The Wall

Just for fun I tried throwing a bunch of videos into one window simultaneously.

And yes, the sound plays ... although it is quite disconcerting and disorienting having all 9 play at once ...

The sources are PAL-format digital recordings transcoded to x264, i'm scaling them using libswscale and then simply using swing labels to display them. It's clocking up between 100% and 150% CPU usage on the java process (100% == 1 core on Linux). So even though i'm hardly trying to make it quick it's hardly taxing the box (albeit a very fast box).

For some reason the timing code gets messed up if I try to sync to the audio timestamp, but that might just be these videos.

Video Player

So this weekend I wrote a video player using jjmpeg. Yesterday I had the basic video working, and today I got sound working - using OpenAL (JOAL) for output. For the most part even using a fairly simple synchronisation mechanism it works fairly well. I'm letting the hardware run the sound at it's native rate, and synchronising the video to the timestamps in the file (using Thread.sleep() no less!) - not perfect but it's a start.I also uploaded a new jjmpegdemo's directory to the jjmpeg project which includes a simple audio player that I used to work out OpenAL.

But the Video player is a bit more complex, I've currently got 1 thread demuxing the input, 1 thread each for video or audio stream decoding, and another thread to synchronise the audio and video. I use some of the nice classes from java.util.concurrent to handle the packet and frame queues which means each bit of code is pretty simple. I'm recycling the AVPacket's, the frames and audio sample buffers so once started it has a pretty low GC load.

I'm having some strange problems though - certain files seem to throw the demuxer right out - I get massive corruption in video and audio and it's getting completely broken data frames. These files play just fine in the AudioPlayer above, so I presume i'm doing something incorrect with my threading which is corrupting something along the way. Other files work just fine though so it doesn't seem to be just a simple problem with invalid code - it also affects different containers and codecs inconsistently. Just the sort of bugs I like ...

Update It seems I had too many threads. I've moved calling the codecs from their own thread to the demux thread. Then I have 2 threads for rendering the audio and video separately instead.

Video List

So one thing i've been mucking about with using jjmpeg for is creating a GUI for listing videos ... which seems a pretty basic starting point for doing anything further.

Yeah it's not much to look at so far but one has to start somewhere. I might look at using Piccolo2d as the rendering surface, although I have to determine how to handle virtual items as I do here with the JList. Apart from general fugliness it flashes white whenever you change the view sort, which is quite unpleasant. As can be seen, I hooked it up to mplayer after you double-click a row, just for a laff ...

Under the bonnet it uses jjmpeg of course to scan the files - it's currently generating 128x128 preview images at 1 minute intervals - of which only the first is shown. I have a separate tool to `import' the videos for the moment but I have code lying about to allow dropping of files, so it wont be hard to add. I'm using Berkeley DB - java edition to hold the meta-data and preview images, and i've hooked it up so the DB is scanned in another thread. I use different secondary indices for each sorted view so they are all just as fast (slow?) as each other - this will also let me query by keyword with a little more code. I'm also using SoftReferences to implement a cache of database items. Unfortunately Berkeley DB JE doesn't let you query by record number like the C version, nor read the secondary database keys without also dereferencing to the primary database (i.e. slower than might otherwise be), but judicious use of threads can help alleviate such issues.

In short: it should scale quite well.

Probably ...

beep

Feeling a bit cold and crappy and had a day of franticly little progress at work so I sat down with jjmpeg for a little while tonight to pick some low hanging fruit to make it feel like I'm getting somewhere.To that end I added audio decoding support. It only requires a couple of functions and field accessors beyond what you need for video, but they needed some extra native functions to make them work. Mainly with decode_audio3 as it takes an AVPacket but may not fully consume it and doesn't update it to indicate this, so you really need to make a copy of it and update the copy's pointers based on the decoding results. At least I did it in a way which requires no allocation activity on the Java side during the decoding loop, I just use the wrapping ByteBuffer's to perform a memcpy and have a single native function to update the data pointer and size on the copy.

I don't have any immediate need for it, but it was easy enough to add and now the day doesn't feel like a total waste.

I'm slowly working toward a loose idea of 'something' that uses this stuff, although I haven't really pinned down what that 'something' might be exactly. Which leaves the options pretty wide open for now.

Sharing GLContexts

Hit a problem yesterday, how to share GL contexts with OpenCL and several GLCanvas objects. i.e. I have a window with a few output windows and they all need to share OpenCL processing.

I eventually found one of the test cases which did the same thing, and I used a similar approach.

• Create a 1x1 off-screen GLPbuffer object using GLDrawableFactory.
• Add an event listener to that.
• Call display() which will invoke the GLEventListener.init() method synchronously - in which I then create my CLGLContext object.
• A few other bits and pieces like using makeCurrent() on the GLContext taken from the CLGLContext() when creating textures to be shared in the OpenCL code.

Bit tricky to debug as most mistakes just lead to clue-free segfaults. It didn't help that I forgot to initialise a width/height variable at one point, and until I did everything 'worked' except that creating the output texture always failed with an error which didn't make sense.

jjmpeg - why java?

As a follow-up to the post about using Java and JNI to access ffmpeg, perhaps the more fundamental query is - why use Java in the first place? After all, Java is slow and crappy and nobody uses it anyway and isn't .NET the way to go and all that?

I used to write Amiga BOOPSI classes in assembler for fuck's sake, so why am I now using Java?

For starters Java is not slow - although as with any language you can (un)intentionally make it slower than necessary. Compared to similar systems with the same application support it isn't bulky; at run-time or on disk. The JVM is mature and stable and the garbage collection is reliable and fast.

Machines are also not slow these days - in-fact they are so fast most of the processing power is wasted much of the time. Likewise for memory. Wasted processing cycles and wasted memory bytes are actually an inefficiency, not necessarily something to chime about. I am no longer developing applications for a 1MB system running a multitasking GUI. Nobody is.

I still enjoy writing C, and I am still concerned with performance and efficiency, but I have been using Java for a few years now and am very happy with it - and I continue to be further pleasantly surprised from time to time. I find it puzzling that far more desktop software isn't written using Java - in my experience it compares well in all the important categories and is generally easier to develop for.

For example, performance is usually within a few % of C for normal scalar, single-threaded C. Most programmers don't seem capable of going beyond that type of code anyway - and those that are will find JNI a piece of piss. It will probably require twice as much memory - but this is simply an artefact of the use of a decently fast garbage collector - nothing comes completely free, but with memory expanding so much in recent years this is about the cheapest cost you could imagine for the huge benefit it provides. And I don't just mean no longer needing to track which pointers to free - I never found that particularly onerous although many people are unable to grok it - the GC is also a very fast memory allocator as well. No need for pool or slice allocators and the whatnot.

By the time you add all of the features of a basic JVM runtime to C (or anything else), you have something like GNOME or KDE which are not very small at all, have large memory footprints themselves, and are still not as easy to work with (speaking of GNOME as of some time ago at least, I haven't tried KDE and in any event loathe C++ so am not about to).

Of course, python (or ruby) seem to be the flavour of the month at the moment, but they have their own issues. Usually they are just ugly front-ends to some C libraries or commands and they have the same problems that tcl/tk scripts had - a specific version dependency, ugly gui's, and meaningless error messages from their inevitable crashes. And for all that they're not particularly robust, nor provide a particularly compact memory footprint.

seek to frame

Seeking to a frame using ffmpeg ...

I knew this was a bit of a pain since I'd tried it before, but oh boy - there went my weekend. And i still don't have a 100% reliable solution. Ho hum.

Some of the issues I found with only a handful of videos I have at hand:

1. An mpeg ts which will wont seek via timestamp. Only byte seeking works.
2. An avi in which byte seeking never works. Only timestamp seeking does.
3. An avi in which the DTS increments forever - so although you can seek by timestamp to a keyframe, you cannot use it to identify specific frames thereafter.
4. A mov file in which byte seeking never works.
5. A mov file which ends with an EPIPE error rather than end of file. It must be closed and re-opened to perform any further operations.

I have something which mostly works now, but I suspect it will never be reliable enough.

jjmpeg - why jni?

I started writing a reply to Michael's comment on the last entry but because I tend to ramble, it ended up so long I thought I'd promote it to a post.

have you considered using JNA? ...

... otherwise gluegen if you want to stay on the JNI road.

I looked at JNA previously some time ago, and found some problems with using it. I can't remember what they were at the moment but I was so displeased with it I know it ruled me out ever bothering with it again. It looks really good on paper but as I chose to write JNI directly at the time (for cross platform code too) there must have been a good reason. FWIW I didn't look into SWIG or any other option either.

For ffmpeg specifically, you need to access random fields of big structures and it would be impractical (or impossible) to map them using jna - many of the fields are private and the public ones are spread out through the structure. So i'd be forced to write a library to define accessors anyway, and then the jna objects to call those, so in the end i'd have to write something twice whereas now i don't even need to write it once (just a simple config file entry, assuming i didn't write a generator for jna - but then there would be no reason to use it).

I tried gluegen because it looks pretty nice and i've had nothing but positive experiences with jocl and jogl, but it's preprocessor and parser just weren't up to the task - the ffmpeg headers are mostly internal headers which have become public, they are not a cleaned-up public/standard api. They contain a ton of cruft that isn't public as well as public stuff that is behind conditional compilation (using expressions) and the like. I tried pre-processing it using cpp -dD (iirc) which preserves the #defines but then the inline code or other stuff threw it and i couldn't even work out which bit of code was the problem from the terse error messages. After giving up on it i found some other tools that might generate a simple/clean enough file to process (e.g. cproto can dump cleaned up types as well as clean prototypes) - but by then i'd moved on.

The perl script is a bit of a mess but most of the binding is automatic. At minimum i only have to write a constructor method for the public class. Accessors and most methods are automatic (once defined in the config file). I only need 2 classes for every wrapped 'object', one auto-generated.

There are a few special cases, but I find JNI pretty easy to use for those - given what it does it's about as simple as could be expected. And having attempted or worked with interfaces for similar purposes in the past I think JNI is actually quite nice. For example .NET's native binding looks really nice on paper too (it's more like a 'built-in' JNA) but there are actually more gotchas because it's trying to automate more - it's good most of the time but can be a real pain when things get complicated.

I'm already spending more time trying to work out how to use the libraries, the binding itself is mostly looking after itself, even if it is still incomplete.

Having said all that .... I realise that I may have made a mistake and there will be outstanding issues yet to resolve. But at least i'm fighting with my own mistakes and not finding the hidden limitations of tools i know little about - which simply makes it a lot more fun for a spare-time project.