pdfcraft/crates/optimize/README.md

pdfcraft-optimize

Layer L4: the PDF Optimizer (execution plan M11.1; Acrobat's Reduce File Size and Optimize PDF ▸ Advanced optimization).

let report = optimize(&mut doc, &Settings::default())?;   // Reduce File Size's choices
let report = optimize(&mut doc, &Preset::Mobile.settings())?; // smaller, for screens
// Or with progress: called before each image and the clean-up; `false` cancels.
let report = optimize_with_progress(&mut doc, &settings, &mut |stage| keep_going)?;

• Presets: Standard (Reduce File Size's choices: images above 225 ppi to 150 ppi, JPEG medium) and Mobile (above 144 ppi to 96 ppi, JPEG high), as in Acrobat's Settings menu.
• Merge image fragments (fragments, on by default): some PDF printers draw a page picture as thousands of images one pixel row high. Runs of upright 8-bit grey or RGB images in one colour space, sharing their left and right edges and stacked edge to edge (at least four, drawn with nothing else between them), become one lossless image at their finest resolution, which the image settings then resample; the strips are dropped by Clean Up. A real page printed this way went from 4.3 MB to 231 KB (Standard) and 161 KB (Mobile). Not merged yet: side-by-side tiles, rotated strips, masks. Content arrays use the shared space boundary policy, so trailing comments, split operands and graphics state keep their meaning after merging.
• Images: the effective resolution of each image is measured where pages draw it (the CTM through form XObjects, up to 12 deep; the smallest over all uses). Page-local /UserUnit scaling is included, so large-format pages are measured in physical inches. If a use can't be measured (forms nested deeper, or drawn too many times), no image is resampled. Colour and grayscale images above a threshold are resampled with a bicubic (Catmull-Rom) filter to a target resolution and stored as JPEG (Acrobat's quality steps) or ZIP; soft masks are resampled with their image. An image is replaced only if the result is smaller. Left alone: CMYK and other colour spaces, image masks, decode arrays, more than 8 bits, JPEG 2000, JBIG2 and CCITT. A JPEG is read with its colour transform (an Adobe marker's, else /ColorTransform, ISO 32000-2 §7.4.8), so one stored untransformed keeps its colours; one whose marker and /ColorTransform disagree is left alone, since readers differ on which wins. Samples are read with /DecodeParms given by reference too, the numbers inside included (Document::with_direct_parms), here and below: without them a predictor's tag bytes would shift every row of an image written back.
• Monochrome images (Settings::mono, CCITT Group 4 by default, or JBIG2): a 1-bit image with one colour component (grey, indexed, a separation) or an image mask, stored with no filter or with general-purpose ones, is stored again as CCITT Group 4 (pdfcraft_filters::encode_g4) or lossless JBIG2 (encode_jbig2_generic; a document below PDF 1.4 gets /Version /1.4 in its catalog) when that is smaller as stored. On 300 dpi text JBIG2 is about a third smaller than Group 4. It is lossless: the samples decode to the same bits, so the colour space, decode array and masks stay. Group 4 wins on scans, whose ragged edges defeat Flate; on clean computer-drawn 1-bit art Flate can be smaller, and the image then stays. Not yet: JBIG2 symbol coding (lossy or lossless), monochrome downsampling, inline images, soft masks and /Mask images, and rows wider than 32,767 samples. Redaction doesn't decode Group 4 yet: a mark over part of an image stored this way removes the whole image (nothing leaks). Nor does anything that exports an image: Export ▸ Images and Save Image As refuse it, Copy Image fails, and Word, PowerPoint and HTML export leave the picture out.
• Discard objects: thumbnails, alternate images, document tags, print settings.
• Clean up: Flate for streams with no filter; for streams with an outer ASCII85 or ASCIIHex layer, that layer is removed when the result is smaller (the inner filters keep their encoded bytes; streams with /DecodeParms, images and object streams keep their ASCII layer. LZW-compressed streams, content and images alike, are decoded and stored with Flate instead (Acrobat's "In streams that use LZW encoding, use Flate instead"): a predictor carries over to Flate without EarlyChange, filters applied before the LZW one go with it and those after it keep their bytes. A stream whose LZW data doesn't decode cleanly is left as it is. Not yet: inline images.

The engine runs Remove Hidden Information (pdfcraft-redact) for the user-data categories, then this, then merges identical objects and writes a full, compressed save (pdfcraft_engine::optimizer::OptimizeJob, which the UI runs on a worker thread with a progress bar). JPEG decoding comes from zune-jpeg (the decoder behind image and the renderer), JPEG encoding and resampling from the image crate.

Target PDF version: partial compatibility

The target selects the header version, clears the catalog /Version, controls object streams (disabled for PDF 1.4), and checks encryption requirements. It does not detect or convert content newer than the target: optional content, JPEG 2000 images, 16-bit images, OpenType fonts and PDF 2.0 features can remain. A PDF 1.4 header therefore does not guarantee compatibility with PDF 1.4 readers or validators. optimize.compatibility-level remains partial until content compatibility is checked.