[00:00.000 --> 00:17.000]  It's easy, you know, because my laptop doesn't really like how it's being disconnected from the laptop.
[00:30.000 --> 00:48.000]  Hello, my name is Paris and my talk is about supporting old proprietary graphic formats.
[00:48.000 --> 00:55.000]  Specifically, we're going to be talking about the WMF and the EMF formats from Windows.
[00:55.000 --> 01:03.000]  So what is a WMF? And this also applies to the EMFs, but the WMF is a Windows metafile.
[01:03.000 --> 01:09.000]  It is a graphics format that supports vector and raster operations, mostly vector.
[01:09.000 --> 01:13.000]  It was introduced in the early 1990s in comparison.
[01:13.000 --> 01:18.000]  A different vector format like the SVG was released in 2001.
[01:18.000 --> 01:23.000]  It is composed of a set of GDI drawing commands and structures.
[01:23.000 --> 01:31.000]  These drawing commands are played back in order to render the graphic within what is known as the playback device context.
[01:31.000 --> 01:35.000]  And it is not as widely supported as SVGs.
[01:35.000 --> 01:43.000]  Essentially, this means that you can code this format into existence by writing some GDI functions.
[01:43.000 --> 01:48.000]  What are the difficulties in supporting this format?
[01:48.000 --> 01:53.000]  So the WMF files are application and device dependent.
[01:53.000 --> 02:04.000]  The EMF files later, that were later introduced, try to solve this issue, but the WMF files are more difficult in that way.
[02:04.000 --> 02:09.000]  The device context that is associated with a WMF file cannot be queried.
[02:09.000 --> 02:15.000]  That is, an application cannot retrieve the device resolution data, font metrics, and so on.
[02:15.000 --> 02:21.000]  So if you made a WMF file for a specific device, you cannot really know.
[02:21.000 --> 02:27.000]  If you tried running on a different device, you don't really know the device it was built for.
[02:27.000 --> 02:32.000]  There is a format specification for this, but a lot of things are missing.
[02:32.000 --> 02:36.000]  And there is some edge cases with undefined behavior.
[02:36.000 --> 02:41.000]  And pinpointing the root cause of a buggy file can be tricky.
[02:42.000 --> 02:44.000]  So how do you debug a WMF?
[02:44.000 --> 02:51.000]  Well, there's a lot of ways. I'm going to present the way that I do it, usually.
[02:51.000 --> 02:55.000]  So you would want to get the drawing commands, the GDI drawing commands,
[02:55.000 --> 02:58.000]  and there is multiple ways of doing so, more than I listed.
[02:58.000 --> 03:00.000]  One is MSO Dumper.
[03:00.000 --> 03:06.000]  It is created and used by LibreOffice developers, and it dumps the drawing commands.
[03:06.000 --> 03:09.000]  Another one is the Metafile Explorer.
[03:09.000 --> 03:13.000]  It allows for viewing and stepping through the drawing commands,
[03:13.000 --> 03:17.000]  so you can easily understand which command does what.
[03:17.000 --> 03:22.000]  And then there is the Metafile GDI function, which is defined in the GDI header,
[03:22.000 --> 03:27.000]  which allows to enumerate the drawing commands in WMF file and call a callback function.
[03:27.000 --> 03:32.000]  A similar function also exists for EMF files.
[03:32.000 --> 03:35.000]  This is an example of an WMF drawing command.
[03:35.000 --> 03:37.000]  WMF drawing commands look very similar, of course.
[03:37.000 --> 03:40.000]  It's the function signature. It takes some parameters,
[03:40.000 --> 03:48.000]  and this is what the record looks within the file if you open it in the aforementioned debugger.
[03:48.000 --> 03:54.000]  So it's very similar to the function signature itself.
[03:54.000 --> 03:59.000]  After you obtain the drawing commands, well, you want to debug.
[03:59.000 --> 04:04.000]  Because WMF is such a platform-dependent graphic format,
[04:04.000 --> 04:12.000]  sometimes it's good to compare with other WMF reader implementations like PowerPoint
[04:12.000 --> 04:20.000]  to understand exactly what the graphic looks like in other implementations.
[04:20.000 --> 04:25.000]  Then you would want to identify which drawing commands cause the bug.
[04:25.000 --> 04:28.000]  You would, you know, the drawing commands that you obtained,
[04:28.000 --> 04:35.000]  you would step through them and try to pinpoint what exactly causes the bug.
[04:35.000 --> 04:40.000]  It is important to also reduce the relevant commands as much as possible.
[04:40.000 --> 04:44.000]  WMF files can contain thousands of drawing commands,
[04:44.000 --> 04:48.000]  and stepping through all of them is very tricky.
[04:48.000 --> 04:51.000]  A way to do this is to simplify the problematic file
[04:51.000 --> 04:56.000]  or make a new one that reproduces the bug in LibreOffice.
[04:56.000 --> 05:02.000]  And then you work around these buggy commands to find out what is wrong.
[05:02.000 --> 05:06.000]  Easier said than done, but yeah.
[05:06.000 --> 05:10.000]  Finally, you want to make sure you didn't break something.
[05:10.000 --> 05:16.000]  It is good to create a unit test for your fix using the minimally reproducible example you created before.
[05:16.000 --> 05:18.000]  You run the appropriate test suits.
[05:18.000 --> 05:21.000]  You probably broke something, so you go back to step one.
[05:21.000 --> 05:25.000]  And then you confirm that round-tripping works as expected.
[05:25.000 --> 05:30.000]  And you also, I should mention, monitor the WMF file,
[05:30.000 --> 05:36.000]  the fix you made for WMF file to make sure it doesn't break something in the future.
[05:36.000 --> 05:39.000]  And that is all. Thank you.