Thursday, 1 March 2012

jjmpeg updates

I finally checked in some jjmpeg changes I had lying around, added seeking to JJMediaReader and an icon creation helper (hmmm, maybe that was api bloat ...) and a few other odds and sods.

I've been playing with some interactive video stuff for work and when the video file is in good condition (i.e. seeking works properly) it's quite amazing to me just how zippy it all is - coming from the days of the C=64 I still can't get over just how fast modern machines are. And Java of course makes the multi-threading required to make the GUI very responsive an absolute doddle.

Absorbing rapid events

One idea I borrowed from my experiments on ReaderZ was a fairly simple mechanism to collapse rapidly incoming events. e.g. for a slider bar calibrated in ms, one can get many many updates as the slider moves; more than can be accommodated whilst panning an e-ink display or seeking around HD video. In the past i've either used a timeout, or some other throttling mechanism on the caller end such as a 'i'm busy' flag. This usually needs some other logic to handle completion cases, perhaps cancelling of jobs and other quite complex synchronisation tasks to ensure valid programme state when it's all done and dusted.

In ReaderZ I tried a different approach in order to simplify serial processing:
  1. Incoming tasks are queued as they arrive into a blocking queue.
  2. A consumer thread waits until something arrives on the queue.
  3. The consumer thread then polls the queue for any other tasks waiting to be processed.
  4. Based on the class of the request, jobs are discarded explicitly. For example, if you have a seek followed by an open or another seek, the first seek can be thrown away. i.e. the command is either kept, changed, or nullified.
  5. Then at most, 1 job each of each class are executed in the correct order.
  6. Repeat, go back to 1.

So basically a simplified broken-apart state-machine with explicit state reduction. If a given job is indivisible/can't be ignored (say, 'save current image'), then the collapse processing is cut short, and it jumps to step 5.

This way there's no need for any locking (apart from the task queue): the code is always called from a single thread, with guaranteed execution order and with simple state management. And even when something does take a while to run; it eventually catches up and never does more than less than one lot of extra work. It does need to ferry ALL sequentially oriented tasks through the command queue, but usually one has a fairly limited number of operations required.

Because the same thread is used to decode and play the video for my video player, if it is in 'play' mode, this just polls the command queue after each frame is displayed rather than waiting for it to contain something; but the overall logic is the same.

Also because it's done in one place I can more easily add a timeout if I really want to make sure the system is idle: rather than a separate timeout callback which needs resetting and gets called once things are done, I can just add a timeout to the Queue.poll() invocation. Or just as easily not, for example if it's been faffing around collapsing too many commands and hasn't updated the output for a while.

Speaking of ReaderZ, although I don't have any other plans for it at the moment, I am waiting for the next version of mupdf to be released at which point I will update PDFZ to match that. It should be quite soon.

Although I wasn't going to look into JavaFX too closely, every time I hit a problem in Swing I keep thinking the solution is a dead-end and the time spent on it is wasted. Unfortunately the one thing I need the GUI toolkit to to: i.e. display an array of pixels generated elsewhere: is one of the major things it cannot do yet! It can only read images from a url/disk, and that feature is targeted for 3.0 - about 18 months away. I suppose i'll just have to wait ...

Damn I wish I wasn't so tired. Must be the weather ... Autumn started very suddenly on the 29th.

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