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ROADMAP.md: kept both progress lines. crates/optimize/README.md: PR's decoder sentence plus the target-version section from main. Cargo.lock: auto-merged.
58 lines
4.1 KiB
Markdown
58 lines
4.1 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. Content arrays use the shared space boundary
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policy, so trailing comments, split operands and graphics state keep their meaning after merging.
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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).
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Page-local `/UserUnit` scaling is included, so large-format pages are measured in physical inches.
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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. A JPEG is read with
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its colour transform (an Adobe marker's, else `/ColorTransform`, ISO 32000-2 §7.4.8), so one
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stored untransformed keeps its colours; one whose marker and `/ColorTransform` disagree is left
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alone, since readers differ on which wins.
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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 keep their ASCII layer. LZW-compressed
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streams, content and images alike, are decoded and stored with Flate instead (Acrobat's "In
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streams that use LZW encoding, use Flate instead"): a predictor carries over to Flate without
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`EarlyChange`, filters applied before the LZW one go with it and those after it keep their
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bytes. A stream whose LZW data doesn't decode cleanly is left as it is. Not yet: inline images.
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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 comes from `zune-jpeg` (the decoder behind `image` and the
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renderer), JPEG encoding and resampling from the `image` crate.
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## Target PDF version: partial compatibility
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The target selects the header version, clears the catalog `/Version`, controls object
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streams (disabled for PDF 1.4), and checks encryption requirements. It does **not**
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detect or convert content newer than the target: optional content, JPEG 2000 images,
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16-bit images, OpenType fonts and PDF 2.0 features can remain. A PDF 1.4 header
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therefore does not guarantee compatibility with PDF 1.4 readers or validators.
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`optimize.compatibility-level` remains partial until content compatibility is checked.
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