% SPDX-FileCopyrightText: 2022 James R. Barlow % SPDX-License-Identifier: CC-BY-SA-4.0

Performance

Some users have noticed that current versions of OCRmyPDF do not run as quickly as some older versions (specifically 6.x and older). This is because OCRmyPDF added image optimization as a postprocessing step, and it is enabled by default.

Speed

If running OCRmyPDF quickly is your main goal, you can use settings such as:

• --optimize 0 to disable file size optimization
• --output-type pdf to disable PDF/A generation
• --fast-web-view 999999 to disable fast web view optimization
• --skip-big to skip large images, if some pages have large images

You can also avoid:

• --force-ocr
• Image preprocessing

Memory

Peak memory is set by the number of pixels in the largest page, not by the size of the PDF on disk. OCRmyPDF rasterizes each page at the resolution of the images it contains, and there is no ceiling on that by default, so a 1200 dpi scan costs four times as much as the same page at 600 dpi.

As a rule of thumb, budget about 15 bytes per pixel of the largest page, per concurrent job. A 34 megapixel page (letter size at 600 dpi) therefore peaks at roughly 500 MB with --jobs 1, and roughly 2 GB with --jobs 4. Three things account for that:

• the OCR engine, which is the largest single consumer -- Tesseract peaks at about 12 bytes per pixel of the image it is given
• rasterizing the page, which holds the renderer's bitmap and the copy taken from it at the same time
• masking and writing the image that goes to OCR

They run one after another rather than at once, so the peak is the largest of them, not their sum -- but each concurrent job pays it separately.

To bound memory on files with very large images, use --max-ocr-image-mpixels. It downsamples the image sent to OCR when a page exceeds the given size, which caps the largest consumer:

ocrmypdf --max-ocr-image-mpixels 8 --jobs 2 input.pdf output.pdf

The visible page is never downsampled, so the appearance of the output is unaffected in every mode, including --force-ocr. What you trade is OCR accuracy on very high resolution scans. Tesseract is tuned for roughly 300 dpi and gains little above 400 dpi, so a cap around 8 megapixels is usually free for letter or A4 pages; on documents with unusually small print, check the results before adopting it.

Two other settings matter:

• --jobs multiplies the peak, since each worker processes its own page. Lowering it is the bluntest way to fit a memory limit.
• --max-image-mpixels is not a memory control. It is Pillow's decompression-bomb guard, and it aborts the job rather than downsampling.