SWWheel Saga

📖 A Brief History

In 1870, German physiologist Ludimar Hermann noticed that faint gray ghost spots appeared at the intersections of white corridors in a black-and-white grid. The spots are vivid in peripheral vision, but vanish the instant you look directly at any intersection — one of the most striking examples of how the visual system actively constructs perception rather than simply recording it.

Hermann’s illusion is explained by lateral inhibition in the retina: ganglion cells at corridor intersections receive inhibitory input from more neighbors than cells along straight corridors, causing the brain to perceive intersections as slightly darker. While later research has proposed additional cortical mechanisms (orientation-selective filters in V1), lateral inhibition remains the dominant classroom explanation.

⚡ How the Illusion Works Here

• White arms on black — thick white SWCross arms fill the canvas, creating white corridors between black square “rooms.” Maximum contrast makes the ghost spots as vivid as possible.
• Sealed lattice — each arm tip meets the opposing tip of the adjacent sign exactly (spacing = 2 × arm length), creating unbroken corridors that the visual system reads as continuous white lines rather than individual plus signs.
• No hub dots — unlike the Lingelbach and scintillating grid variants, the pure Hermann Grid has nothing special at the intersections. The ghost spots arise entirely from corridor geometry.
• SQUARE arm caps — flat stroke ends ensure perfectly clean geometric joints, reinforcing the seamless-corridor percept.

🔧 What Was Built

• buildLingelbachGrid() (reused from Stage 4) computes grid dimensions from ARM_LENGTH and HALF_SPAN dynamically — no manual row/column count needed. The arm-length slider automatically rebuilds the grid.
• Hub drawing is guarded by HUB_DIAMETER > 0, so the default state is a pure Hermann Grid. Students can slide the Hub Size control up to explore the Lingelbach or scintillating-grid variants.
• Background, arm color, arm thickness, and arm opacity are all independently adjustable — letting students test how contrast and line weight affect the strength of the illusion.

🎮 Controls

• Arm length — larger arms widen both the black “rooms” and the white corridors equally (touching constraint is maintained).
• Arm weight — thicker arms = wider corridors = more vivid ghost spots. Very thin arms weaken or eliminate the illusion.
• Arm color / opacity — try reducing opacity or changing arm color to see how contrast affects the ghost-spot intensity.
• Hub size — slide up from 0 to add a colored dot at each intersection. This transitions the display toward the Lingelbach effect (small hubs) or the scintillating grid (larger white hubs).
• G — toggle the coordinate grid (keep off for best effect).

🧠 The Science

Each ganglion cell in the retina has a circular receptive field with an excitatory centre and an inhibitory surround (centre–surround organisation). A cell whose centre overlaps a straight corridor receives inhibitory input mainly from cells along that one corridor. A cell centered on an intersection receives inhibitory input from cells running in four directions simultaneously — far more total inhibition. The result is a weaker output signal, which the brain interprets as a slightly dimmer (grayish) patch at each intersection.

The fovea’s ganglion cells are extremely small and densely packed, so direct fixation on an intersection stimulates many cells with very precise centre–surround balance — and the ghost spot disappears. The illusion persists only in the periphery, where ganglion cells are larger and their surrounds span more corridor-crossing territory.