Skipping Frames Backward in VD or other editors

[danswano]

Hello,
I'm trying to drag the seeking bar frame by frame but sometimes it gets stuck when i go backward and do not display any frame while froward it works always.

How do i fix this frustrating issue?

Thanks.

[jagabo]

Use source videos with shorter GOPs.

[danswano]

Can you explain further please?

[jagabo]

Most high compression codecs use temporal compression techniques. Those include a key frame (a complete image compressed much like a JPG file) followed by many subsequent frames which only contain changes from the previous (or future) frames. The GOP size is the distance between key frames. Some codecs use GOPs as long as 300 frames, even more. To decompress frame 299 of a 300 frame GOP requires that the processor go back to the first frame of the GOP, then reconstruct 299 frames. Then to go back one more frame to frame 298 requires the CPU go back to the first frame of the GOP and reconstruct 298 frames. To randomly seek to frame 150 of that 300 frame GOP requires the CPU decompress the first frame of the GOP then reconstruct every frame up to frame 150. Etc. This is why seeking backward is so slow, especially with high definition resuolutions.

There's basically no way around this problem except to avoid long GOP videos in the first place, or to recompress the video to short GOPs or even all key frames before editing. Some editors allow for proxy editing where they first create a small, all keyframe, video that's used while editing, then, when your done, apply all those edits to the source video.

[danswano]

Thank you.

[jagabo]

Note I added a bit to my previous post. VirtualDub doesn't support proxy editing but some commercial editors do.

[pippas]

If you install either the Canopus HQ codec, or the Cineform HD codec (both are available free online) then you can use Virtuadub to convert your existing high definition (HD) video file to an all keyframe format with good quality results, but without the huge file size you would get from converting to uncompressed video.
(Convert using a DV codec, if your video is a standard definition (SD) format)

Files in either of those formats are then - for the reasons jajabo has outlined - much easier to 'scroll' through smoothly in Virtualdub...and of course easier to edit as well!

[danswano]

How to use proxy editing, is there something i must set externally in the system? (i'm trying an trial copy of Video Mastering Works)