img2pdf

Lossless conversion of images to PDF without unnecessarily re-encoding JPEG and JPEG2000 files. Thus, no loss of quality and no unnecessary large output file.

Background

PDF is able to embed JPEG and JPEG2000 images as they are without re-encoding them (and hence loosing quality) but I was missing a tool to do this automatically, thus I wrote this piece of python code.

If you know how to embed JPEG and JPEG2000 images into a PDF container without recompression, using existing tools, please contact me so that I can put this code into the garbage bin :D

Functionality

The program will take image filenames from commandline arguments and output a PDF file with them embedded into it. If the input image is a JPEG or JPEG2000 file, it will be included as-is without any processing. If it is in any other format, the image will be included as zip-encoded RGB. As a result, this tool will be able to lossless wrap any image into a PDF container while performing better (in terms of quality/filesize ratio) than existing tools in case the input image is a JPEG or JPEG2000 file.

For example, imagemagick will re-encode the input JPEG image and thus change its content:

$ convert img.jpg img.pdf
$ pdfimages img.pdf img.extr # not using -j to be extra sure there is no recompression
$ compare -metric AE img.jpg img.extr-000.ppm null:
1.6301e+06

If one wants to do a lossless conversion from any format to PDF with imagemagick then one has to use zip-encoding:

$ convert input.jpg -compress Zip output.pdf
$ pdfimages img.pdf img.extr # not using -j to be extra sure there is no recompression
$ compare -metric AE img.jpg img.extr-000.ppm null:
0

The downside is, that using imagemagick like this will make the resulting PDF files a few times bigger than the input JPEG or JPEG2000 file and can also not output a multipage PDF.

img2pdf is able to output a PDF with multiple pages if more than one input image is given, losslessly embed JPEG and JPEG2000 files into a PDF container without adding more overhead than the PDF structure itself and will save all other graphics formats using lossless zip-compression.

Another nifty advantage: Since no re-encoding is done in case of JPEG images, the conversion is many (ten to hundred) times faster with img2pdf compared to imagemagick. While a run of above convert command with a 2.8MB JPEG takes 27 seconds (on average) on my machine, conversion using img2pdf takes just a fraction of a second.

Commandline Arguments

At least one input file argument must be given as img2pdf needs to seek in the file descriptor which would not be possible for stdin.

Specify the dpi with the -d or --dpi options instead of reading it from the image or falling back to 96.0.

Specify the output file with -o or --output. By default output will be done to stdout.

Specify metadata using the --title, --author, --creator, --producer, --creationdate, --moddate, --subject and --keywords options (or their short forms).

Specify -C or --colorspace to force a colorspace using PIL short handles like 'RGB', 'L' or '1'.

More help is available with the -h or --help option.

Bugs

If you find a JPEG or JPEG2000 file that, when embedded can not be read by the Adobe Acrobat Reader, please contact me.

For lossless conversion of other formats than JPEG or JPEG2000 files, zip/flate encoding is used. This choice is based on a number of tests I did on images. I converted them into PDF using imagemagick and all compressions it has to offer and then compared the output size of the lossless variants. In all my tests, zip/flate encoding performed best. You can verify my findings using the test_comp.sh script with any input image given as a commandline argument. If you find an input file that is outperformed by another lossless compression, contact me.

I have not yet figured out how to read the colorspace from jpeg2000 files. Therefor jpeg2000 files use DeviceRGB per default. If your jpeg2000 files are of any other colorspace you must force it using the --colorspace option. Like -C L for DeviceGray.

Installation

You can install the package using:

$ pip install img2pdf

If you want to install from source code simply use:

$ cd img2pdf/
$ pip install .

To test the console script without installing the package on your system, simply use virtualenv:

$ cd img2pdf/
$ virtualenv ve
$ ve/bin/pip install .

You can then test the converter using:

$ ve/bin/img2pdf -o test.pdf src/tests/test.jpg

Note that the package can also be used as a library as follows:

import img2pdf
pdf_bytes = img2pdf('test.jpg', dpi=150)