v17.6.0

- 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`