double mm;
    double dX,dY,dZ;
    dX=camera.xto-camera.x;
    dY=camera.yto-camera.y;
    dZ=camera.zto-camera.z;
    mm=sqrt(dX*dX+dY*dY+dZ*dZ);
    dX/=mm;
    dY/=mm;
    dZ/=mm;
    double uX,uY,uZ;
    uX=camera.xup;
    uY=camera.yup;
    uZ=camera.zup;
    mm=uX*dX+uY*dY+uZ*dZ;
    uX-=mm*dX;
    uY-=mm*dY;
    uZ-=mm*dZ;
    mm=sqrt(uX*uX+uY*uY+uZ*uZ);
    uX/=mm;
    uY/=mm;
    uZ/=mm;
    double tFOV;
    tFOV=tan(camera.fov*PI/360);
    uX*=tFOV;
    uY*=tFOV;
    uZ*=tFOV;
    double vX,vY,vZ;
    vX=uY*dZ-dY*uZ;
    vY=uZ*dX-dZ*uX;
    vZ=uX*dY-dX*uY;
    vX*=SCREEN_WIDTH/SCREEN_HEIGHT;
    vY*=SCREEN_WIDTH/SCREEN_HEIGHT;
    vZ*=SCREEN_WIDTH/SCREEN_HEIGHT;

    double pX,pY,pZ;
    pX=x1-camera.x;
    pY=y1-camera.y;
    pZ=z1-camera.z;
    mm=pX*dX+pY*dY+pZ*dZ;

    double fx1,fy1;

    if(mm>0)
    {
        pX/=mm;
        pY/=mm;
        pZ/=mm;

        mm=(pX*vX + pY*vY + pZ*vZ) / pow((SCREEN_WIDTH/SCREEN_HEIGHT)*tan(camera.fov*PI/360),2);
        fx1=(mm+1)/2*SCREEN_WIDTH;
        mm=(pX*uX + pY*uY + pZ*uZ) / pow(tan(camera.fov*PI/360),2);
        fy1=(1-mm)/2*SCREEN_HEIGHT;
    }
    else
    {
        drawit++;
        //TODO: behind the camera code
    }