Netstream and step() or seek()?

问题

I'm on an AS3 project, playing a video (H264). I want, for some special reasons, to go to a certain position.

a) I try it with NetStream.seek(). There it only goes to keyframes. In my current setting, this means, i can find a position every 1 second. (for a better resolution, i'd have to encode the movie with as many keyframes as possible, aka every frame a keyframe)

this is definetly not my favourite way, because I don't want to reencode all the vids.

b) I try it with NetStream.step(). This should give me the opportunity to step slowly from frame to frame. But in the documentation it says:

This method is available only when data is streaming from Flash Media Server 3.5.3 or higher and when NetStream.inBufferSeek is true.

http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/net/NetStream.html#step()

Does this mean, it is not possible with Air for Desktop? When I try it, nothing works.

Any suggestions, how to solve this problem?

Greetings & Thank you! Nicolas

回答1:

Flash video can only be advanced by seconds unless you have Flash Media Server hosting your video. Technically, that means that you can have it working as intended in Air, however, the video would have to be streaming (silly adobe...).

You have two options:

1) Import the footage as a movieclip. The Flash IDE has a wizard for this, and if you're developing exclusively in non-FlashIDE environment, you can convert and export as an external asset such as an SWF or SWC. This would then be embedded or runtime loaded into your app giving you access to the per-frame steppable methods of MovieClip. This, however, does come with some audio syncing issues (iirc). Also, scrubbing backwards is not an MC's forté.

2) Write your own video object that loads an image sequence and displays each frame in order. You'd have to setup your own audio syncing abilities, but it might be the most direct solution apart from FLVComponent or NetStream.

回答2:

I've noticed that flash player 9 scrubs nice and smooth but in players 10+ I get this no scrub problem.

My fix, was to limit frequency the calls to the seek function to <= 200ms. This fixed scrubbing but is much less smooth as player 9. Perhaps because of the "Flash video can only be advanced by seconds" limitation? I used a timer to tigger the function that calls seek() for the video.

    private var scrubInterval:Timer = new Timer(200);

    private function  videoScrubberTouch():void {
        _ns.pause();

        var bounds:Rectangle = new Rectangle(0,0,340,0);

        scrubInterval.addEventListener(TimerEvent.TIMER, scrubTimeline);
        scrubInterval.start();

        videoThumb.startDrag(false, bounds);


    }
    private function  scrubTimeline(e:TimerEvent):void {
        var amt:Number = Math.floor((videoThumb.x / 340) * duration);
        trace("SCRUB duration: "+duration+" videoThumb.x: "+videoThumb.x+" amt "+amt);

        _ns.seek(amt);

    }

回答3:

Please check this Demo link (or get the SWF file to test outside of browser via desktop Flash Player).
Note: Demo requires FLV with H.264 video codec and AAC or MP3 audio codec.

The source code for that is here: Github link

In the above demo there is (bytes-based) seeking and frame by frame stepping. The functions you want to study mainly are:

Append_SEEK ( position amount ) - This will got to the specified position in bytes and search for the nearest available keyframe.
get_frame_TAG - This will extract a tag holding one frame of data. Audio can be in frames too but lets assume you have video-only. That function is your opportunity to adjust timestamps. When it's run it will also append the tag (so each "get_frame_TAG" is also a "frame step").

For example : You have a 25fps video, you want the third-frame at 4 seconds into playback...

1000 milisecs / 25 fps = 40 units for each timestamp.
So 4000 ms == 4 secs + add the 40 x 3rd frame == an expected timestamp of 4120.

So getting that frame means... First find a keyframe. Then step through each frame checking the timestamps that represent a frame you want. If it isnt then change it to the same as most recent keyframe timestamp (this forces Flash to fast-forward through the frames to keep things in sync as it assumes the frame [with smaller than expected timestamp] should have been played by that time). You can "hide" the video object during this process if you don't like the look of fast-forwarding.

来源：https://stackoverflow.com/questions/9365479/netstream-and-step-or-seek

标签

actionscript-3

actionscript

seek

netstream