OCRmyPDF v17.6.0 release notes

When the optimizer encounters an image it cannot process (for example, an exotic colorspace that cannot be transcoded), it now logs a concise warning that the image was left unchanged rather than printing an alarming traceback. The output file was already valid in these cases; only the reporting was misleading. The full traceback is still available at debug verbosity (-v 1) ({issue}846).
--pdfa-image-compression=auto (the default) now selects lossless image compression at -O0 so Ghostscript no longer transcodes lossless images to JPEG during PDF/A generation. At -O1 and above, auto continues to defer to Ghostscript's heuristic, which may recompress images lossily. -O1 (the default level) is kept as a historical exception because coercing it to lossless can substantially bloat output; users who want guaranteed lossless image handling should pass --pdfa-image-compression=lossless or use -O0 ({issue}1124).
--pdfa-image-compression=lossless now passes existing JPEG images through unchanged rather than re-encoding them with a lossless codec. Re-encoding an already-lossy JPEG losslessly cannot recover quality and only inflates the file, so JPEGs are preserved while non-JPEG images are encoded losslessly.
OCRmyPDF now validates and repairs malformed page-boundary boxes (/MediaBox, /CropBox, /TrimBox, /ArtBox, /BleedBox) in its input, following the PDF 2.0 specification. Coordinates written in invalid exponential notation are reinterpreted ({issue}1398); rectangles whose corners are given in reversed order are normalized, which previously crashed with NegativeDimensionError ({issue}1526); and a crop/trim/art/bleed box that falls outside the MediaBox is clamped to their intersection, or discarded when that intersection is empty, which previously produced an output with a zero-height effective page that some viewers refused to open ({issue}1400). When a box is discarded, clamped, or reinterpreted, OCRmyPDF logs a warning recommending visual inspection of the output. Thanks @ajdlinux for the initial fix in PR #1691.
OCRmyPDF now discards an embedded Adobe full-text search index (/Root/PieceInfo/SearchIndex) from its output. This proprietary index, produced by Acrobat's "Embed Index" feature, is read only by Adobe Acrobat; other viewers ignore it and search the text on the fly. Because any change to a PDF invalidates the index, retaining it after OCRmyPDF rewrites the document would leave a stale index that returns incorrect search results in Acrobat. Modern viewers rebuild a search index on demand, so there is no loss of search capability.
OCRmyPDF now discards embedded per-page thumbnail images (the optional /Thumb image XObject on a page) from its output. OCRmyPDF alters page appearance (deskew, clean, rasterize, re-render) and plugins may edit pages arbitrarily, so a retained thumbnail would be stale and no longer match its page. Embedded thumbnails are a navigation aid that modern viewers generate on demand, so there is no loss of functionality.
Fixed a regression in OCR quality for PDFs that paint a 1-bit image mask (stencil) with a gray or colored fill color. Previously such pages were rasterized as 1-bit black-and-white before OCR, so Ghostscript dithered mid-tone text into an unreadable stipple and Tesseract failed to recognize it. The rasterizer now inspects the fill color used to paint a mask and promotes the page to grayscale or full color as needed, so the distinction is preserved for the OCR engine. This applies to both the Ghostscript and pypdfium rasterizers. {issue}1688
The default 1-bit raster device for Ghostscript is now pngmonod (error-diffusion) instead of pngmono (ordered dithering). It produces better input for OCR on faint or anti-aliased scans at negligible cost and no change to output file size, since the rasterized image is an intermediate that is discarded after OCR.
When rasterizing pages with Ghostscript, OCRmyPDF now enables text and graphics anti-aliasing (-dTextAlphaBits=4 -dGraphicsAlphaBits=4) for the grayscale and color raster devices. Ghostscript 10.x renders aliased glyphs that OCR frequently misreads as extra word breaks or substituted characters; anti-aliasing materially improves OCR accuracy on the Ghostscript rasterization path, especially for small fonts at moderate resolution. The 1-bit monochrome devices are unaffected, since they perform their own anti-aliased downscaling and older Ghostscript versions reject alpha-bit options on them. Note that the default rasterizer (--rasterizer auto) prefers pypdfium2, which already anti-aliases; this change benefits users who select --rasterizer ghostscript or do not have pypdfium2 installed. OCRmyPDF now also logs which rasterizer rendered each page at debug verbosity (-v 1), and the --rasterizer help text explains the OCR-quality trade-off, to make such reports easier to diagnose. {issue}1439
When Tesseract reports a page with many diacritics, OCRmyPDF still logs its interpreted "lots of diacritics - possibly poor OCR" hint, but now also emits Tesseract's raw message at debug verbosity (-v 1) so the original wording is available for diagnosis. {issue}1566
Added --mode strip, which removes the invisible OCR text layer from a PDF in place. Unlike --ocr-engine none --force-ocr, it does not rasterize the page, so images and visible content are preserved unchanged and the output is smaller rather than larger. Only text drawn as invisible (PDF text render mode 3) is removed; some OCR engines -- and OCRmyPDF v2.2 and earlier -- express text as visible glyphs covered by an opaque image, and that text cannot be removed this way. {issue}1435

OCRmyPDF v17.6.0

Upgraded? How did it go?

Discussion