1.2.1.1 Understanding POVRay's Coordinate System
First, we have to tell POVRay where our camera is and where it is looking. To do this, we use 3D coordinates. The
usual coordinate system for POVRay has the positive yaxis pointing up, the positive xaxis pointing to the right,
and the positive zaxis pointing into the screen as follows:
This kind of coordinate system is called a lefthanded coordinate system. If we use our left hand's fingers we can
easily see why it is called lefthanded. We just point our thumb in the direction of the positive xaxis (to the
right), the index finger in the direction of the positive yaxis (straight up) and the middle finger in the positive
zaxis direction (forward). We can only do this with our left hand. If we had used our right hand we would not have
been able to point the middle finger in the correct direction.
The left hand can also be used to determine rotation directions. To do this we must perform the famous "Computer
Graphics Aerobics" exercise. We hold up our left hand and point our thumb in the positive direction of the
axis of rotation. Our fingers will curl in the positive direction of rotation. Similarly if we point our thumb in the
negative direction of the axis our fingers will curl in the negative direction of rotation.
In the above illustration, the left hand is curling around the xaxis. The thumb points in the positive x direction
and the fingers curl over in the positive rotation direction.
If we want to use a righthanded system, as some CAD systems and modelers do, the right
vector in the camera specification needs to be changed. See the detailed description in "Handedness".
In a righthanded system we use our right hand for the "Aerobics".
There is some controversy over whether POVRay's method of doing a righthanded system is really proper. To avoid
problems we stick with the lefthanded system which is not in dispute.
