Crosspoint

Justinian 825ef56ad8 feat: X3 grayscale antialiasing improvements (#1607 )

## Summary
Improves text antialiasing quality on the Xteink X3 (SSD1677) display to
bring it closer to X4 rendering quality. Addresses white lines through
letter strokes and ghosting artifacts during page turns and screen
transitions.

## Changes

### Display Driver (open-x4-sdk)
- Dedicated X3 grayscale LUTs with tuned VDL drive strengths for dark
gray (2 time units) and light gray (3 time units), with active GND hold
on non-gray transitions to prevent floating source crosstalk
- Tight scan timing: TP2/TP3 reduced to 1 (total gate-on 7 units vs 17),
minimizing parasitic charge leakage that caused white lines through
letter strokes
- Fast diff BB reinforcement: Added mild VDH reinforcing pulse to
lut_x3_bb_full so black pixels are actively driven during differential
refreshes, clearing gray residue/ghosting
- displayGrayBuffer() updated to use the dedicated gray LUT bank instead
of full refresh LUTs for X3

### Rendering Pipeline
- Re-enabled light gray rendering for X3 text and images, now safe with
dedicated gray LUTs providing proper 4-level gray
- Removed isLightGrayRestricted() gating that was limiting X3 to 3-level
gray
- Runtime display dimensions in DirectPixelWriter and ScreenshotUtil,
replaced hardcoded constants with runtime getters to support X3 792x528
resolution

### Note
The open-x4-sdk submodule references a commit on
juicecultus/community-sdk. A corresponding PR to
open-x4-epaper/community-sdk should be merged first so the submodule ref
resolves on upstream.

## Testing
Tested on physical X3 hardware. White lines through letters
significantly reduced, in-book ghosting improved via BB reinforcement,
antialiasing visually closer to X4 quality.

## AI Disclosure
Yes, AI was used to assist with the development of these changes.

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2026-04-08 21:58:59 -04:00

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