touch-packages team mailing list archive

[Michi Henning <michi.henning@xxxxxxxxxxxxx>]

So, here is some history of this. The old thumbnailer, when presented
with (-1,-1), delivered a 512 thumbnail. That wasn't really by design,
but a side-effect of how the code was written. When we changed to the
new thumbnailer, we decided to return the full-size image for an invalid
QSize.

That turned out to be a mistake: when we ran a bunch of apps and scopes
against the new thumbnailer, we found that there were lots of cases were
the caller passed an uninitialized QSize, and we returned a full-size
image in response. To the caller, everything appears to be working
correctly, but it's working horribly inefficiently, both in terms of
disk space and in terms of scaling.

If the caller wants a full-size image, it can do that, by asking for (0,
0). The advantage is that asking for a full-size image becomes an
explicit operation, so it won't happen by accident.

I don't quite understand why you need to know the aspect ratio
beforehand. Surely, there must be some display size into which the
thumbnail is expected to fit? If we get something like QSize(512, 0) or
(0, 512), we assume a 512x512 bounding box and scale accordingly. (We
always preserve aspect ratio.) So, why not use that? And, if you really
want a full-size image, just ask for (0,0) and you'll get it. But we
can't return a full-size image for an uninitialized QSize; all that does
is hide errors and it causes things to run horribly inefficiently.

Returning some other size for (-1,-1) will be wrong most of the time for
most callers, no matter what size we pick. It'll be too small and look
bad, or it'll be too large and inefficient.

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Touch seeded packages, which is subscribed to thumbnailer in Ubuntu.
https://bugs.launchpad.net/bugs/1467740

Title:
  Lots of thumbnail requests with invalid size

Status in thumbnailer package in Ubuntu:
  New
Status in unity8 package in Ubuntu:
  New

Bug description:
  When using scopes, the thumbnailer gets loads of requests with
  QSize(-1,-1) from the shell. For example, with a bunch of videos
  recorded, run up the video scope and go to "My Videos". The shell asks
  for a thumbnail at QSize(-1,-1) for the tiny thumbnail that appears to
  the left of each list entry.

  The problem with this is that the thumbnailer interprets this to mean
  "give me the largest size you can (limited to a 1920x1920 bounding
  box). That's very expensive, especially in terms of disk space,
  because that 1920 "thumbnail" ends up going into the cache, needlessly
  hogging space.

  We are about to add a qWarning message to the QML side that reports
  invalid QSize requests. For now, we are going to retain the old
  behavior, but this will turn into an error soon.

  The most effective way to use the thumbnailer is to simply ask for an
  image in the desired size, with neither width nor height of -1. The
  thumbnailer will efficiently produce a thumbnail for that. (We do lots
  of internal caching to avoid extracting or downloading a thumbnail
  unnecessarily.)

  The thumbnailer may deliver a thumbnail that is smaller than what was
  asked for (because it never up-scales) so, if asked for a thumbnail of
  size 256, it's guaranteed not to be larger, but might be smaller (if
  the original image is smaller than what was asked for).

  Could you please adjust the shell behavior to ask for specific, valid
  sizes only?

  Running a tail -f on ~/.cache/upstart/dbus.log allows you to see the
  requests as they are made. Each request shows the size that was asked
  for.

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