How to convert pixels to gray scale?

Question

Ok, I am using Processing which allows me to access pixels of any image as int[]. What I now want to do is to convert the image to gray-scale. Each pixel has a structure as shown below:

...........PIXEL............
[red | green | blue | alpha]
<-8--><--8---><--8--><--8-->  

Now, what transformations do I need to apply to individual RGB values to make the image gray-scale ??
What I mean is, how much do I add / subtract to make the image gray-scale ?

Update

I found a few methods here: http://www.johndcook.com/blog/2009/08/24/algorithms-convert-color-grayscale/

Answer 1

For each pixel, the value for the red, green and blue channels should be their averages. Like this:

int red = pixel.R;
int green = pixel.G;
int blue = pixel.B;

pixel.R = pixel.G = pixel.B = (red + green + blue) / 3;

Since in your case the pixel colors seem to be stored in an array rather than in properties, your code could end up looking like:

int red = pixel[0];
int green = pixel[1];
int blue = pixel[2];

pixel[0] = pixel[1] = pixel[2] = (red + green + blue) / 3;

The general idea is that when you have a gray scale image, each pixel's color measures only the intensity of light at that point - and the way we perceive that is the average of the intensity for each color channel.

Answer 2

The following code loads an image and cycle through its pixels, changing the saturation to zero and keeping the same hue and brightness values.

PImage img;

void setup () {
    colorMode(HSB, 100);
    img = loadImage ("img.png");
    size(img.width,img.height);
    color sat = color (0,0,0);

    img.loadPixels();

    for (int i = 0; i < width * height; i++) {
        img.pixels[i]=color (hue(img.pixels[i]), sat, brightness(img.pixels[i]));
    }

    img.updatePixels();
    image(img,0,0);
}