PDF bridge (RecursiveAI.OneOcr.Pdf)

How a PDF page becomes an oneocr.page page. All PDF parsing is OnePDF's and all drawing is the OneImage PDF bridge's; this package only converts, decides what to trust, and calls the engine's page entry point (OcrEngine.Ocr) on rendered regions.

Versions

RecursiveAI.OnePdf 1.10.1 and RecursiveAI.OneImage.Pdf 1.10.4 (the versions in src/OneOcr.Pdf/OneOcr.Pdf.csproj). OneImage.Pdf pins the exact RecursiveAI.OneImage version, so 1.10.4 is the one that matches the core package's OneImage 1.10.4 floor.

Coordinates

A PDF page is OcrPage with Width/Height in points of the viewed page (box, /Rotate and /UserUnit applied; the crop box is used), UnitsPerInch 72, origin top-left, y down. OnePDF's extraction is y up in the same upright space, so a text-layer box [l, b, r, t] becomes [l, H − t, r, H − b]. ToPdf maps a page-unit point back to user space (x' = a x + c y + e, y' = b x + d y + f), for the crop box [l b r t], width w, height h, user unit u and k = 1/u:

Text-layer glyph boxes are OnePDF's: 0.9 em above the baseline to 0.2 em below, from the glyph origin to its advance. Mapped through ToPdf a word's left edge lands on the glyph origin and its edges on the baseline ∓ those distances; the tests hold this to 0.6 pt in user space for upright, rotated, cropped and /UserUnit pages. Recognized words come from a render at RenderDpi and carry the recognizer's ink boxes, so their left edge is only held to within 4 pt (CTC alignment).

What is read how

1. The text layer is extracted in reading order with glyphs retained. Words, lines, blocks, roles (header/footer), tables and cell starts carry over; words and characters have Source = TextLayer, confidence 1 (the file says so; it is not a calibrated probability).
2. Invisible text (render mode 3 or fully transparent) is somebody else's OCR layer. By default it is dropped and every image it sat on is read by the recognizer. TrustExistingOcrLayer keeps it as TextLayer and the images under it are not read.
3. Garbled text: a line with a word that contains U+FFFD, a private-use code point or a control character is dropped and its box (padded by 15% of its height) is rendered and read.
4. Cross-check: up to CrossCheckLines (3) evenly spaced, visible, left-to-right lines are rendered, read, and compared with the text layer on letters and digits (case-folded, Levenshtein distance more than 40% of the longer string = disagreement). If at least half the checked lines disagree, the text layer is dropped and the whole page is read from pixels. This catches a ToUnicode map that is wrong but plausible.
5. Images: every image the page draws (from the interpreter's device calls, not inside Type 3 glyphs) of at least MinImagePoints (24) in both sizes, whose area carries less than 10% text-layer word boxes and fewer than three trusted invisible words, is read. A page with no text at all therefore reads its images, and a scan drawn full-page is one region.
6. A region is the crop of the page rendered at RenderDpi (300), passed to OcrEngine.Ocr with that DPI, and its blocks, tables and regions are scaled back to points. Recognized words that overlap an accepted word by at least 40% of the smaller box (text drawn over a picture, or two regions that overlap) are dropped, text layer first; tables of a region that lost words are not kept.
7. Order: text-layer blocks keep OnePDF's reading order; each recognized region goes before the first text-layer block that starts below its top. Blocks, lines and words keep their own Source.

Failures (a page that cannot be rendered, an image that is too large, a region over an OCR limit) leave a diagnostic on the page and the rest of the page is still read; Strict throws instead. Damaged files raise OcrFormatException, files over a limit OcrLimitException; nothing else comes out of a public method except argument errors, I/O errors and cancellation.

Not done: outlined text (glyphs drawn as paths), annotations (they are neither read nor rendered), vector text-as-art.