// Atkinson dither translated from a Python source

PImage img;

void setup()
{
    img = loadImage("ball.jpg");  // add some image here!

    size( img.width, img.height );
}

void draw () {

    int[] threshold = new int[128+128];
    for ( int i = 0; i<threshold.length; i++ )
    {
        threshold[i] = i < 128 ? 0 : 255;
    }
            int[] idx = new int[]{
                1,
                2,
                - 1 + img.width,
                img.width,
                1 + img.width,
                img.width + img.width
            };

    for ( int iy = 0 ; iy < img.height; iy++ )
    {
        for ( int ix = 0 ; ix < img.width; ix++ )
        {
            int index = iy*img.width + ix;

            int old = toGrey(img.pixels[index]);    // blue only ..
            int now = threshold[old];
            int err = (old - now) >> 3; // devide by 8

            img.pixels[index] = color( now );
            for ( int i = 0; i<idx.length; i++ )
            {
                int ii = index + idx[i];
                if ( ii < img.pixels.length ) img.pixels[ii] = color( toGrey(img.pixels[ii]) + err );
            }
        }
    }

    image( img, 0, 0 );

    noLoop();
}

int toGrey ( int c )
{
    return (((c>>16) & 0xFF) + ((c>>8) & 0xFF) + (c & 0xFF)) / 3;
}