Where your eye stands, the lines follow

The horizon is always at your eye level

Perspective is not a set of rules laid over a drawing; it is a description of where you are standing. Every line that runs away from you in the same direction appears to meet at a single point, and every such point for lines lying flat sits on one line: the horizon, which is simply the height of your own eye. Move your eye and the whole world rearranges itself around it.

How to use it

• One-point: face an object square on. Only the lines that run away from you converge, to one point straight ahead. Raise your eye above the cube and its top appears; lower it and the top disappears into its own edge.
• Two-point: turn the object. Its edges split into two families, each with its own point on the horizon. At 45° the points sit evenly; turn further and one rushes close while the other flies off the page.
• Three-point: tilt your gaze up at a tall building or down from a height, and the verticals converge too, to a third point far above or below.
• Distance: come close and the lines converge steeply; step back and they relax. Wide, dramatic perspective is a sign of a close viewpoint.
• Cylinders and spheres: turn on the construction box. Every ellipse in a painting sits inside a square in perspective; draw the box first, and the ellipse follows.

What each slider means, in the room

Imagine yourself standing in a room with one object on the floor in front of you. Every slider is something you could actually do in that room.

• Your eye level is the height of your eye above the floor. Crouch and it drops; stand on a chair and it rises. The horizon is not out there in the distance, it is at your eye, so it rises and falls with you. Below the floor (negative values) you are looking up from beneath a glass floor.
• Move the object left · right slides the object across the floor while you keep facing straight ahead. It is the same as stepping sideways yourself: perspective only knows where the object is relative to your eye. The vanishing point of a one-point view stays directly in front of you, so a sideways object shows one of its sides.
• Distance is how far you stand from the object. Walk closer and it grows and its lines converge sharply; walk away and it shrinks and its lines relax.
• Zoom changes nothing in the room. It is the size of your sheet of paper, or how far you hold the drawing from you. Use it to bring distant vanishing points onto the page.
• Turn the object is a turntable under it. At 0° you face it square on (one-point); turn it and two families of edges run to two points on the horizon; at 45° they sit evenly on either side.
• Tilt your gaze tips your head up or down. The moment your gaze is no longer level, the verticals stop being parallel on paper and converge to a third point: above you when you look up at a tower, below you when you look down from a height.
• Depth of field is focus, not position: the band of distance a lens or eye keeps crisp. Painters use the same idea with softer edges and cooler, paler colour to push things back.
• Where you are shows two small maps in the corner: a side view (your eye height against the object) and a top view (where you stand and which way you look). Use them whenever you lose track of which of you moved.

Find your eye level first. Everything else in the drawing is measured from it.