[[!meta title="Optimizing XSane's scanned PDFs (also: PDF internals)"]] [[!meta author="rohieb"]] [[!meta license="CC-BY-SA 3.0"]] [[!img defaults size=x200]] [[!toc levels=2]] Problem ------- I use [XSane][xsane] to scan documents for my digital archive. I want them to be in PDF format and have a reasonable resolution (better than 200 dpi, so I can try OCRing them afterwards). However, the PDFs created by XSane’s multipage mode are too large, about 250 MB for a 20-page document scanned at 200 dpi. [xsane]: http://www.xsane.org/ "XSane homepage" [[!img xsane-multipage-mode.png caption="XSane’s Multipage mode"]] First (non-optimal) solution -------------- At first, I tried to optimize the PDF using [GhostScript][gs]. I [[already wrote|use-ghostscript-to-convert-pdf-files]] about how GhostScript’s `-dPDFSETTINGS` option can be used to minimize PDFs by redering the pictures to a smaller resolution. In fact, there are [multiple rendering modes][gs-ps-pdf] (`screen` for 96 dpi, `ebook` for 150 dpi, `printer` for 300 dpi, and `prepress` for color-preserving 300 dpi), but they are pre-defined, and for my 200 dpi images, `ebook` was not enough (I would lose resolution), while `printer` was too high and would only enlarge the PDF. [gs]: http://ghostscript.com "Ghostscript homepage" [gs-ps-pdf]: http://milan.kupcevic.net/ghostscript-ps-pdf/#refs "Ghostscript PDF Reference & Tips" Interlude: PDF Internals ------------------ The best thing to do was to find out how the images were embedded in the PDF. Since most PDF files are also partly human-readable, I opened my file with vim. (Also, I was surprised that [vim has syntax highlighting for PDF](vim-syntax-highlighting.png).) Before we continue, I'll give a short introduction to the PDF file format (for the long version, see [Adobe’s PDF reference][pdf-ref]). [pdf-ref]: http://partners.adobe.com/public/developer/en/pdf/PDFReference.pdf "Adobe Portable Document Format, Version 1.4" ### Building Blocks ### Every PDF file starts with the [magic string][magic] that identifies the version of the standard which the document conforms to, like `%PDF-1.4`. After that, a PDF document is made up of the following objects: [magic]: https://en.wikipedia.org/wiki/Magic_number_(programming)#Magic_numbers_in_files "Wikipedia: Magic numbers in files" Boolean values : `true` and `false` Integers and floating-point numbers : for example, `1337`, `-23.42` and `.1415` Strings : * interpreted as literal characters when enclosed in parentheses: `(This is a string.)` These can contain escaped characters, particularly escaped closing braces and control characters: `(This string contains a literal \) and some\n newlines.\n)`. * interpreted as hexadecimal data when enclosed in angled brackets: `<53 61 6D 70 6C 65>` equals `(Sample)`. Names : starting with a forward slash, like `/Type`. You can think of them like identifiers in programming languages. Arrays : enclosed in square brackets: `[ -1 4 6 (A String) /AName [ (strings in arrays in arrays!) ] ]` Dictionaries : key-value stores, which are enclosed in double angled brackets. The key must be a name, the value can be any object. Keys and values are given in turns, beginning with the first key: `<< /FirstKey (First Value) /SecondKey 3.14 /ThirdKey /ANameAsValue >>` Usually, the first key is `/Type` and defines what the dictionary actually describes. Stream Objects : a collection of bytes. In contrast to strings, stream objects are usually used for large amount of data which may not be read entirely, while strings are always read as a whole. For example, streams can be used to embed images or metadata. : Streams consist of a dictionary, followed by the keyword `stream`, the raw content of the stream, and the keyword `endstream`. The dictionary describes the stream’s length and the filters that have been applied to it, which basically define the encoding the data is stored in. For example, data streams can be compressed with various algorithms. The Null Object : Represented by the literal string `null`. Indirect Objects : Every object in a PDF document can also be stored as a indirect object, which means that it is given a label and can be used multiple times in the document. The label consists of two numbers, a positive *object number* (which makes the object unique) and a non-negative *generation number* (which allows to incrementally update objects by appending to the file). : Indirect objects are defined by their object number, followed by their generation number, the keyword `obj`, the contents of the object, and the keyword `endobj`. Example: `1 0 obj (I'm an object!) endobj` defines the indirect object with object number 1 and generation number 0, which consists only of the string “I'm an object!”. Likewise, more complex data structures can be labeled with indirect objects. : Referencing an indirect object works by giving the object and generation number, followed by an uppercase R: `1 0 R` references the object created above. References can be used everywhere where a (direct) object could be used instead. Using these object, a PDF document builds up a tree structure, starting from the root object, which has the object number 1 and is a dictionary with the value `/Catalog` assigned to the key `/Type`. The other values of this dictionary point to the objects describing the outlines and pages of the document, which in turn reference other objects describing single pages, which point to objects describing drawing operations or text blocks, etc. ### Dissecting the PDFs created by XSane ### Now that we know how a PDF document looks like, we can go back to out initial problem and try to find out why my PDF file was so huge. I will walk you through the PDF object by object. [[!format pdf <> endobj EOF]] This is just the magic string declaring the document as PDF-1.4, and the root object with object number 1, which references objects number 2 for Outlines and number 3 for Pages. We're not interested in outlines, let's look at the pages. [[!format pdf <> endobj EOF]] OK, apparently this document has four pages, which are referenced by objects number 6, 8, 10 and 12. This makes sense, since I scanned four pages ;-) Let's start with object number 6: [[!format pdf <> >> endobj EOF]] We see that object number 6 is a page object, and the actual content is in object number 7. More redirection, yay! [[!format pdf <> stream q 1 0 0 1 0 0 cm 1.000000 0.000000 -0.000000 1.000000 0 0 cm 595.080017 0 0 841.679993 0 0 cm BI /W 1653 /H 2338 /CS /G /BPC 8 /F /FlateDecode ID x$¼[$;¾åù!fú¥¡aæátq.4§ [ ...byte stream shortened... ] EI Q endstream endobj EOF]] Aha, here is where the magic happens. Object number 7 is a stream object of 2,678,332 bytes (about 2 MB) and contains drawing operations! After skipping around a bit in Adobe’s PDF reference (chapters 3 and 4), here is the annotated version of the stream content: [[!format pdf <> stream % [ raw byte data ] endstream EOF]] The filter `/RunLengthDecode` indicates that the stream data is compressed with [Run-length encoding][RLE], another simple lossless compression. Not what I wanted. (Apart from that, `convert` embeds images as XObjects, but there is not much difference to the inline images described above.) [RLE]: https://en.wikipedia.org/wiki/Run-length_encoding "Wikipedia: Run-length encoding" ### Converting PNM to JPG, then to PDF ### Next, I converted the PNMs to JPG, then to PDF. $ convert image*.pnm image.jpg $ convert image*jpg document.pdf (The first command creates the output files `image-1.jpg`, `image-2.jpg`, etc., since JPG does not support multiple pages in one file.) When looking at the PDF, we see that we now have DCT-compressed images inside the PDF: [[!format pdf <> stream % [ raw byte data ] endstream EOF]] ### Converting PNM to JPG, then to PDF, and fix page size ### However, the pages in `document.pdf` are 82.47×58.31 cm, which results in about 72 dpi in respect to the size of the original images. But `convert` also allows us to specify the pixel density, so we'll set that to 200 dpi in X and Y direction, which was the resolution at which the images were scanned: $ convert image*jpg -density 200x200 document.pdf *Update:* You can also use the [`-page` parameter][page] to set the page size directly. It takes a multitude of predefined paper formats (see link) and will do the pixel density calculation for you, as well as adding any neccessary offset if the image ratio is not quite exact: $ convert image*jpg -page A4 document.pdf With that approach, I could reduce the size of my PDF from 250 MB with losslessly compressed images to 38 MB with DCT compression. *Another update (2023):* Marcus notified me that it is possible to use ImageMagick's `-compress jpeg` option, this way we can leave out the intermediate step and convert PNM to PDF directly: $ convert image*.pnm -compress jpeg -quality 85 output.pdf You can also play around with the `-quality` parameter to set the JPEG compression level (100% makes almost pristine, but huge images; 1% makes very small, very blocky images), 85% should still be readable for most documents in that resolution. Too long, didn’t read ----------------- Here’s the gist for you: * Read the article above, it’s very comprehensive :P * Use `convert` on XSane’s multipage images and specify your scanning resolution: $ convert image*.pnm image.jpg $ convert image*jpg -density 200x200 document.pdf Further reading ------------- There is probably software out there which does those thing for you, with a shiny user interface, but I could not find one quickly. What I did find though, was [this detailed article][scan-to-pdfa], which describes how to get high-resolution scans wihh OCR information in PDF/A and DjVu format, using `scantailor` and `unpaper`. Also, Didier Stevens helped me understand stream objects in in his [illustrated blogpost][pdf-stream-objects]. He seems to write about PDF more often, and it was fun to poke around in his blog. There is also a nice script, [`pdf-parser`][pdf-tools], which helps you visualize the structure of a PDF document. [scan-to-pdfa]: http://blog.konradvoelkel.de/2013/03/scan-to-pdfa/ "Konrad Voelkel: Linux, OCR and PDF: Scan to PDF/A" [pdf-stream-objects]: http://blog.didierstevens.com/2008/05/19/pdf-stream-objects/ "Didier Stevens: PDF Stream Objects" [pdf-tools]: http://blog.didierstevens.com/programs/pdf-tools/ "Didier Stevens: PDF Tools" [page]: http://www.imagemagick.org/script/command-line-options.php#page "ImageMagick: Command-line Options" [[!tag PDF note_to_self howto ImageMagic convert file_formats reference longpost]]