Scott - thank you for clarifying that - I think that is what I have been wondering about - I-Frames that is
So can you think of situations where you need more or fewer I-frames than the default "every 15 frames"
Is this what is being referred to in VJ apps for better scratching, scrubbing or whatever you want to call it?

fultro

Forgive an old fart, VJ is what? Video Jockey? Like in Video DJ using one of those neat new DVD players from Pioneer?

Remember that the number being talked about is the number of full frames of video per every I frame. With 15 you you get one I frame followed by 14 B/P frames that describe the changes from the I frame .

Since you can only jump around by GOPs (since you need the I frame to figure out how to interpret the 14 b/p frames that follow it), then we're talking 1/2 second jumps (in NTSCland - 15 frames at 29.9 frames/second = 1/2 second), so I can see that it would be useful for doing certain types of scrubbing to have a resolution better than 1/2 second. Lowering the number of video frames per I frame could be usefu since at 3 (for example) you have resolution at 1/10 of a seocnd rather than 1/2. I can't see going higher than 15 being all that useful.

But that's just me.

--Scott

Got it - that makes sense to me also - about the fast scrubbing aspect of this - but I wonder if you think having more I-frames than the default has any downside - say - in terms if normal playback and editing.
And now that I think about it -what of audio? If you need to edit the audio track with a greater resolution than a half second (and who doesn't ) - what happens to the video - in fact I 'm not aware of any problems occuring with the video when I have edited based on the needs of the audio track alone - or simply split the event with no regard for the I-frame. I can't believe I have lucked out every time and found that I-frame as my edit point - wouldn't Vegas playbak complain if I cut the stream between I-frames?
Yes - being an old fart myself - I had to look VJ myself not too long ago - it's actually VJay
And I have started using an app called Resolume which has all kinds of free filters available for it that are MIDI controllable which can be a lot of fun - but also quite useful for creating textures and odd effects David

I don't normally edit in MPEG-2, but I believe that Vegas will reconstruct each frame shown on the timeline from the I, B & P frames, and when you go to render again, all of the information is uncomressed and then recompressed. Hence no restirction on where you edit.

So far I have not found any drawback to having more I frames except the resulting file size, but since I only drop the value down to 3 when I'm rendering MPEG for a menu background that I want to have seemless looping, which is usually of short duration, then it's not a major concern.

--Scott

Actually... I've always thought it was better to render DVD menu backgrounds to AVI... as invariably you overlay text/buttons etc onto that menu background in DVDA. This results in it being re-compressed right? In which case I had thought it was always better to be rendering FROM an AVI file rather than MPEG.

For DVD Architect you are entirely correct - for your menu video render to AVI, not MPEG; because 99% of the time DVDA is going to re-render the menu video anyway (even though there are situations where it doesn't need too, it still does).

For something like DVD Lab or DVD Lab Pro (which does menus quite differently than DVDA) you want to go to MPEG (M2V actually).

--Scott

Thanks - a useful discussion from a number of angles here
I should have realized I-frames apply to mpeg , not the avi that I normally edit
And if I understand you correctly - for looping video to be seemless it might be necessary to increase the number I-frames

David

You'll never get an entirely seemless loop no matter what you do. However, by increasing the number of I frames you do have more control over where you set the loop point which makes it easier to design your audio and video content. That is if you are using authoring software that lets you set up a menu loop.

How do you know you are setting the loop point on an I-frame ? I take it that that is specific to the DVD authoring software one uses...?
David

Ive had no problems creating seemless looping menu backgrounds. The hardest part is creating a motion background that is loopable.
Back to the original question though. It would seem with WMV they're refered toas keyframes and with mpeg "I" frames. Same hosre, different name.
As has been suggested you need to go back to the previous I frame and read all the subsequent frames to derive the current frame. This does create more work for a NLE. Increasing the number of I frames doesn;t have any direct downside except ths file gets bigger and the average bitrate goes UP. The last part can be a big issue. SOme high end broadcast systems like IMX use very short GOPs, this can mean you have to deal with very high datarates at the expense of ease of editing.