Artwork artifact
Dematerialization Near the Nose of Nero, Salvador Dali, 1947
Picture artifact
Odegaard library stairs
The camera position is represented in these by a white sphere.
The camera position is represented in these by a white sphere.
Much of the modeller operates using the idea of a "point stack". You can push points onto the stack by pointing at a point and pressing the spacebar. Similarly, you pop points off the stack by pointing at any other location on the screen and pushing the spacebar. It will become clear later exactly what it means for a point to be on the stack.
The first step in creating a model is to set up the coordinate system.
This involves two steps: first, you need to choose a bunch of
x, y, and z lines in the image.
Second, you need to choose the
x, y, and z unit vectors in the scene.
This is done by choosing the endpoints of the three unit vectors
and placing them on the stack in order.
For example, in the picture below, the origin is as shown and so
the unit x vector is denoted by the number 0, the unit
y axis is denoted by the number 1, and the unit z axis
is denoted by the number 2.
Once you have selected the appropriate points, you can then run the
"Compute VPs" command from the Tools menu to set the axes.
Note that after you do this, the points get little boxes around them, like
this: 
This denotes that the points are "known";
that is, you can use them to compute the 3D positions of other points.
Once you have chosen a coordinate axis, you can use these points to determine the positions of other points. For example, suppose you wanted to determine the position of other points in the ground plane. Then you can simply push the x unit vector onto the stack, push your new point onto the stack, and use the "same Z plane" command from the tools menu. If you have a point that you know is located directly above one of your other points, you can use the "same XY" command from the tools menu. You also have the option of specifying that a point lies in an arbitrary plane, by pushing 3 other points from the same plane onto the stack and then using the "same plane" command.
You can use guidelines to enforce line constraints by enabling them
in the tools menu. When guidelines are enabled, whenever the cursor is
near enough to the line connecting either two points on the stack, or
one of the points on the stack and a vanishing point, the line will
appear in the window and you can use it to guide your work. If you
want to snap to the line, hold down Ctrl while plaing your point.
Guidelines are especially useful in combination with the "same plane"
feature; with guidelines you can place points in the centers of boxes,
etc., as shown below.
We made the following additions to the basic requirements: