To be able to do perspective drawing the viewing angle needs to be defined. Bring up the Vanishing Point Dialog (F6). In the dialog you see a set of cubes. By rotating these cubes, you define the viewing angle of your scene.
First click the cubes to redirect keyboard input to the cubes. Use the keyboard to rotate the cubes: The arrow keys are used to rotate the view, A and D tilt the view.
Once you are happy with the rotation, click the "Add Vanishing Point" button. A new entry appears in the list. Select the new vanishing point and close the vanishing point dialog. The editor now draws 3D guides that follow your cursor.
Once the vanishing points have been set up, the 3D cursor keeps you aware of the perspective. You have 3 vanishing points: Left, Right and Up/Down. Strokes can be locked to vanishing point by pressing and holding one of the keys A, S or D. These correspond to the left, up/down and right vanishing points. (If you have a non qwerty keyboard layout you probably want to change the shortcuts for selecting vanishing points.)
The stroke is locked to the vanishing point at the location of the cursor when you press the key. A circle appears on the screen to indicate the lock position. When you draw a stroke with your stylus, the stroke will appear on the line from the vanishing point to the lock point.
The lock point controls the scaling of the brush. The scaling is 100% at the lock point, and decreases towards 0% as you get closer to the vanishing point. Drawing further away from the lock point scales up the brush (>100%).
Masks can also be created using vanishing points using the 3D Mask tool. The idea is similar to how brush strokes are drawn in perspective. Press one of the VP selection keys (A, S, D) to select which VP is used. Next click and drag to draw the mask. A triangular mask is created, with the tip of the triangle at the selected VP and the other corners at infinity.
Step 1: The right VP is selected by pressing D. A triangular selection is created by clicking and dragging.
To get a 3D surface selected, you need to select another VP, and cut out the sides. Keeping ctrl pressed while drawing a mask subtracts from the current mask. Holding shift pressed while drawing adds to the mask.
Step 2: The left VP is selected by pressing A. Ctrl is held down while drawing another triangular mask, causing the mask to be subtracted.
Step 3: Another subtract operation is done on the other side leaving us with a rectangular mask in perspective.
