NetterSilverandClaude Opus 5.5 2f4a6d906a Speed up PDF export by reusing equal slide backdrops ...

PDF export rasterises every slide as a backdrop and a content layer and
hands both to the writer. Profiling (gdb stack sampling) showed most of
the main thread outside rendering going to per-pixel work on those
rasters: compositing the backdrop onto white (not vectorised), the
writer hashing every backdrop to deduplicate it, and the straight-alpha
conversion scanning every content pixel.

- Composite the backdrop with saturating adds and skip 16-pixel blocks
  that are already opaque; skip blocks with no partly transparent pixel
  in the straight-alpha conversion. Same formulas, same bytes.
- Keep the last opaque backdrops (up to 32 MiB of pixels) and hand the
  writer the same image again when a slide renders an identical
  backdrop. The writer stored equal backdrops once already; now it also
  skips hashing and compressing the copies. Output is byte-identical.

deckcraft-cli bench, export .pdf step, 15 interleaved runs each on a
16-thread Ryzen (A/A noise +0.9 %):
- sample x4 (32 slides): median 531 -> 322 ms (-39 %), min 529 -> 317 ms
- same deck, every slide a different background: 557 -> 441 ms (-21 %)
- sample (8 slides): 149 -> 103 ms (-31 %)
Exported PDFs are byte-identical.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2026-10-27 17:39:31 +01:00

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lib.rs

Speed up PDF export by reusing equal slide backdrops

2026-10-27 17:39:31 +01:00

tests.rs

Speed up PDF export by reusing equal slide backdrops

2026-10-27 17:39:31 +01:00