50 lines
2.9 KiB
Markdown
50 lines
2.9 KiB
Markdown
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# Letterbox Fill: Replace Dithering with Edge Replication ("Extend Edges")
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**Date:** 2026-02-13
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## Task Description
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After implementing Bayer ordered dithering for the "Blended" letterbox fill mode (replacing earlier noise dithering), the user reported that the problematic cover (*The World in a Grain* by Vince Beiser) looked even worse. Investigation revealed that **any dithering technique** is fundamentally incompatible with the e-ink multi-pass rendering pipeline for large uniform fill areas:
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- The BW HALF_REFRESH pass creates a visible dark/white pattern in the letterbox
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- The subsequent greyscale correction can't cleanly handle mixed-level patterns in large uniform areas
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- This causes crosstalk artifacts that can affect adjacent cover rendering
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- Uniform fills (all same level, like Solid mode) work fine because all pixels go through identical correction
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The user's key insight: the display already renders the cover's grayscale correctly (including Atkinson-dithered mixed levels). Instead of re-dithering an averaged color, replicate the cover's actual boundary pixels into the letterbox -- letting the display render them the same way it renders the cover itself.
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## Changes Made
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### `src/CrossPointSettings.h`
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- Renamed `LETTERBOX_BLENDED` (value 2) → `LETTERBOX_EXTENDED`
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- Updated comment to reflect "None / Solid / Extend Edges"
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### `src/SettingsList.h`
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- Changed UI label from `"Blended"` → `"Extend Edges"`
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### `src/activities/boot_sleep/SleepActivity.cpp`
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- **Removed**: `BAYER_4X4` matrix, `quantizeBayerDither()` function (all Bayer dithering code)
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- **Added**: `getPackedPixel()`, `setPackedPixel()` helpers for packed 2-bit array access
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- **Rewrote** `LetterboxFillData` struct:
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- Added `edgeA`/`edgeB` (dynamically allocated packed 2-bit arrays) for per-pixel edge data
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- Added `edgePixelCount`, `scale` for coordinate mapping
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- Added `freeEdgeData()` cleanup method
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- **Renamed** `computeEdgeAverages()` → `computeEdgeData()`:
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- New `captureEdgePixels` parameter controls whether to allocate and fill edge arrays
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- For horizontal letterboxing: captures first/last visible BMP rows
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- For vertical letterboxing: captures leftmost/rightmost pixel per visible row
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- Still computes averages for SOLID mode in the same pass
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- **Rewrote** `drawLetterboxFill()`:
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- SOLID mode: unchanged (uniform fill with snapped level)
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- EXTENDED mode: maps screen coordinates → BMP coordinates via scale factor, looks up stored 2-bit pixel values from the cover's boundary row/column
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- **Updated** `renderBitmapSleepScreen()`: new log messages, `freeEdgeData()` call at end
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## Backward Compatibility
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- Enum value 2 is unchanged (was `LETTERBOX_BLENDED`, now `LETTERBOX_EXTENDED`)
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- Serialized settings files continue to work without migration
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## Follow-up Items
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- Test "Extend Edges" mode with both covers (Power Broker and World in a Grain)
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- Test vertical letterboxing (left/right) if applicable covers are available
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- Verify SOLID mode still works as expected
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