This sets full trace for the thumbnailer:

gsettings set com.canonical.Unity.Thumbnailer log-level 2

If you want to start out with a cold cache, you can run

thumbnailer-admin clear

Then run a trace on ~phablet/.cache/upstart/dbus.log

The trace shows the requested dimensions for each thumbnail. Check
whether the dimensions being asked for are correct. If too small, the
problem is outside the thumbnailer. If dimensions are as expected, and
the video contains embedded cover art, the embedded cover art will be
returned scaled down to the requested size; if the dimensions of the
embedded cover art are smaller than the requested dimensions, the cover
art will be returned at the size it was found in the video.

Otherwise, if the video does not contain cover art, the thumbnailer will
extract a still frame and return that still frame, scaled down to the
requested size. In this case, the only way the image would be blurry if
the still frame itself is blurry, or if the video has ridiculously small
dimensions.

To confirm what's going on, run

/usr/lib/x86_64-linux-gnu/thumbnailer/vs-thumb <absolute/path/to/video>
video.tiff

Then check video.tiff by looking at it with a browser or image viewer.
This completely bypasses the thumbnailer and verifies whether the image
is being extracted correctly and omits any scaling; the command simply
dumps whatever image was found as cover art, or whatever screenshot was
extracted.

You can also talk to the thumbnailer:

thumbnailer-admin get <absolute/path/to/video>

This drops the thumbnail at the largest available resolution into the
current directory.

To request a specific size, use

thumbnail-admin get --size=200 <absolute/path/to/video>

This adds the image to the cache and returns it (or returns it from the
cache if it was cached earlier) after running it through the scaler.

man thumbnailer

and

man thumbnailer-service

provide more detail.

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https://bugs.launchpad.net/bugs/1542083

Title:
  broken video clip has extremely blurry icon when navigating to the
  camera roll

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