Offset direction and magnitude need per-model factory calibration, not one shared parameter

What it is

Parallax geometry depends on this panel’s glass thickness, bonding, and the translation and rotation of the digitizer relative to the pixel array. Those quantities differ across models and sometimes across lots of the same model. A vector hard-coded as “all tablets compensate 3 px left” will straighten some units and skew others. Direction and magnitude must be factory-calibrated per model (better, per unit), and the software must load that unit’s table.

Why it happens

Digitizer and display coordinates differ by translation, scale, rotation, and a little nonlinearity (edge electrode distortion). Factory jigs put the nib on known pixel targets and fit the table. A table shared by a whole series ignores glass and bonding tolerances of different SKUs; no table at all leaves apps assuming the two frames already coincide. Calibration also includes which pen: nib length changes where contact sits relative to the sensor, so a different tip is a different lever. User film is not in the factory table; if the system allows film, it needs an extra “I applied film” offset rather than editing the model table. A lost or corrupted table after a firmware flash looks like “the pen is inaccurate”; the table is gone. Edge nonlinearity cannot be fixed by one global vector; a grid is required, or the centre lines up and the corners do not.

Studying it

Measure static alignment error across several models and lots. Compare no table, a series-wide table, a per-unit jig table, and a four-point user calibration.

Independent variables: calibration grain (series / model / unit), edge grid, nib swap. Dependent measures: static error at centre and corners, whether error jumps after an SKU change, how much field calibration recovers.

Changing model without changing software is the critical control.

Where it stops holding

A stable model from one mould and bonding line may be fine at model grain without a per-unit jig. Field user calibration can cover film, but completion of four targets by ordinary note-takers is low; it cannot replace factory. A third-party nib length is not in the OEM table, and error falls back into the “one parameter” failure mode.

Applying it

• Store a table per SKU and load it from a hardware id at boot; flashes must keep or rewrite that table.
• Use a grid at the corners, not only one vector at the centre.
• Offer optional user trim (film); default remains the factory table, not blank.
• How to check: the same software on two machines of different thickness/bonding, each with its table—static crosshairs should align. Copy A’s table onto B; error should jump. Measure corners, not only centre. Swap an official nib and remeasure the lever.

Related

• Same group: C5.12.1 Glass-to-display gap causes parallax · C5.12.2 Parallax offset changes with viewing angle · C5.12.4 Parallax and latency lag are independent and easy to confuse
• Adjacent: C5.17 Pen Pairing, Battery, and Loss · C5.01 Pressure Sensitivity
• Search: digitizer calibration · per-device alignment · SKU offset