> Well, consider the cost of scaling down from a full-size photo to a 
> thumbnail. It takes a quarter of a second on a Nexus 4.
Yes, that should be done as few times as possible.

> Surely, it's not too much to ask the caller what size thumbnail it wants?
It actually is, i have no idea of the image i want unless you tell me the 
aspect ratio and since i can't know the aspect ratio i need the full image.

>  The meaning of (-1, -1) is "I don't know what I need, just give me something.
You're making this up, the API documentation explicitly says "sourceSize can be 
cleared to the natural size of the image by setting sourceSize to undefined."

> The point here is that the thumbnailer has no business setting policy.
You say you have no business setting the policy but then go and set a policy 
that says "i'm not accepting undefined sizes even if the API says i should", 
please do not do that.

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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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