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perf: Reduce overall flash usage by 30.7% by compressing built-in fonts (#831)

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## Summary

**What is the goal of this PR?**

Compress reader font bitmaps to reduce flash usage by 30.7%.

**What changes are included?**

- New `EpdFontGroup` struct and extended `EpdFontData` with
`groups`/`groupCount` fields
- `--compress` flag in `fontconvert.py`: groups glyphs (ASCII base group
+ groups of 8) and compresses each with raw DEFLATE
- `FontDecompressor` class with 4-slot LRU cache for on-demand
decompression during rendering
- `GfxRenderer` transparently routes bitmap access through
`getGlyphBitmap()` (compressed or direct flash)
- Uses `uzlib` for decompression with minimal heap overhead.
- 48 reader fonts (Bookerly, NotoSans 12-18pt, OpenDyslexic) regenerated
with compression; 5 UI fonts unchanged
- Round-trip verification script (`verify_compression.py`) runs as part
of font generation
## Additional Context

## Flash & RAM

| | baseline | font-compression | Difference |
|--|--------|-----------------|------------|
| Flash (ELF) | 6,302,476 B (96.2%) | 4,365,022 B (66.6%) | -1,937,454 B
(-30.7%) |
| firmware.bin | 6,468,192 B | 4,531,008 B | -1,937,184 B (-29.9%) |
| RAM | 101,700 B (31.0%) | 103,076 B (31.5%) | +1,376 B (+0.5%) |

## Script-Based Grouping (Cold Cache)

Comparison of uncompressed baseline vs script-based group compression
(4-slot LRU cache, cleared each page). Glyphs are grouped by Unicode
block (ASCII, Latin-1, Latin Extended-A, Combining Marks, Cyrillic,
General Punctuation, etc.) instead of sequential groups of 8.

### Render Time

| | Baseline | Compressed (cold cache) | Difference |
|---|---|---|---|
| **Median** | 414.9 ms | 431.6 ms | +16.7 ms (+4.0%) |
| **Pages** | 37 | 37 | |

### Memory Usage

| | Baseline | Compressed (cold cache) | Difference |
|---|---|---|---|
| **Heap free (median)** | 187.0 KB | 176.3 KB | -10.7 KB |
| **Heap free (min)** | 186.0 KB | 166.5 KB | -19.5 KB |
| **Largest block (median)** | 148.0 KB | 128.0 KB | -20.0 KB |
| **Largest block (min)** | 148.0 KB | 120.0 KB | -28.0 KB |

### Cache Effectiveness

| | Misses/page | Hit rate |
|---|---|---|
| **Compressed (cold cache)** | 2.1 | 99.85% |

------

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

  Did you use AI tools to help write this code? _**YES**_
Implementation was done by Claude Code (Opus 4.6) based on a plan
developed collaboratively. All generated font headers were verified with
an automated round-trip decompression test. The firmware was compiled
successfully but has not yet been tested on-device.

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
Adrian Wilkins-Caruana
2026-02-19 20:30:15 +11:00
committed by GitHub
parent f16c0e52fd
commit 47aa0dda76
73 changed files with 126417 additions and 246010 deletions
Download Patch File Download Diff File Expand all files Collapse all files

13
lib/EpdFont/EpdFontData.h
View File

@@ -12,9 +12,18 @@ typedef struct {
int16_t left; ///< X dist from cursor pos to UL corner
int16_t top; ///< Y dist from cursor pos to UL corner
uint16_t dataLength; ///< Size of the font data.
uint32_t dataOffset; ///< Pointer into EpdFont->bitmap
uint32_t dataOffset; ///< Pointer into EpdFont->bitmap (or within-group offset for compressed fonts)
} EpdGlyph;
/// Compressed font group: a DEFLATE-compressed block of glyph bitmaps
typedef struct {
uint32_t compressedOffset; ///< Byte offset into compressed data array
uint32_t compressedSize; ///< Compressed DEFLATE stream size
uint32_t uncompressedSize; ///< Decompressed size
uint16_t glyphCount; ///< Number of glyphs in this group
uint16_t firstGlyphIndex; ///< First glyph index in the global glyph array
} EpdFontGroup;
/// Glyph interval structure
typedef struct {
uint32_t first; ///< The first unicode code point of the interval
@@ -32,4 +41,6 @@ typedef struct {
int ascender; ///< Maximal height of a glyph above the base line
int descender; ///< Maximal height of a glyph below the base line
bool is2Bit;
const EpdFontGroup* groups; ///< NULL for uncompressed fonts
uint16_t groupCount; ///< 0 for uncompressed fonts
} EpdFontData;