Friday, December 14, 2018

GIFs in GNOME

Here is the story of how I fell down a rabbit hole and ended up learning far more about the GIF image format than I ever expected...
We had a problem with users viewing a promoted snap using GNOME Software. When they opened the details page they'd have huge CPU and memory usage. Watching the GIF in Firefox didn't show a problem - it showed a fairly simple screencast demoing the app without any issues.
I had a look at the GIF file and determined:
  • It was quite large for a GIF (13Mb).
  • It had a lot of frames (625).
  • It was quite high resolution (1790×1060 pixels).
  • It appeared the GIF was generated from a compressed video stream, so most of the frame data was just compression artifacts. GIF is lossless so it was faithfully reproducing details you could barely notice.
GNOME Software uses GTK+, which uses gdk-pixbuf to render images. So I had a look a the GIF loading code. It turns out that all the frames are loaded into memory. That comes to 625×1790×1060×4 bytes. OK, that's about 4.4Gb... I think I see where the problem is. There's a nice comment in the gdk-pixbuf source that sums up the situation well:

 /* The below reflects the "use hell of a lot of RAM" philosophy of coding */

They weren't kidding. 🙂

While this particular example is hopefully not the normal case the GIF format has has somewhat come back from the dead in recent years to be a popular format. So it would be nice if gdk-pixbuf could handle these cases well. This was going to be a fairly major change to make.

The first step in refactoring is making sure you aren't going to break any existing behaviour when you make changes. To do this the code being refactored should have comprehensive tests around it to detect any breakages. There are a good number of GIF tests currently in gdk-pixbuf, but they are mostly around ensuring particular bugs don't regress rather than checking all cases.

I went looking for a GIF test suite that we could use, but what was out there was mostly just collections of GIFs people had made over the years. This would give some good real world examples but no certainty that all cases were covered or why you code was breaking if a test failed.

If you can't find what you want, you have to build it. So I wrote PyGIF - a library to generate and decode GIF files and made sure it had a full test suite. I was pleasantly surprised that GIF actually has a very well written specification, and so implementation was not too hard. Diversion done, it was time to get back to gdk-pixbuf.

Tests plugged in, and the existing code actually has a number of issues. I fixed them, but this took a lot of sanity to do so. It would have been easier to replace the code with new code that met the test suite, but I wanted the patches to be back-portable to stable releases (i.e. Ubuntu 16.04 and 18.04 LTS).

And with a better foundation, I could now make GIF frames load on demand. May you GIF viewing in GNOME continue to be awesome.
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