The physical gap between cover glass and the emissive layer causes parallax offset
Aliases: parallax · glass gap · digitizer parallax · cover-glass offset
What it is
The nib rests on the top of the cover glass; pixels emit from a layer below. Between them sit glass thickness, an air gap, the touch sensor, and polarizers. When eye, nib, and pixel are not collinear, ink does not look under the nib—parallax offset. It is there when the pen is still, and it is not the “ink chasing from behind” of latency. A hover cursor drawn in display coordinates is under the same geometry.
Why it happens
The cover lifts the tactile nib above the display plane. A line of sight from above and to the side projects the nib and the lit pixel to different places on the retina; the split grows with thickness and incidence angle. The digitizer usually reports a position on the sensor plane, and ink is drawn on the pixel plane; if those planes were calibrated only along the normal, a side view opens a seam. Film and a case add thickness, so a factory alignment computed on a bare panel is pushed apart. Optical bonding (removing the air gap) shrinks but does not zero the effect, because the glass itself still has thickness. An opaque nib also hides the pixel it “should” hit, so people aim with the nib’s outline and the offset mixes aiming strategy with geometry.
Studying it
Hold the nib still on a crosshair. Photograph the crosshair-to-nib relation head-on and from a camera at a known angle. Repeat with films of different thickness.
Independent variables: total cover+film thickness, viewing angle, optical bonding. Dependent measures: static pixel offset of ink/crosshair relative to the nib, whether offset follows simple parallax geometry with thickness and angle.
Only a static measurement separates parallax from latency. A moving seam is not evidence for this claim.
Where it stops holding
Head-on, almost along the normal, parallax is smallest; a demo pose undercounts ordinary writing from a slanted grip. Thick paper-feel film helps haptics and hurts parallax. Projected writing, or watching the screen through a camera, is another geometry; tablet bonding numbers do not transfer.
Applying it
- Optically bond where possible to shrink the air gap. When film is required, fold the extra thickness into alignment rather than pretending the panel is still bare.
- Calibrate and demo with at least one slanted writing view; do not shoot the campaign only along the camera normal.
- Accept static press on a crosshair: ink under the nib, with a stated pixel budget.
- How to check: static press, one head-on still and one at about 30°. Slanted offset should grow with thickness. Compare before and after film. If a fast stroke’s seam still changes with speed, that is latency, not thickness.