Frame tables

When thinking a bit about video editing, I thought it might be nice to have the timestamps of all video frames in advance, i.e. without decoding the whole file. With that you can e.g. seek to the Nth frame or for a given absolute time you can find the closest video frame even for variable framerate files.

The seeking functions always take a (scaled) time as argument and I saw no reason to change that.

So what I needed was a table which allows the translation of framecounts to/from PTS values even for variable framerate files. For many fileformats a similar information is already stored internally (as a file index) so the task was only to convert the info into something usable and export it through the public API.

Since this feature is very generic and might get used in both gmerlin and gmerlin-avdecoder, I decided to put the stuff into gavl.

Implicit table compression
One very smart feature in the Quicktime format is the stts atom. That's because it stores a table of frame_count/frame_duration pairs. The nice thing is, that for constant framerate files (the vast majority) the table consists of just one entry and translating framecounts to/from timestamps becomes trivial. Only for variable framerate streams, the table has more entries and the translation functions need longer.

The implementation
The frame table is called gavl_frame_table_t and is defined in gavl.h. Note: The structure is public at present but might become private before the next public release.
Here, there are also the translation functions gavl_frame_table_frame_to_time(), gavl_frame_table_time_to_frame() and gavl_frame_table_num_frames().

If you use gmerlin-avdecoder for decoding files, you can use bgav_get_frame_table() to obtain a frame table of a video stream. It can be called after the stream has been fully initialized. Naturally you will want to use sample accurate decoding mode before obtaining the frame table. If you want to reuse the frame table after the file was closed, use gavl_frame_table_save() and gavl_frame_table_load().

Future extension
An also interesting information are the timecodes. First of all, what's the difference between timecodes and timestamps in gmerlin terminology? Timestamps are used to synchronize multiple streams (audio, video, subtitles) of a file with each other for playback. They usually start at zero or another small value and play an important role e.g. after seeking in the files. Timecodes are usually given for each video frame and are used to identify scenes in a bunch of footage. They can resemble e.g. recording time/date.

Timecodes will also be supported by the frame table, but this isn't implemented yet.

Back from Malta

After a number of adventure- and/or conference trips, I decided to make a more ordinary vacation. Since the goal was to extend the summer a bit, we decided to travel in the first 2 September weeks (when German weather can already start to suck) and move to the southernmost country, which can be reached without many difficulties. Easy traveling for Europeans means to stay within the EU (and Euro zone), so the destination was chosen to be Malta.

I knew a bit about Malta from TV documentaries, and while reading the Wikipedia article a bit, I became even more curious.

Here are some facts I figured out. For pictures click on the Wikipedia links, they have higher quality than the ones from my crappy mobile.

• It was damn hot and humid. Not all people from the Northern countries can withstand that. I was happy to find out that after surviving 4 weeks in the monsoon season in Southern India, I'm more heat resistant than my Greek companion.
• It is indeed very stress-free because everyone speaks English (unlike in other Southern European countries) and the islands are small enough to reach practically every destination by bus. Just make sure you stay in the capital Valletta, almost all bus routes start there. On the smaller island Gozo, all busses start in Victoria.
  
• It has an extremely rich 7000 years old history. You can visit prehistoric temples (probably the oldest of their kind worldwide), churches, fortresses and remains from the Romans and Phoenicians. I already saw many churches from the inside before, I was not really a fan of Latin language and Roman history in school, and my companion didn't want to bother looking for sight seeing destinations at all. So I decided to concentrate on the prehistoric stuff.
  
• We saw the temples of Ħaġar Qim, Mnajdra (both covered with giant protective tents now), Tarxien and Ġgantija. The latter ones were the most impressive, especially after we reached them after a long walk in the afternoon heat 20 minutes before the last admission :)
  
• The temples of Skorba are the oldest ones, but not very spectacular and not accessible for the public.
  
• No chance to get into the Hypogeum of Ħal-Saflieni. It was booked out 4 weeks in advance. That sucked.
  
• If you like beaches, Malta is definitely not the #1 destination for you. There are just a few, a nice one we visited is Ramla Bay.
  
• If you like diving at cliffs, Malta is a paradise for you.
  
• Funny is the bit of British culture (Malta was British until 1964). People are driving on the left, you get chips with most meals, and the phone booths and mailboxes are red, like in the UK.
  
• Some Maltese people know more German soccer teams than me.
  

Shortly after coming back home, German weather started to suck.