# Letterbox Fill: Hash-Based Block Dithering Fix & Cleanup

**Date:** 2026-02-13

## Task Description

Resolved an e-ink display crosstalk issue where a specific book cover ("The World in a Grain") became completely washed out and invisible when using "Dithered" letterbox fill mode. The root cause was pixel-level Bayer dithering creating a high-frequency checkerboard pattern in the BW pass for gray values in the 171-254 range (level-2/level-3 boundary), which caused display crosstalk during HALF_REFRESH.

## Solution

Hash-based block dithering with 2x2 pixel blocks for gray values in the problematic BW-boundary range (171-254). Each 2x2 block gets a uniform level (2 or 3) determined by a spatial hash, with the proportion approximating the target gray. Standard Bayer dithering is used for all other gray ranges.

## Changes Made

### Modified Files
- **`src/activities/boot_sleep/SleepActivity.cpp`** — Added `bayerCrossesBwBoundary()` and `hashBlockDither()` functions; `drawLetterboxFill()` uses hash-based block dithering for BW-boundary gray values, standard Bayer for everything else. Removed all debug instrumentation (H1-H20 logs, frame buffer checksums, edge histograms, rewind verification).
- **`src/CrossPointSettings.h`** — Reordered letterbox fill enum to `DITHERED=0, SOLID=1, NONE=2` with `DITHERED` as default.
- **`src/SettingsList.h`** — Updated settings UI labels to match new enum order.
- **`lib/GfxRenderer/GfxRenderer.h`** — Removed `getRenderMode()` getter (was only needed by debug instrumentation).

### Deleted Files
- 16 debug log files from `.cursor/` directory

## Key Findings

- Single-pixel alternation (block=1) causes display crosstalk regardless of pattern regularity (Bayer or hash).
- 2x2 minimum pixel runs are sufficient to avoid crosstalk on this e-ink display.
- The irregular hash pattern is less visually noticeable than a regular Bayer grid at the same block size.
- The issue only affects gray values 171-254 where Bayer produces a level-2/level-3 mix (the BW pass boundary).

## Follow-up Items

- None. Feature is stable and working for all tested book covers.