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Michael Zucchi

 B.E. (Comp. Sys. Eng.)

  also known as zed
  & handle of notzed

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Tuesday, 19 December 2017, 20:26

jjmpeg, jni, javafx

So I guess the mood took me, I somehow ended poking away until the very late morning hours (4am) the last couple of nights hacking on jjmpeg. Just one more small problem to solve ... that never ended. Today I should've been working but i've given up and will write it off, it's nearly xmas break anyway so there's no rush, and i'm ahead of the curve anyway.

JJMediaReader

I got this ported over and playing video fairly easily, and then went through on a cleanup spree. I removed all the BufferedImage, multi-buffering, and scaling stuff and a few other experiments which never worked. Some api changes allowed me to consolidate more code into a base class, and some changes to AVStream necessitated a different approach to initialising the AVCodecContext (using AVCodecParameters). I made a few other little tweaks on the way.

The reason I removed the BufferedImage code is because I didn't want to pollute it with "platform specific" code. i.e. swing, javafx, etc. I've moved that functionality into a separate namespace (module?).

My first cut just took the BufferedImage code and put it into another class which provides the functionality by taking the current AVFrame from the JJMediaReader video stream. This'll probably do but when working on similar functionality for JavaFX I took a completely different approach - implementing a native PixelReader() so that the native code can decide the best way to write to the buffer. This is perhaps a little more work but is a lot cleaner to use.

swscale

jjmpeg1 lets you scale images 'directly' to/from primitive arrays or direct ByteBuffers in addition to AVFrame. Since they have no structure description (size, format), this either has to be passed in to the functions (messy) or stored in the object (also messy). jjmpeg1 used the latter option and for now I simply haven't implemented them.

The PixelReader mentioned above does implement it internally but for code re-use it might make sense to implement them with the structure information as explicit parameters, and use higher level objects such as PixelReader/Writer to track such information. On the other hand the native code has access to more information so it also makes sense to leave it there.

I went a bit further and created a re-usable super-class that does most of the work and toolkit specific routines only have to tweak the invocation. This approach hides libswscale behind another api. The slice conversions don't work properly but they're not necessary.

jni

So far I had public constructors and `finalisers' because otherwise the reflection code failed. That's a bit too ugly (and `dangerous') so I made them private. The reflection code just had to look up the methods and set them Accessible.

    Constructor cc = jtype.getDeclaredConstructor(Long.TYPE);

    cc.setAccessible(true);

    return cc.newInstance(p);

Whilst working on JJMediaReader I hit a snag with the issue of ownership. In most cases objects are either created anew and released (or gc'd) by the Java code, or are simply references to data managed elsewhere. I was addressing the latter problem by simply having an empty release() method for the instance, but that isn't flexible enough because some objects are created or referenced the the context determines which.

So I expanded the Java-side object tracking to include a `refer' method in addition to the 'resolve' method. `resolve' either creates a new instance or returns and existing one with a weak-reference object which will invoke the static release method when it gets finalised. `refer' on the other hand does the same thing but uses a different weak-reference object which does nothing.

I then noticed (the rather obvious) that if an object is created, it can't possibly 'go away' from the object tracking if it is still alive; therefore the `resolve' method was doing redundant work. So I created another `create' method which assumes the object is always a new one and simply adds it to the table. It can also do some checking but i'm pretty sure it can't fail ...

If on the other hand the underlying data was reference counted then the `resolve' method would be useful since it would be possible to lookup an existing object despite it being `released'. So i'll keep it in CObject.

As part of this change I also improved CObject in other ways.

I was storing the weak reference to the object itself inside the object so I could implement explicit release and to avoid copying the pointer. I removed that reference and only store the pointer now. The WeakReference it already tracked in a hash table so I just look it up if I need it. This lets me change the jni code to use a field lookup rather than a function call to retrieve it (I doubt it makes much perf difference but I will profile it at some point).

I also had some pretty messy "cross-layer" use of static variables and messy synchronisation code. I moved all map references to outside of the weak reference routine and use a synchronised map for the pointer to object table.

For explicit release I simply call .clear() and .enqueue() on the WeakReference - which seems to do the right thing, and simplifies the release code (at least conceptually) since it always runs on the same thread.

Tagged hacking, java, javafx, jjmpeg.
damned enums | `parallel' streams
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