Stacking panels in depth does not increase effective capacity

What it is

A second and third panel placed further along z look like extra windows. Along the line of sight they compete for the same readable visual angle: the front covers the rear, and even a strip of the rear still needs the front to yield before it can be read. Depth stacking capacity barely increases — the z-axis is not a new bookshelf column, it is another book on the same slot. That sits next to the physiological fact of how many depth slots the eyes can keep alive. The question here is the information ledger: after stacking, did the number of words that can be finished at once go up. It did not. It often went down, because occlusion and layer-switching fragment the band.

Why it happens

Projection along the line of sight folds layers back onto one retinal patch. An opaque front sets rear capacity to zero; a translucent front mixes rear type with front type and both readable capacities fall. Reading the rear is not “look in another direction”; it is deal with the front first (move it, see through it, switch layer), so instantaneous bandwidth becomes time-shared. Desktop window stacks at least have a taskbar and a switcher to recall the buried layer; in space the rear often has no recall affordance except a hand to push the front aside. Stacking therefore manufactures fake inventory: board count up, settlable words not up. Side by side at one depth can still be read in one scan; side by side in z cannot even use that scan — vergence has to change.

Studying it

The same material that must be finished, compared as one board, two boards side by side at one depth, and two or three stacked in z. Do not allow the front to be closed unless that operation is part of the layout being tested.

Independent variables: layout topology (single / side-by-side / z-stack), front transparency, whether an action exists to take the front away. Dependent variables: time to finish reading, miss rate on the rear, occluded area of the rear, number of layer switches.

Capacity is “how much unique information was read correctly”, not “how many panels are in the scene”. The latter is exactly the fake metric stacking wants to sell.

Where it stops holding

A rear layer used only as background or presence, never read, does not claim extra reading capacity and may exist. Card-style “draw one layer, read one layer” is a sequential UI: instantaneous capacity is still one, inventory just happens to be expressed in z — call it switching, not parallelism. Fully transparent wireframe stacks used as explode views are about structure, not body text, and the ledger is different. Monocular, non-stereo viewing makes z-stacking even more like flat window stacking, and the capacity claim faker still, because even the depth cue for switching is weak.

Applying it

• Do not pad currently read content with another body-text layer in z. Add a page by placing it beside, by tab-switching, or by an explicit “draw the front away”.
• If a stack is required, the rear is not readable while the front is open; it enters the band only after the front is closed or moved.
• Write the acceptance metric as “unique items read correctly in one foveation”, not as “panels visible at once”.
• How to check: run the same reading on a three-deep stack and on two boards side by side. If the stack is no faster and misses the rear more, capacity did not grow with z — delete the rear from “readable at once”.

Related

• Same group: N3.14.1 Space got larger; the readable angular range did not · N3.14.3 Spatial layout has no automatic typesetting; overlap is constrained only by rules · N3.14.4 The extra placement freedom is also a tidying cost
• Nearby: N3.09 Interface Distance and Comfort Viewing Distance · N3.06 Transparency and Occlusion
• Search terms: depth stacking · information capacity · occlusion of UI