
r[x][y] = ((values[index] & 0x00ff0000) >> 16);
index++;
return r;
Could you please please explain how this code works? brief step-by-step explaination would be satisfied.
Please explain explicitly how "r[x][y] = ((values[index] & 0x00ff0000) >> 16);" this works.
Thank you so much!! :)

Answer 1

Consider the following :
In this case a color is stored as 4 bytes. Each byte holds a color component, namely Alpha, Red, Green and Blue. In that order.
Now, 4 bytes = 1 int. The color is stored in an int, so bits 16-23 hold the red component. If you want to isolate that byte you'll need to do two things :
1) Isolate the "red" bits
2) Move them from bits 16-23 to 0-7
So, if we take the color value in bits, where A, R G and B signify a single bit reserved for an alpha, red, green or blue color component respectively :
AAAAAAAARRRRRRRRGGGGGGGGBBBBBBBBStep 1) is isolating the red :
00000000RRRRRRRR0000000000000000You do this by doing a bitwise AND with operand 1 being the color int, and operands 2 being an integer whose 16-23 bits are set :
color & 0x00ff0000Step 2) is moving the red bits to 0-7 :
(color & 0x00ff0000) >> 16
000000000000000000000000RRRRRRRRNow it's a valid integer value ranging from 0-255 and you can use it, as done in your code snippet.