It's possible, but a bit more difficult, and only works with P3D I think.

Code:

PGraphics3 pg3;

void setup()
{
  size(400,400,P3D);
  pg3=(PGraphics3)g;
}

void draw()
{
  .. stuff ..
  color c=get(x,y); 
  float z=pg3.zbuffer[x+y*width]; // z-value is a float
} 

However the value of the zbuffer is a bit hard to fathom out.. though thing further away have bigger z-values from near ones, and undrawn points have z values equal to MAX_FLOAT.

Oh and the range of z-values seems to depend somehow on the height of the sketch... I've still to work out how they're calculated from x/y/z coordinates...

John,
Thank you. This helps.
I now need to workout the inverse transformation to get the corresponding 3D point in model space from the pixel coordinates and the zbuffer value.
- Jarek