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
// 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)?;

• Images: the effective resolution of each image is measured where pages draw it (the CTM through form XObjects; the smallest over all uses). 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.
• 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 are left alone.

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 and encoding and resampling come from the image crate.