DGIndex: video corruption on single step preview/play

[FlimsyFeet]

I've noticed what could be a bug in DGIndex when using Options -> Playback Speed -> Single Step (for example, to locate a specific frame for which you want to save a BMP).

If you find the desired start position and press [, then the end postion and press ], then go to File -> Preview, the first couple of frames end up with video corruption in the form of large blocks over the image.

Using v1.5.8, and it's not related to the file, it seems to happen on every VOB/MPG/M2V file I try.

The AviSynth script is OK, but like I said I just wanted to use DGIndex as a quick method of getting a screenshot of an exact frame.

[Hagbard23]

Read a bit about MPEG Compression. I Think what you have happens due to GOP's. If you want to get a frame inside a GOP and want it to be displayed alone, this will not work. Most likely you have chosen a frame which is dependent to its predecessors (B-Frame), so spoken lightly: "A B-Frame is no Frame at itself - it is some kind of intermediate process between the previous Keyframe and the following one. In Fact that B-Frame is dependent on the other "Frames" of this particular GOP.

As said before, read about it, because it is difficult for me to put it into words.

It is NO Bug of DGIndex, i think.

[Guest]

Yes, it is due to open GOPs in your stream and is not a bug. From the manual:

"The Play command does not decode frames prior to the current position to obtain reference frames, so you may see macroblocking on the first few frames of a Play operation if you jump around on the timeline."

The reason I do this rather than suppress them is that it is useful for *seeing* in DGIndex whether your GOPs are open.

[FlimsyFeet]

Oh I see, thanks. I thought that when scanning with the slider, or using the < and > keys, that DGIndex snaps to I frames (key frames). But if I understand correctly, it actually snaps to the first frame in a GOP, which for open GOPs may not neceassrily be an I frame.

[Guest]

When stepping by GOP or jumping around on the timeline (not in play/preview), it steps to the first I frame of the GOP. When doing play, preview, or stepping by frames, it doesn't.

[laserfan]

I think this has come-up before as "no", but sorry I do not recall exactly: does not DGIndex know what kind of frame it's on, and if yes, could an I-i-P-B be shown accordingly?

Neither of course do the frame types appear in Vdbub but I'm not sure why not. :o

[Guest]

It's shown in the Info dialog: Coding Type.

[laserfan]

It's shown in the Info dialog: Coding Type.

Ouch. Thank you. Yep there it is. How'd I miss that...argh. :o