SWWheel Saga

Troxler’s Effect (1804) is an optical illusion in which peripheral, stationary, low-contrast stimuli fade from conscious perception when you hold your gaze very still on a central point. Your visual cortex stops reporting signals that never change — a form of neural adaptation that saves processing bandwidth.

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👁 How the Illusion Works

• Fixate — stare at the black dot without moving your eyes. Within 5–15 seconds the peripheral plus signs should begin to ghost out and may disappear entirely.
• Move your eyes — the signs instantly snap back. The illusion requires a perfectly stable gaze.
• Contrast matters — drag the Arm opacity slider down (lower contrast = faster fading) or up (higher contrast = fading is harder).
• Motion breaks it — even a tiny rotation speed (5–10°/s) is enough to prevent the signs from fading at all, because motion continuously refreshes the peripheral signal.

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🔧 What Was Built in Stage 3

• A flat array of SWCross instances is created by buildCrossGrid(), with each sign positioned at a new SWPoint(cx, cy) in user-space and drawn via ps.drawOnGrid(grid).
• The centre cell at user (0, 0) is left empty (null in the array) so the fixation dot is never covered.
• The fixation dot is drawn in screen-pixel coordinates using grid.userToScreen(0, 0) — it always sits exactly at the canvas centre regardless of the grid mapping.
• Layout changes (columns, rows, spacing) call buildCrossGrid() to recreate the array. Appearance changes (colour, alpha, thickness) call applyAppearanceToAll(), which updates each sign in-place without rebuilding.

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🎮 Controls

• Columns / Rows — change the grid dimensions. Values are forced odd so a column/row always lands on the origin.
• Spacing — distance between sign centres in user units.
• Arm opacity — single most important parameter; lower values give stronger fading.
• Rotation — demonstrate that motion prevents fading.
• Fixation dot size — enlarge for younger students.
• G — toggle the coordinate grid (off = best effect).

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🧠 The Science

The fading occurs because retinal ganglion cells and higher visual neurons adapt to unchanged inputs. When a stimulus has been constant for several seconds, the cells reduce their firing rate — the brain interprets this as “nothing new to report” and suppresses the perception. This is the same mechanism behind why you stop noticing a constant background sound or smell.

Moving stimuli, or moving eyes, constantly shift the stimulus onto fresh photoreceptors, preventing adaptation and keeping the signal strong — which is why the illusion breaks the instant you look away from the fixation dot.