DIY · Card 62 · Perception · Motion
A shoebox, two cardboard tubes and a long strip of paper. Turn the crank and the scenery rolls past a cut-out car, which seems to speed down the road, though it never moves an inch.
01The brief
Make a picture travel without moving it.
2
rollers, carrying one loop of hand-drawn scenery
44 cm
of belt for rollers 15 cm apart and 4.5 cm across
0
centimetres the cut-out car actually moves
Put a toy car on a table and pull the tablecloth slowly from under it. Watch only the car and the cloth, and it looks as if the car is driving. Your eyes can see that the two are moving apart, but not which one is doing the moving.
This little cinema uses that blind spot. A loop of scenery rolls behind a still cut-out, and the cut-out comes alive. It is the trick behind Victorian panoramas, studio car chases and side-scrolling games. It belongs with the 3D hand illusion, another way to fool the eye, and the cardboard periscope, another box that plays with what you see.
02Kit
A box, two tubes and a long strip of paper to draw on.
Tick off what you have
0 of 6 ready
03Method
Straight rollers and a snug belt make the scenery glide.
Step 1 of 8 · Minute 0
You need a shoebox or small cardboard box, two toilet-roll tubes, two wooden dowels or long pencils a little longer than the box is tall, a strip of paper about 50 cm long, card, glue, double-sided tape, scissors and a paper clip.
The belt slips on the rollers.
Why
The loop is too loose.
Fix
Shorten the belt a little, or wrap a strip of masking tape round each roller to give it grip.
The belt creeps up or down and falls off.
Why
The rollers aren’t parallel.
Fix
Check the holes line up exactly, top above bottom, and that both are the same distance from the front of the box.
The crank is hard to turn.
Why
The belt is too tight or the holes too small.
Fix
Loosen the belt slightly and widen the holes with a pencil so the dowels turn freely.
04The illusion
Motion is always measured against something. A car passing trees moves relative to the trees; trees passing a car move relative to the car. On its own, the eye only sees the difference. When the background is big and the object small, the brain usually decides the background is still and the object is moving, even when the opposite is true.
Crank the belt, change the roller, and switch between the two views:
Belt speed
8.5 cm/s
Belt length, rollers 15 cm apart
44 cm
The car seems to go
15 km/h
Switch between the two views. In one the car stays still and the world slides past; in the other the world stays still and the car drives. Watching the car, you can hardly tell them apart: all your eye can measure is how fast the car and the scenery move relative to each other. If the scenery is drawn at 1 : 50, a toy car, the cinema’s car seems to be doing 15 km/h.
Add more belts at different distances and the scene gains depth, as in a side-scrolling game:
Mountains, 8 units
8 px/s
Hills, 3 units
20 px/s
Fence, 1 unit away
60 px/s
Something twice as far away seems to move half as fast. Your brain uses that difference to judge distance, so three flat layers moving at different speeds look like a deep landscape. Add a second, slower belt behind your first and the cinema gains depth.
05The big idea
All motion
is relative.
There is no such thing as simply ‘moving’: only moving compared with something else. Galileo pointed out that a sailor below deck on a smoothly sailing ship couldn’t tell whether it was moving at all.
Your cinema shows the same idea at the kitchen table. Car still and road moving, or road still and car moving: from where you sit, the two are the same thing. It is a short step from here to the physics of Galileo and Einstein.
06The math
1 · Belt speed
v = π d n
Roller diameter d, n turns a second.
2 · Belt length
L = 2a + π d
Rollers a apart: two straight runs and once round.
3 · Parallax
v ∝ 1 ⁄ z
Apparent speed of scenery at distance z.
Turn a 4.5 cm roller once every two seconds and the belt moves π × 4.5 × 0.5 ≈ 7 cm a second. If your scenery is drawn at about 1 : 50, toy-car scale, that is like driving at 7 × 50 = 350 cm/s, about 13 km/h: a bike ride. Crank three times faster and you’re up to about 38 km/h.
For depth, a second belt of mountains should move more slowly. If the mountains are meant to be 8 times further away than the road, move that belt 8 times more slowly, for example with a roller 8 times thinner on the same axle.
07Real life
Painted panoramas, film studios, stages and screens.
Moving panoramas
In the 1800s, audiences paid to watch enormous painted scenes, some said to be hundreds of metres long, wound slowly from one giant roller to another, often with music and a narrator: a journey down a river without leaving your seat.
Is my train moving?
Sitting in a still train, you see the train next to you start, and for a moment you are sure it is your train that is moving. Your brain has the same problem as with your cinema car.
Studio car scenes
For decades film-makers shot driving scenes in the studio: the car stood still while a film of moving streets was projected or scrolled behind it, exactly like your cut-out car and its paper road.
Side-scrolling games
Arcade games of the early 1980s learned to slide several layers of scenery at different speeds. Moon Patrol, in 1982, used three, and the flat screen suddenly looked deep.
The multiplane camera
Walt Disney’s studio built a camera that filmed painted scenery on several glass layers at different distances, each moved separately. It made its debut in the 1937 short The Old Mill and gave cartoons real depth.
Treadmills on stage
When the play Ben-Hur opened in New York in 1899, its famous chariot race was run by real horses galloping on treadmills, with a painted backdrop rolling the other way behind them.
1937
Cartoons used to look flat: one painted background slid behind the characters. Walt Disney’s studio built a towering camera that looked down through several sheets of glass, each painted with part of the scene, and moved each layer at its own speed.
Its first outing, the 1937 short The Old Mill, showed a forest and a windmill with real depth. It is your conveyor cinema with extra belts, the same parallax trick that later made side-scrolling video games look deep.
08Is and isn’t
You can always tell what is really moving.
Only by comparison. With no fixed reference in view, your eyes can only see that two things move relative to each other, which is why the still car seems to drive.
A faster crank makes the car look closer.
It makes it look faster. What makes things look near or far is parallax: near layers sliding faster than far ones.
Films are moving pictures.
Films are still pictures shown quickly one after another. Your conveyor cinema is different: here, the picture really does move, continuously.
The background has to be realistic.
Simple stripes work almost as well. The illusion comes from the motion, not from the detail of the drawing.
09Go further
A
Add a second belt
Fit a second, wider pair of rollers further back with distant mountains, geared to turn more slowly. Does the scene look deeper?
B
Reverse it
Turn the crank backwards. Does the car seem to reverse, or does it look as though it is driving the other way?
C
A flying scene
Draw clouds and birds on the belt, and a cut-out plane in front. Run the belt up and down instead of sideways.
D
Time the drive
Count crank turns for 10 seconds. Use the calculator to work out how fast your car seems to be going.
E
Film it
Record a video with the phone close to the opening. Does the illusion look stronger on screen? Why might that be?
10Questions
Stand an open box on its end. Fit two cardboard tubes on dowels through holes in its top and bottom, as rollers. Wrap a long strip of paper with a landscape drawn on it round both rollers and tape it into a loop. Put a cut-out picture, such as a car, in front, and turn a paper-clip crank: the scenery rolls past and the car seems to drive.
Your eyes judge motion mostly by how things move relative to each other. When the background slides past a still object, the brain often decides the object is the one moving, the way a stationary train can seem to pull away when the train beside it starts. Psychologists call this induced motion.
Twice the distance between the rollers plus once round a roller: L = 2a + πd. With rollers 15 cm apart and 4.5 cm across, that is 2 × 15 + π × 4.5 ≈ 44 cm. Join two strips of paper if one isn’t long enough.
Add a second, slower belt further back, for distant hills or clouds. Far-away things seem to move more slowly than near ones, and that difference, called parallax, is one of the strongest clues the brain uses to judge depth.
Nineteenth-century audiences watched huge painted panoramas unrolled between rollers. Film-makers used moving backdrops and rear projection to film car scenes in the studio, and side-scrolling video games move layers of scenery at different speeds, a technique made famous by the 1982 arcade game Moon Patrol.
Safety · A grown-up should make the pin holes and do any cutting with a craft knife. If you use a hot-glue gun, let a grown-up handle it.
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