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3f25d55a96
The effective-resolution walk skipped any form XObject already visited once when it was nested more than four deep, with one seen-set for the whole document. A later, larger use of the same form was never measured, so Reduce File Size and Optimize PDF resampled the image for its smallest use: a 1200 px image drawn 1 in and 8 in wide through six nested forms came out 150 px wide, about 19 ppi on the large copy. - Cycles are detected on the current path of forms, not document-wide. - A form with its own resources drawn again at the same scale (only moved) is skipped, as that can't change any image's resolution; forms without resources draw their caller's images and are always walked. - Repeat walks are capped at 100,000. If the cap or the 12-level depth limit leaves a use unmeasured, every image gets INFINITY: recompressed as the settings say, never resampled. Fixes #1811 Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com> Co-authored-by: Brandon Thomas <echelon@gmail.com>
39 lines
2.6 KiB
Markdown
39 lines
2.6 KiB
Markdown
# pdfcraft-optimize
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Layer L4: the PDF Optimizer (execution plan M11.1; Acrobat's Reduce File Size and Optimize PDF ▸
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Advanced optimization).
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```rust
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let report = optimize(&mut doc, &Settings::default())?; // Reduce File Size's choices
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let report = optimize(&mut doc, &Preset::Mobile.settings())?; // smaller, for screens
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// Or with progress: called before each image and the clean-up; `false` cancels.
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let report = optimize_with_progress(&mut doc, &settings, &mut |stage| keep_going)?;
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```
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- **Presets:** Standard (Reduce File Size's choices: images above 225 ppi to 150 ppi, JPEG
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medium) and Mobile (above 144 ppi to 96 ppi, JPEG high), as in Acrobat's Settings menu.
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- **Merge image fragments** (`fragments`, on by default): some PDF printers draw a page picture
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as thousands of images one pixel row high. Runs of upright 8-bit grey or RGB images in one
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colour space, sharing their left and right edges and stacked edge to edge (at least four,
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drawn with nothing else between them), become one lossless image at their finest resolution,
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which the image settings then resample; the strips are dropped by Clean Up. A real page
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printed this way went from 4.3 MB to 231 KB (Standard) and 161 KB (Mobile). Not merged yet:
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side-by-side tiles, rotated strips, masks.
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- **Images:** the effective resolution of each image is measured where pages draw it (the CTM
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through form XObjects, up to 12 deep; the smallest over all uses). If a use can't be
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measured (forms nested deeper, or drawn too many times), no image is resampled. Colour and grayscale images above a
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threshold are resampled with a bicubic (Catmull-Rom) filter to a target resolution and
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stored as JPEG (Acrobat's quality steps) or ZIP; soft masks are resampled with their image.
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An image is replaced only if the result is smaller. Left alone: CMYK and other colour spaces,
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image masks, decode arrays, more than 8 bits, JPEG 2000, JBIG2 and CCITT.
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- **Discard objects:** thumbnails, alternate images, document tags, print settings.
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- **Clean up:** Flate for streams with no filter; for streams with an outer ASCII85 or ASCIIHex
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layer, that layer is removed when the result is smaller (the inner filters keep their encoded
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bytes). Streams with `/DecodeParms`, images and object streams are left alone.
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The engine runs Remove Hidden Information (`pdfcraft-redact`) for the user-data categories,
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then this, then merges identical objects and writes a full, compressed save
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(`pdfcraft_engine::optimizer::OptimizeJob`, which the UI runs on a worker thread with a
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progress bar). JPEG decoding and
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encoding and resampling come from the `image` crate.
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