if (@P.z>0 && @P.y>0){
    @density*=1;
} else {
    @density=0;
}

f@density *= length(v@P) < 0.4;            // sphere, radius 0.4
f@density *= max(abs(v@P)) < 0.4;          // cube, size 0.4
f@density *= abs(v@P.y) < 0.04;            // plane, thickness 0.04
f@density *= length(v@P * {1,0,1}) < 0.1;  // cylinder, radius 0.1

f@density *= v@P.y > 0.0;                  // top half
f@density *= min(v@P.y, v@P.x) > 0;        // one quarter
f@density *= max(v@P.y, v@P.x) > 0;        // three quarter
f@density *= avg(v@P.y, v@P.x) > 0;        // diagonal half