How does VirtualDub2 open MKVs/MP4s instantly without having to create an index?

[wonkey_monkey]

Title, really... in AviSynth you have to wait for an index to be created the first time you open a file with ffmpegsource, or you have to create an index with DGIndex first to use DGDecNV, etc, but VirtualDub2 opens the files instantly.

Is seeking not accurate in that case?

[Emulgator]

avlib-1.vdplugin seems to rely on ffmpeg which seems to rely on container info being properly written.
After a few early attempts I have only ever used Avisynth source scripts which relied on "proper" indexing source filters on VD like LWLibav,
avoiding any shortcut like LSmash which rely on trusting the container...1 frame off and work is invalid.
Direct throwing onto VD I have used only for the occasional quick preview, not for processing.
But I can not say I compared and can tell failure for sure...

[shekh]

Yes there is enough indexing info in certain files which can be used for instant access. Unfortunately the support for such access in FFMPEG was missing/terrible (probably still is). Also you can meet all kinds of malformed files with wrong or missing metadata. In the end depending on the file it may work perfectly or not work at all or seem to work until you notice.