Cover Art Support

Last week: July 13 - July 19

As I explained last week, the cover art project must be disabled for some features. After some research and discussions with other developers, I notice that the best solution so far would be having a signal "enableCoverArtDisplay(bool)" in the active() method of all features. As this method is called always when the user switch the views (loading a new view) it is working fine now.

Supported features
* analysis
* auto dj
* crates
* library
* history
* playlist
Non-supported features:
* banshee
* browse
* itunes
* recording
* rhythmbox
* traktor

Currently we are identifying covers by looking for md5hash and it must be unique (obvious reasons).

In cases when the user overwrite the cover file in the trackdir, it will have the same location but an different md5hash. In this way, COVERDAO would try to store the new cover (new md5hash) but it would not be possible since it would have the same location as the old image... so, as the md5hash already is unique, we do not need to maintain unique locations as well...

New revision:

  <revision version="24" min_compatible="3">
    <description>
      Add cover art support
    </description>
    <sql>
      CREATE TABLE IF NOT EXISTS cover_art (
    id INTEGER primary key AUTOINCREMENT,
    location TEXT,
    md5 TEXT UNIQUE
      );
      ALTER TABLE library ADD COLUMN cover_art INTEGER;
    </sql>
  </revision>

Some improvements done during the week:

Copying covers
As I pointed out last week, it was important keep all new covers (chosen by the user) in the track directory. So, now Mixxx will check if the new cover comes from an external directory and decide if it have to be copied to the track directory.

Redesign dlgtrackinfo
It is very important that all fields have the same vertical spacing. I noticed that the only way to ensure it is drawing all fields in the same box_layout and it had an excessive number of them which could cause some bugs on Windows. Now it is also loading the covers on the right side.

Bug when showing smaller covers
The QPushButton will rescale just covers which are bigger than the icon bounds, so it is important rescale all of then to ensure that it will always use as much space as possible...

This week I also spent a lot of time studying the best ways to load the covers in the table view. Actually it was working since the last week (coverdelegate), however it had many issues yet. So, I did several improvements in the CoverArtDelegate and I do not see any performance issue any more...

Now it is cropping the covers in a working thread and storing them into the PixmapCache. It is being done in the CoverArtCache class and a new parameter (croppedPixmap) was added to the requestPixmap method.

Basically, if the request comes from CoverDelegate (table view), it'll want to get a cropped cover which is ready to be drawn in the table view (cover art column).

It's very important to keep the cropped covers in cache because it avoids having to rescale+crop it ALWAYS (which brings a lot of performance issues).

I extended the same logic that was used to delay covers for the cover widget to also delay the coverdelegate loading... Basically, now we have a member to lock/unlock coverdelegate calls.

Obviously, as the CoverDelegate calls are done when a new row is painted (always) it is be very dangerous when the user scroll very fast, because many requests would be done at the same time... So, now it is also delaying covers (search and loading) when the user is scrolling (it means that while the user is scrolling, the table view will show just covers which are already in the pixmapcache)...

If the CoverDelegate is locked, it must not try to load and search covers. It means that in this cases it will just draw covers which are already in the PixmapCache.

This week I also spent a lot of time studying some APIs to download covers. The good point is that the best available APIs use a very similar logic to request and reply the queries. In this way, we could put more effort in just one API and spend more time working on the new dialog (dlgcoverfetcher). After that we could analyse if we would need to integrate other API, but it would be much easier...

Initially my plan is download covers from Last.fm. I found a nice library which could make this integration a bit easier (liblastfm), however I noticed that is possible do the queries (that we need for this purpose) just using some Qt functions and it is very nice because it avoids having to add new dependencies to Mixxx.

Now the Last.fm search is already working and I also implemented a initial layout to show the results.

https://github.com/cardinot/mixxx/tree/coverArtSupport_4

Next week: July 27 - August 02

Although it is already working, it needs more work around the download management. It is using a QMap to store the search results, but it makes necessary doing many iterations to handle the results. A better approach would be having a simple list to store just the urls (download queue)...

The current layout is very basic and it has already many issues with the table view. It should also display the search status (error/no_results/searching/...).

Saving downloaded covers
After a search, all covers are downloaded and stored as a pixmap in a list. If the user choose any of them, we just need to store it as a file into the track directory. So, it could use the same logic that we are using when the user change the cover with the file browser...

Fetching missing covers
Should we have an option in the context-menu to auto fetch covers? (as we have for tags)

Automatic downloads
Should we have an option to enable/disable automatic download for missing covers?