feat(glyph): add --pixel-order option for LSB/MSB bit ordering - #153

Summary

Add a --pixel-order CLI option to control pixel bit arrangement within each byte of glyph bitmap data. Defaults to MSB (current behavior), with LSB as a new option.

Motivation

Some embedded LCD controllers (e.g., SSD1306 in page-addressing mode, certain SPI-driven monochrome displays) expect pixel data in LSB-first bit ordering. Currently, users must post-process the font output to reverse bit order per byte, which is error-prone and adds complexity to the build pipeline.

This option enables direct compatibility without post-processing.

Changes

Design decisions

• Byte-level post-processing: Reordering is applied after pixel packing, keeping the core BitStream logic untouched. This minimizes risk to existing functionality.
• LSB + compression mutual exclusion: RLE compression operates on MSB-ordered data. Allowing LSB with compression would produce incorrect output, so the CLI validates and rejects this combination.
• 8 BPP passthrough: At 8 bits per pixel, each byte holds exactly one pixel — no reordering is needed regardless of the setting.
• Default MSB: Fully backward compatible. Existing users see no change.

Usage

# LSB ordering (requires --no-compress)
lv_font_conv --font my_font.ttf -r 0x20-0x7F --size 16 --bpp 1 \
  --format bin --no-compress --pixel-order LSB -o my_font.bin

# MSB ordering (default, same as before)
lv_font_conv --font my_font.ttf -r 0x20-0x7F --size 16 --bpp 1 \
  --format bin -o my_font.bin

Testing

• Property-based tests using fast-check covering all BPP × order combinations
• Specific bit-pattern tests for each BPP (1, 2, 4, 8)
• CLI integration tests: LSB+compress rejection, LSB+no-compress acceptance, MSB default behavior, invalid value rejection
• All existing tests continue to pass

Notes

package-lock.json has minor "peer": true field removals due to npm version differences. The only actual dependency additions are fast-check and its dependency pure-rand.

Summary by cubic

Add a --pixel-order CLI option to control per-byte pixel bit order in glyph bitmaps. Default stays MSB; LSB is available for devices that read LSB-first.

• New Features

  • Add --pixel-order {MSB,LSB} to choose pixel bit ordering when packing glyphs.
  • Byte-level reordering happens after packing and only on full pixel bytes to avoid touching glyph headers; supports 1/2/4/8 BPP (8 BPP is a no-op).
  • CLI prevents --pixel-order LSB with compression; use with --no-compress.

• Migration

  • For LSB-first displays (e.g., SSD1306), run with --pixel-order LSB --no-compress.

^{Written for commit c0a47c6. Summary will update on new commits.}