PDF pages · oneocr cookbook

Read a PDF whose page has both real text and a scanned image: the text layer is used where it exists, the image is read with OCR, and every word says which.

Terminal

dotnet add package RecursiveAI.OneOcr --version 1.0.0
dotnet add package RecursiveAI.OneOcr.Pdf --version 1.0.0

expense.pdf is a one-page US Letter PDF printed from HTML: a heading and a paragraph as text, and the grainy receipt as an embedded image.

using OneOcr;
using OneOcr.Pdf;
 
OcrEngine engine = OcrEngine.LoadBundled();
IReadOnlyList<OcrPage> pages = PdfOcr.Read("expense.pdf", engine);
 
foreach (OcrPage page in pages)
{
    Console.WriteLine($"page {page.Index + 1}: {page.Width:F0} x {page.Height:F0} pt, {page.Words.Count} words");
    foreach (var group in page.Words.GroupBy(w => w.Source))
        Console.WriteLine($"  {group.Key}: {string.Join(" ", group.Take(8).Select(w => w.Text))} ...");
    foreach (OcrDiagnostic diagnostic in page.Diagnostics)
        Console.WriteLine($"  {diagnostic.Code}: {diagnostic.Message}");
}

Text

page 1: 612 x 792 pt, 64 words
  TextLayer: Expense report EX-118 Submitted by Dana Whitfield on ...
  Ocr: Corner Market 1402 Oak Street 78704 Austin +1 ...
  OCR301: 1 regions were read from pixels at 300 DPI (47 words).

Each PDF page is one OcrPage in PDF points, y down; ToPdf maps coordinates back to PDF user space.
Image regions the text doesn’t cover are rendered at PdfOcrOptions.RenderDpi (300 by default) through OneImage.Pdf and read with the local engine; OCR301 says how many.
A text layer isn’t trusted blindly: an invisible layer over a scan (an earlier OCR pass) is read again unless you set TrustExistingOcrLayer, and garbled lines are read again from a render. See PDF pages.
For a PdfPage or PdfDocument you already hold from OnePDF, the extension methods Ocr, ToText, ToMarkdown, ExtractInvoice and ExtractReceipt do the same. Text drawn as outlined glyphs and annotations aren’t read yet.