# Sleep Cover: Double FAST_REFRESH for Dithered Letterbox

**Date:** 2026-02-19

## Task

Apply the "double FAST_REFRESH" technique (proven for inline EPUB images via PR #556) to sleep cover rendering when dithered letterboxing is enabled. This replaces the corruption-prone HALF_REFRESH with two FAST_REFRESH passes through a white intermediate state.

Also reverted the hash-based block dithering workaround back to standard Bayer dithering for all gray ranges, confirming the root cause was HALF_REFRESH rather than the dithering pattern.

## Changes Made

### `src/activities/boot_sleep/SleepActivity.cpp`

- **Added `USE_SLEEP_DOUBLE_FAST_REFRESH` define** (set to 1): compile-time toggle for easy A/B testing.
- **Removed `bayerCrossesBwBoundary()` and `hashBlockDither()`**: These were the HALF_REFRESH crosstalk workaround (2x2 hash blocks for gray 171-254). Removed entirely since double FAST_REFRESH addresses the root cause.
- **Simplified `drawLetterboxFill()`**: Dithered mode now always uses `quantizeBayerDither()` for all gray ranges -- same algorithm used for non-boundary grays. No more hash/Bayer branching.
- **Modified `renderBitmapSleepScreen()` BW pass**: When dithered letterbox is active and `USE_SLEEP_DOUBLE_FAST_REFRESH` is enabled:
  1. Clear screen to white + FAST_REFRESH (establishes clean baseline)
  2. Render letterbox fill + cover + FAST_REFRESH (shows actual content)
- **Letterbox fill included in all greyscale passes**: Initially skipped to avoid scan coupling, but re-enabled after testing confirmed the double FAST_REFRESH baseline prevents corruption. This ensures the letterbox color matches the cover edge after the greyscale LUT is applied.
- **Standard HALF_REFRESH path preserved** for letterbox=none, letterbox=solid, or when define is disabled.

## Build

Successfully compiled with `pio run` (0 errors, 0 warnings).

## Testing

Confirmed working on-device -- dithered letterbox renders cleanly without corruption, and letterbox color matches the cover edge including after greyscale layer application.

## Commit

`0fda903` on branch `mod/sleep-screen-tweaks-on-1.1.0-rc-double-fast`