About setting I-frames in a GOP

[dynamique]

I'm just curious about the GOP structure. In my encoding software, I can set i-frame keyframes. But can I really set them as I want? Could I set every frame as an i-frame and hence have a mjpeg stream? Can I set whatever frames as I want as i-frames? Or is there a limit to how close they can be? I tried once to set like ten i-frames in a row, but the file could not be opened by other programs.
Any place on the net that explains these things without digging twenty miles down the earth is appreciated.

[Cornucopia]

I'll answer what I can:

#1

But can I really set them as I want?

Depends on the encoding software, but usually (for the better ones), YES.

#2

Could I set every frame as an i-frame

Same as above, YES. Note that in doing so, you'll need a good deal higher average bitrate in order to maintain the same overall quality.

#3

and hence have a mjpeg stream

No, you'd have an I-frame MPEG stream. There ARE differences (however minor) between I-frame MPEG (and MPEG stills) and JPEG, even though both use DCT. MPEG, being ratified after JPEG, has a few areas where it can be more efficient than JPEG.

#4

Can I set whatever frames as I want as i-frames

See #1. This is where manual control is really important in pro use of hardware/software encoders to optimize the quality.

#5

there a limit to how close they can be

Not really, although this depends upon playback BUFFERS and expectations, unless you're talking about MultiAngle/ILVB/Playlist/Parental clips, then YES.

#6

tried once to set like ten i-frames in a row, but the file could not be opened by other programs.

Problem with that is that you set up the equivalent of a much higher bitrate (and probably overran the buffers). This has to be taken into account. Try Bitrate Viewer to see what a sequence like yours is doing.
It's best to start with a stock sequence: IIIIIII or IPPIPPIPPI or IBBPBBPBBPBBPBBI and only modify the few cases where you HAVE to (e.g. jumpcuts). [[edit: and/or start with "AutoSceneDetect" and save/reload manually modify from there]]

Try to find "DVD Authoring and Production, by Ralph LaBarge" or read Scenarist or Maestro encoding manuals (if you can get a hold of them).

Scott

[dynamique]

Thanks

[jman98]

It's also worth noting that setting every frame to an I-frame is invalid for DVD (should you care about that) and will increase the size of your files as the B and P frames are what is used to save space in MPEG encoding.

[Cornucopia]

jman98,

You got me wondering...
After checking the manuals available to me, I don't see ANYTHING that specifically says that I-frame only is invalid.

They do mention "low delay", but that doesn't necessarily mean that I-frame only is low delay.
And I tested an I-frame only clip I made with TMPGEnc. Both DVDMaestro and DVDArchitect happily authored DVDs (that played OK), without any error message or anything.

That's not to say it's recommended, as you have a quality (or bitrate) penalty. But this could be dependant upon the kind of material you're working with.

Plus, if you think about it, MPEG stills are valid and they're I-frames.

Scott

[Geckoid]

Low delay mode is separate from I-frame only mode. In low delay mode you don't have to have a frame every expected interval. Usually used for trick states like fast forward or rewind, which often are I-frame only, but may not have any frames between them for simplicity. The player knows to simply play the first frame, then continue showing it (possibly deinterlaced) until the next frame comes along.

I-frames are intra coded. Which means that every macro block needs to be specified. If you kept the bit rate the same, and forced all frames to be I-frames the quality would go down radically as you forced the nice efficient B and P-frames out of the stream, and replaced them with I-frames. The I-frames would be forced, on average, to be the same size as the B and P-frames that they were replacing, and things would get very blocky. Might even turn out to be impossible if the bit rate were low enough, either resulting in an encoder failure, or buffer over or underflows, or skipped frames, or some other error depending on the encoder. The resulting stream might play back okay on a player, or it might not.

[SLK001]

They do mention "low delay", but that doesn't necessarily mean that I-frame only is low delay.

But it is low delay. The "delay" comes from the processing delay needed for the B pictures, which need information from pictures "yet to come" - thus there is significant processing delay for these pictures (they can't be processed until all parts are available). They are even sent temporally out of sequence, I guess to help speed up the processing. If you look at the actual sequence of the pictures in a GOP, you'll see something like this: 1,4,2,3,7,5,6,9,7,8,12,10,11. The actual display sequence would be 1,2,3,4,5,6,7,8,9,10,11,12.

I've always heard (from those who are supposed to know) that in the DVD "bible" (a $5000 book of technical specs available only after signing a "non-disclosure" agreement), a "low delay" stream is not permitted. As for those stills, they technically are not video streams, but are M2V pictures.

And no, I don't have access, nor have I ever seen, this book. And those here who have, can't comment on this (due to the non-disclosure thing). So, I guess we'll never know...