img2pdf/README.md

img2pdf
=======

Lossless conversion of raster images to PDF. You should use img2pdf if your
priorities are (in this order):

 1. **always lossless**: the image embedded in the PDF will always have the
    exact same color information for every pixel as the input
 2. **small**: if possible, the difference in filesize between the input image
    and the output PDF will only be the overhead of the PDF container itself
 3. **fast**: if possible, the input image is just pasted into the PDF document
    as-is without any CPU hungry re-encoding of the pixel data

Conventional conversion software (like ImageMagick) would either:

 1. not be lossless because lossy re-encoding to JPEG
 2. not be small because using wasteful flate encoding of raw pixel data
 3. not be fast because input data gets re-encoded

Another advantage of not having to re-encode the input in most common
situations is, that img2pdf is able to handle much larger input than other
software.

The following table shows how img2pdf handles different input depending on the
input file format and image color space.

| Format               | Colorspace                     | Result        |
| -------------------- | ------------------------------ | ------------- |
| JPEG                 | any                            | direct        |
| JPEG2000             | any                            | direct        |
| PNG (non-interlaced) | any                            | direct        |
| any                  | any except CMYK and monochrome | PNG Paeth     |
| any                  | monochrome                     | CCITT Group 4 |
| any                  | CMYK                           | flate         |

For JPEG, JPEG2000 and non-interlaced PNG input, img2pdf directly embeds the
image data into the PDF without re-encoding it. It thus treats the PDF format
merely as a container format for the image data. In these cases, img2pdf only
increases the filesize by the size of the PDF container (typically around 500
to 700 bytes). Since data is only copied and not re-encoded, img2pdf is also
typically faster than other solutions for these input formats.

For all other input types, img2pdf first has to transform the pixel data to
make it compatible with PDF. In most cases, the PNG Paeth filter is applied to
the pixel data. For monochrome input, CCITT Group 4 is used instead. Only for
CMYK input no filter is applied before finally applying flate compression.

Usage
-----

The images must be provided as files because img2pdf needs to seek in the file
descriptor.

If no output file is specified with the `-o`/`--output` option, output will be
done to stdout. A typical invocation is:

	img2pdf img1.png img2.jpg -o out.pdf

The detailed documentation can be accessed by running:

	img2pdf --help

Bugs
----

If you find a JPEG, JPEG2000 or PNG file that, when embedded into the PDF
cannot be read by the Adobe Acrobat Reader, please contact me.

I have not yet figured out how to determine the colorspace of JPEG2000 files.
Therefore JPEG2000 files use DeviceRGB by default. For JPEG2000 files with
other colorspaces, you must explicitly specify it using the `--colorspace`
option.

Input images with alpha channels are not allowed. PDF doesn't support alpha
channels in images and thus, the alpha channel of the input would have to be
discarded. But img2pdf will always be lossless and thus, input images must not
carry transparency information.

img2pdf uses PIL (or Pillow) to obtain image meta data and to convert the input
if necessary. To prevent decompression bomb denial of service attacks, Pillow
limits the maximum number of pixels an input image is allowed to have. If you
are sure that you know what you are doing, then you can disable this safeguard
by passing the `--pillow-limit-break` option to img2pdf. This allows one to
process even very large input images.

Installation
------------

On a Debian- and Ubuntu-based systems, img2pdf can be installed from the
official repositories:

	$ apt install img2pdf

If you want to install it using pip, you can run:

	$ pip3 install img2pdf

If you prefer to install from source code use:

	$ cd img2pdf/
	$ pip3 install .

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

	$ cd img2pdf/
	$ virtualenv ve
	$ ve/bin/pip3 install .

You can then test the converter using:

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

The package can also be used as a library:

	import img2pdf

	# opening from filename
	with open("name.pdf","wb") as f:
		f.write(img2pdf.convert('test.jpg'))

	# opening from file handle
	with open("name.pdf","wb") as f1, open("test.jpg") as f2:
		f1.write(img2pdf.convert(f2))

	# using in-memory image data
	with open("name.pdf","wb") as f:
		f.write(img2pdf.convert("\x89PNG...")

	# multiple inputs (variant 1)
	with open("name.pdf","wb") as f:
		f.write(img2pdf.convert("test1.jpg", "test2.png"))

	# multiple inputs (variant 2)
	with open("name.pdf","wb") as f:
		f.write(img2pdf.convert(["test1.jpg", "test2.png"]))

	# writing to file descriptor
	with open("name.pdf","wb") as f1, open("test.jpg") as f2:
		img2pdf.convert(f2, outputstream=f1)

	# specify paper size (A4)
	a4inpt = (img2pdf.mm_to_pt(210),img2pdf.mm_to_pt(297))
	layout_fun = img2pdf.get_layout_fun(a4inpt)
	with open("name.pdf","wb") as f:
		f.write(img2pdf.convert('test.jpg', layout_fun=layout_fun))