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File Formats
============

`unpaper` gets its file input and output code support from the
[ffmpeg][1] library, but not all image file formats
supported by the library are supported by `unpaper`.

The reason for the complexity is to be found in the different pixel
formats that need to be supported. At the time of writing, the
supported pixel formats are `gray8`, `rgb24`, `monoblack`,
`monowhite`, `y400a` (aka `ya8`) and `pal8` (_caveat emptor_, see
notes in the output formats).

If you have unsupported files that you think should be supported (for
instance because they are generated by some scanning tool or
hardware), please [open an issue][2].

Output Formats
--------------

`unpaper` still supports only output of files in any of the supported
PNM formats. Support for different formats will likely follow in
future major versions.

As it is, the output format will try to match the pixel format of the
source material, so for a `gray8` or `ya8` file, the output will be
`pgm`, while for a `rgb24` it'll be a `ppm`. Both `monoblack` and
`monowhite` will output a `pbm`.

Because of the way palettes are implemented, an input file in `pal8`
format will output `ppm` files by default. At the time of writing,
this include all grayscale TIFF files with libav versions
preceding 11.

Input Formats
-------------

### PNM Family

The PNM family of formats is the original file format supported by
`unpaper` and it includes `pbm` (Portable Bit Map), `pgm` (Portable
Grayscale Map) and `ppm` (Portable Pixel Map) formats. It is used by
Linux scanning tools such as `scanimage` and `scanadf`.

Input support is limited to the most common sub-formats:

 - black and white (`pbm`);
 - grayscale up to 8-bit (`pgm`);
 - RGB colour, up to 24-bit (`ppm`).

Both grayscale and RGB colour images allows a definition of a `MAXVAL`
property that defines the depth of the image. Images at a lower depth
than those supported (8-bit for grayscale, 24-bit for RGB), will be
upconverted. Images at a higher depth are not supported.

While the PNM family supports YUV images, these are not supported by
`unpaper` and no plan is currently out to support them.

### TIFF

The TIFF format consists of a long list of sub-formats that are not
compatible with each other, so input files in TIFF file formats might
or might not be supported depending on the version of `unpaper` and
`libav`.

At the time of writing, libav 9 and 10 will treat all 8-bit
grayscale files as `pal8`, which will then be considered 24-bit RGB
instead. This is fixed in version 11 of libav.

Version 11 of libav also introduces support for images at 8-bit plus
alpha, as well as (not yet supported by `unpaper`) 16-bit plus alpha
and 48-bit RGB.

Notable missing features at the time of writing, for libav `master`
are JPEG and LZMA compression, 4-bit grayscale images, and multi-pages
files.

PDF Generation
--------------

PDF support is not currently available and not planned. You can
generate PDFs out of the processed images by using tools such as
`pnm2tiff`, `tiffcp` and `tiff2pdf`.

[1]: https://www.ffmpeg.org/
[2]: https://github.com/unpaper/unpaper/issues

