Occlusion is the strongest depth cue and must not be violated

What it is

Near covers far, and order is settled. Occlusion as a depth cue outranks disparity, relative size, and perspective at almost every distance. If a farther panel covers a nearer hand, or a rear label covers a front button, people do not become “slightly unsure” — they invert the spatial relation, or they judge the scene broken. Semi-transparency can weaken the cue; the issue here is which opaque silhouette cuts which. A reversed order is very hard for other cues to walk back.

Why it happens

Cutting and Vishton rank occlusion as the strongest ordinal cue in near space: it states that A is in front of B, not how many metres, and that bit of order is almost never overturned. Vision treats covering as a contact relation between opaque volumes. Geometrically wrong occlusion — opaque pixels of a far object painted over a near one — fights vergence and stereo matching at once. People may briefly explain it as “a HUD layer”; once volumetric contours such as a hand, a table edge, or a shoulder are also sliced by a farther panel, the explanation collapses and spatial credibility drops as a block. Wrong occlusion is worse than missing occlusion: missing is a lack of information, wrong is a lie.

Studying it

A depth-order judgement: two objects, with the system controlling who should cover whom and whether the covering is deliberately reversed. Ask which is nearer, not how comfortable the view feels — comfort mixes occlusion error with sickness.

Independent variables: occlusion relation (correct / reversed / none), stereo on or off, motion parallax on or off. Dependent variables: two-alternative accuracy, reaction time, a rating of whether the scene is believable.

Pictorial-cue weighting experiments and XR detections of virtual–real occlusion errors both belong here. Flicker from z-fighting can be a stimulus on its own; it is more conspicuous than a stable wrong order.

Where it stops holding

In far landscape layers occlusion still works, but people no longer use it to place a UI. A flat overlay that never intersects scene silhouettes is treated as another screen and the occlusion rule is briefly waived; the waiver dies the moment a contour cuts a near object. Monocular users, lacking stereo, lean harder on occlusion, so reversals hurt more. When transparency is high enough that the rear surface is clearly seen, strict occlusion downgrades to “also visible” and order must be carried by other cues.

Applying it

• Keep opaque depth order identical to geometry: near cuts far; far must not cut near.
• Hands, controllers, and other people’s head–shoulder contours get correct occlusion first; do not let a menu cut a hand so the menu stays visible.
• Elements that must float in front of everything live on a layer that does not participate in scene occlusion, and their silhouettes should not cut near objects.
• How to check: reach in close, then walk around the panel. Any frame in which a hand is cut by a farther panel, or a far label covers a near button, is a violated occlusion cue.

Related

• Same group: N3.06.1 A semi-transparent panel mixes with the background · N3.06.3 Transparency must adapt to background complexity · N3.06.4 Cast and contact shadows declare depth better than transparency · N3.06.5 Transparency carries a pass-through meaning, not just a visual style · N3.06.6 Stacked semi-transparent panels multiply opacity · N3.06.7 On optical see-through, black equals transparent; a dark UI cannot hide reality
• Nearby: N3.03 Depth Conflict · N5.02 Virtual–Real Occlusion
• Search terms: occlusion depth cue · depth order · pictorial depth cue