When launching multiple processes of vi3o, it is possible for a race to trigger in the creation of
the cache folder.

vi3o segfaults when requesting video with grey=True and the image height isn't dividable by 16 and the vertical subsampling is 2 (i.e. 4:2:0 format). The same would probably happen with 4:2:2 if the image height isn't dividable by 8.

The reason for this seems to be that jpeg_read_raw_data might increase output_scanline beyond the image height. The libjpeg documentation states intervall "0 .. output_height-1", so this indicates either a bug in libjpeg or miss use of it.

My _mjpeg.so links to libjpeg.so.62 and I have the following libjpeg installed

libjpeg-dev/oldstable,now 1:1.3.1-12 all [installed]
libjpeg8/now 8d1-2 amd64 [installed,local]

but also libjpeg-turbo

libjpeg62-turbo/oldstable,now 1:1.3.1-12 amd64 [installed,automatic]
libjpeg62-turbo-dev/oldstable,now 1:1.3.1-12 amd64 [installed,automatic]

I have tested with two recordings available at the following links, segfault using the first but not second:
http://da-robotteknik.se/shared/video_1920x1080.mjpg
http://da-robotteknik.se/shared/video_1024x768.mjpg

Not sure if this is related to issue: #25

cd test && python -c "import vi3o; v = vi3o.Video('t.mkv'); v[-1]; v[0]"

..leads to:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/danielfa/src/vi3o/vi3o/mkv.py", line 104, in __getitem__
    for img in self.myiter:
  File "/home/danielfa/src/vi3o/vi3o/mkv.py", line 229, in __next__
    return self.next()
  File "/home/danielfa/src/vi3o/vi3o/mkv.py", line 219, in next
    raise DecodeError
vi3o.mkv.DecodeError

It does work with a.mkv, b.mkv, c.mkv and systime.mkv. This might indicate one of the two:

1. It's related to the encoding from the Axis camera's
2. It's depending of if there is a "nice" end of the file or if it's a cut off web stream

mjpg files and mkv files using mjpg codec seems unaffected.

This seems to be a problem when running ubuntu, not debian so probably depending of the installed version of libavcodec?

With the video systime.mkv the first frame is OK, then there is 31 frames which is fully green, then it's OK again. This aligns with the keyframes in the video (1 and 33).

python -c "from vi3o import Video; from vi3o.image import imview; [imview(f) for f in Video('systime.mkv')]"

With a.mkv it does also align with the keyframes (1 and 9)

However, with t.mkv it does not. Only keyframe is 1 but the frames are:
1 OK
2,3,4 green
5 OK
6,7,8 green
9,... OK

keyframes extracted with:
ffprobe -loglevel error -skip_frame nokey -select_streams v:0 -show_entries frame=pkt_pts_time -of csv=print_section=0 systime.mkv

I have not seen the problem with mjpg files or mkv files using mjpg codec.