Müller–Lyer illusion
Use the length slider to chase the point of subjective equality
Arrowheads and arrowtails push the endpoints of a line inward or outward. Slide the length offset until both shafts look equal, then scroll to reveal how each component modulates the illusion.
Live canvas
Step 1 · Length offset
Match the shafts by eye
Keep the lengths locked and adjust the offset until the two shafts appear identical. The required offset is your personal point of subjective equality (PSE).
Step 2 · Classic pairs
Switch between canonical Müller–Lyer configurations
Each preset sets both the top and bottom figures. Jump between arrowheads, arrowtails, t-caps, and other combinations to see how the illusion changes, then choose whether the shafts stay physically equal.
Step 3 · Global framing
Adjust the canvas margin, colours, and striped backgrounds
Framing and context alter how strong the illusion feels. Striped backgrounds can add assimilation cues that bias your judgement.
Step 4 · Perspective underlay
Test Gregory’s 3‑D interpretation
Add an inside or outside corner to simulate depth. Tuning the opacity and vanishing-point exaggeration shows how implied 3‑D context modulates the illusion.
Step 5 · Top figure
Shape the upper figure
Experiment with arrowheads, arrowtails, caps, circles, and feathered fins. Each option changes how far the endpoints feel from the shaft.
Step 6 · Bottom figure
Independently adjust the lower figure
Give the lower shaft its own configuration to create asymmetric pairs, or match the top for comparison. Try mixing feathers with plain lines.
Step 7 · Overlays & export
Toggle measurement aids and save the scene
Use ticks and bisection marks to prove the shafts are equal. Export a PNG or SVG when you want to capture a specific configuration.