TMPGEnc Problem - navigating one frame at a time

[ereiamjh]

I'm having a problem that I can't get my head around

When I'm in cut-edit mode, I want to navigate through the video file one frame at a time. If however, I go back one frame, to one I have previously just viewed.. it is not what it should be.

For example.. lets say frames 1-150 are of a dog, at which point it changes scene and frames 151-300 are of a cat. I can be moving along one frame at a time, frame #150 will display a dog, and stepping to #151 will show a cat (as expected). However, if I then step back to frame #150, I'll still see the cat! I'll need to go back around another five frames (#145) until the dog reappears.

I used to use TMPGEnc a lot, but haven't used it for a while. I'm certain I never used to experience this!

Can anyone shed any light?

[filmboss80]

If your video file is MPEG-based (compressed, with long GOP), it is likely landing on the nearest I-frame. Unless you can also advance to specific P or B frames, your edit points are always going to land on the I-frame, at the start of each GOP (group of pictures).

[ereiamjh]

Thanks for the quick response.
Unfortunately, what you've said has gone a bit over my head.

It seems to happen regardless of what video files I am using. And I can load the same files in VirtualDub and can easily step back and forth from one frame to another with that program, without the same problem occuring.

[Alex_ander]

That depends on decomressor the program uses to import video. To navigate in both directions, that decompressor must keep at least all the current GOP in some memory, since to decode a random frame, it is necessary to have I-frames (independently encoded) and other reference frames (B- and P-type) of that GOP. Some decoders have such memory, some don't. VDub always uses VFW decoders from system, TMPGEnc might use DirectShow etc. The longer are GOPs in your video (up to 300 for compessed avi's), the more annoying is searching backwards in some players (like WinDVD) and other programs.
In case you make cuts for further encoding, you'll have to first find out the wanted frame numbers, then set edit edit points by navigating only in playback direction. In case you want to just cut/join without re-encoding, the cuts are only possible at GOP borders. Programs like TMPGEnc MPEG Editor allow for cutting within GOP, but that GOP will be re-encoded.

[filmboss80]

Sorry to be so technical earlier. If your video is something like DV-AVI, then you have (in the UK) 25 separate video frames per second to access. However, because so much information needs to be devoted to each frame, the file size is huge. In order to squeeze 2-hour movies onto DVD discs, brilliant engineers came out with a scheme to compress videos footage into clusters called "group of pictures" (GOP). Instead of dedicating a whole new set of pixels to each and every video frame, pixels were only allocated to the portions of a video frame that changed. (For example, a blue sky in a 15-frame scene might use the same blue pixels throughout the entire 15-frames.) Newer codecs, such as MPEG-4, compress things even more. The great thing is that you can get nice looking video into smaller-sized files. The not-so-great thing is that the compression makes frame-accurate editing difficult with most free editing software. Edit points on compressed files are usually going to land on the I-frame, which is the full video frame at the start of each GOP.

VirtualDub will allow you to view individual P and B frames, but my version won't allow you to cut MPEG-based video on anything other than the I-frame. As Alex_ander has stated, there are MPEG Editors that will allow cutting of frames inside a GOP, but must re-encode, creating a new I-frame at the edit point, followed by P and B frames. (I think the Womble MPEG Video Wizard will do this. I know Vegas will. Apparently, TMPGEnc MPEG Editor does too. So, you have a few choices.)