From pixels[] > get() in the Reference:

Getting the color of a single pixel with get(x, y) is easy, but not as fast as grabbing the data directly from pixels[] . The equivalent statement to "get(x, y)" using pixels[] is "pixels[y*width+x]".

This should give you a clue ;)

Nice one, I'll give it a go :)

Many thanks

There is also a neat processing trick

if you declare a color [] and set it to the image pixel array in question.  It's even faster for some reason

e.g.

PImage a;
color [] A;
void setup(){

size(255,255,P3D);
loadPixels();
a=createImage(width,height,ARGB);
A= a.pixels;
}


void draw(){
for  (int x=0;x<a.width;x++){
for (int y=0;y<a.height;y++){
int pos = x+a.width*y;
A[pos]=color (a.height-x,x-y,a.width-x);
}
}
arraycopy(A,pixels);

}