/** Draw atom */
void drawAtom(Graphics g, int iz) {
int i = zOrder[iz];
Atom atom = atoms[i];
if (atom == null) return;
double zMin = screenXYZ[zOrder[0]][2]; // fartherest from the viewer
double zMax = screenXYZ[zOrder[np - 1]][2]; // closest to the viewer
int greyScale = Atom.nBalls - 1;
if (zMin != zMax) // zMin == zMax means the two-dimensional case
greyScale = (int) (Atom.nBalls * (screenXYZ[i][2] - zMin) / (zMax - zMin) - 1e-6);
// the atom closest to the viewer has a greyScale of Atom.nBalls - 1
// the atom fartherest from the viewer has a greyScale of 0
double radius = ballSize * atom.relRadius * real2Screen;
atom.paint(g, screenXYZ[i][0], screenXYZ[i][1], greyScale, radius);
}
