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how to force i-frames

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Bruce Schwartz
how to force i-frames
on Aug 4, 2009 at 5:36:21 pm

Using Final Cut and Compressor is there a way to force an i-frame/keyframe at a particular point in the video so that it is efficient to seek to that point during playback? I know I can force keyframes every N seconds but I don't want that. I want relatively few keyframes except for the places I specify. I've tried setting Chapter markers at those points but that didn't cause keyframes to be generated.

Thanks!


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Brian Alexander
Re: how to force i-frames
on Aug 4, 2009 at 5:55:52 pm

[Bruce Schwartz] "is there a way to force an i-frame/keyframe at a particular point in the video so that it is efficient to seek to that point during playback?"

Huh?

What are you trying to do? What codec are you trying to do it with? Are you trying to make interactivity within QuickTime? (Please explain)

You may be getting the whole point of Keyframes mixed up with something else you are trying to accomplish.

[Bruce Schwartz] "want relatively few keyframes except for the places I specify."

Some codecs use keyframes at set intervals in a GOP structure like HDV or MPEG2. Other codec keyframes can be set to a specific number of frames (like you pointed out). When choosing an interval for keyframes the encoder will produce a new keyframe every time there is a scene change.

When scrubbing GOP based videos in QuickTime you will most likely see just the Keyframes. I would suggest using a chapter markers to get around your video. If you can provide more info that would be a big help.

Thanks.


--
Brian Alexander
Sr Video Engineer
Freeman AVS


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Bruce Schwartz
Re: how to force i-frames
on Aug 4, 2009 at 6:56:29 pm

Thanks for the response! More details on what I'm trying to accomplish.

I'm using FCP to encode an MP4 using the H.264 codec. I'll be playing it back later in flash and I want to be able to seek to a particular frame in the video. So I want to insure that that frame is an i-frame. I trued to do this using Chapter markers but that didn't create i-frames that matched the chapter markers.

As a test I set the keyframe interval to be 300 so that there was an i-frame pretty much every 10 seconds. However, I had chapter markers every 3 seconds or so. When I tried to seek in flash I could only land on the 10 second intervals. If possible, I'd like to have i-frames every 10 seconds in addition to forcing each chapter marker to also be an i-frame.

I believe the codec supports this, but do the tools?


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Alex Beckman
Re: how to force i-frames
on Aug 4, 2009 at 7:14:21 pm

I have had the same issue. I am trying to use the h.264 video codex in a flash player, which uses iframes to seek in video. When I set my keyframe interval to be quite often (keyframe every 8 frames), my seeks are accurate, but at the expense of file size.

the more key frames i generate, the bigger my file.

In an flv video workflow, you can create "cue points", which generate iframes. This allows for a better keyframe interval, which creates smaller file sizes.

I want to divorce myself from flv and f4v, and use only the h.264 codec. Better quality, better interactivity, all around.

In FCP, chapter markers seem to do 90% of what I need, but I would love to know if anyone has figured out how to force iframes in a FCP workflow that is intended for online viewing.

Thanks



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Craig Seeman
Re: how to force i-frames
on Aug 4, 2009 at 7:28:10 pm

[Alex Beckman] "I want to divorce myself from flv and f4v, and use only the h.264 codec. Better quality, better interactivity, all around. "

f4v is H.264 (which can be mov or mp4 amongst others as well).



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Craig Seeman
Re: how to force i-frames
on Aug 4, 2009 at 7:26:49 pm

You may need Flash Cue Points as Navigation points. I'm not sure if this is implemented in H.264 Flash spec yet.

http://support.on2.com/h264_faq.php

Additionally, Alpha channel and cue point features in Flash Player are only supported within VP6. The powerful Alpha channel feature allows content creators to create Hollywood-style "green screen" special effects, among others, and is popular among content creators, ranging from advertisers to podcasters. Cue points allow content creators to add events to a video file or live stream at points where they want the video to interact with another element on a web page. You may, for example, want to display text, synchronize with a Flash animation, seek a different point in time in the video, or switch to a different video file altogether.



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Bruce Schwartz
Re: how to force i-frames
on Aug 4, 2009 at 7:40:03 pm

[Craig Seeman] "You may need Flash Cue Points as Navigation points. I'm not sure if this is implemented in H.264 Flash spec yet.
"


If I can only convince Compressor to insert an i-frame at the location of each Chapter Marker then I'll be able to seek to those locations in flash. So I don't really need any special Adobe stuff, just garden variety i-frames at the location I specify.


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Daniel Low
Re: how to force i-frames
on Aug 4, 2009 at 9:06:31 pm

Chapter markers are generally used by DVD Studio Pro, as not only for Chapter point on a DVD but also as compression markers.

At this time, this is the only way of using the markers as compression markers as far as I know.

As far as I know, there is no other way (yet) of doing what you want, you'll have to build your interactive movies using parts, the ins and outs of those parts with be your 'chapters'

__________________________________________________________________
"There's no chance that the iPhone is going to get any significant market share. No chance."

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Bruce Schwartz
Re: how to force i-frames
on Aug 4, 2009 at 10:14:06 pm

Hey thanks! Compression markers did the trick. Your post lead me to Ken Stone's page where he explains a bunch of stuff.

http://www.kenstone.net/fcp_homepage/compression_chapter_markers_stone.html


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Daniel Low
Re: how to force i-frames
on Aug 4, 2009 at 10:29:46 pm

So, you got this to work with H.264 and Flash?

__________________________________________________________________
"There's no chance that the iPhone is going to get any significant market share. No chance."

Steve Ballmer To USA Today


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