unpaper and libav status update

The other day I wrote about unpaper and the fact that I was working on making it use libav for file input. I have now finished converting unpaper (in a branch) so that it does not use its own image structure, but rather the same AVFrame structure that libav uses internally and externally. This meant not only supporting stripes, but using the libav allocation functions and pixel formats.

This also enabled me to use libav for file output as well as input. While for the input I decided to add support for formats that unpaper did not read before, for output at the moment I’m sticking with the same formats as before. Mostly because the one type of output file I’d like to support is not currently supported by libav properly, so it’ll take me quite a bit longer to be able to use it. For the curious, the format I’m referring to is multipage TIFF. Right now libav only supports single-page TIFF and it does not support JPEG-compressed TIFF images, so there.

Originally, I planned to drop compatibility with previous unpaper version, mostly because to drop the internal structure I was going to lose the input format information for 1-bit black and white images. At the end I was actually able to reimplement the same feature in a different way, and so I restored that support. The only compatibility issue right now is that the -depth parameter is no longer present, mostly because it and -type constrained the same value (the output format).

To reintroduce the -depth parameter, I want to support 16-bit gray. Unfortunately to do so I need to make more fundamental changes to the code, as right now it expects to be able to get the full value at most at 24 bit — and I’m not sure how to scale a 16-bit grayscale to 24-bit RGB and maintain proper values.

While I had to add almost as much code to support the libav formats and their conversion as there was there to load the files, I think this is still a net win. The first point is that there is no format parsing code in unpaper, which means that as long as the pixel format is something that I can process, any file that libav supports now or will support in the future will do. Then there is the fact that I ended up making the code “less smart” by removing codepath optimizations such as “input and output sizes match, so I won’t be touching it, instead I’ll copy one structure on top of the other”, which means that yes, I probably lost some performance, but I also gained some sanity. The code was horribly complicated before.

Unfortunately, as I said in the previous post, there are a couple of features that I would have preferred if they were implemented in libav, as that would mean they’d be kept optimized without me having to bother with assembly or intrinsics. Namely pixel format conversion (which should be part of the proposed libavscale, still not reified), and drawing primitives, including bitblitting. I think part of this is actually implemented within libavfilter but as far as I know it’s not exposed for other software to use. Having optimized blitting, especially “copy this area of the image over to that other image” would be definitely useful, but it’s not a necessary condition for me to release the current state of the code.

So current work in progress is to support grayscale TIFF files (PAL8 pixel format), and then I’ll probably turn to libav and try to implement JPEG-encoded TIFF files, if I can find the time and motivation to do so. What I’m afraid of is having to write conversion functions between YUV and RGB, I really don’t look forward to that. In the mean time, I’ll keep playing Tales of Graces f because I love those kind of games.

Also, for those who’re curious, the development of this version of unpaper is done fully on my ZenBook — I note this because it’s the first time I use a low-power device to work on a project that actually requires some processing power to build, but the results are not bad at all. I only had to make sure I had swap enabled: 4GB of RAM are no longer enough to have Chrome open with a dozen tabs, and a compiler in the background.

3 thoughts on “unpaper and libav status update

  1. Other than some API warts… why not use libswscale for this?Indeed, what even is the rationale for a libavscale?

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