The vmix (VMX1) decoder does not decode interlaced files

[ianshade]

Summary of the bug:
When trying to use FFmpeg to transcode a 50i AVI file recorded with vMix's own codec, also known as VMX1, the following error occurs:

Error submitting packet to decoder: Invalid data found when processing input

I have tried numerous 1080i50 vMix AVI files, and the results were always the same.

Those files are decoded fine by vMix itself. They can also be decoded in other software using a DirectShow filter graph. (Both on a machine where vMix and its codec are installed)

A similar file, but recorded in 1080p25 is decoded correctly by FFmpeg - that's why I'm assuming it has something to do with interlacing.

How to reproduce:

.\ffmpeg.exe -i .\capture_50i.avi output.mov

Version:

ffmpeg version 2025-07-21-git-8cdb47e47a-full_build-www.gyan.dev

Files attached (via google drive):

1. capture_50i.avi - sample 50i file that fails to decode in FFmpeg, but decodes fine in vMix
2. still.png - still frame from the file in question, for reference
3. capture_25p.avi - sample 25p file that decodes correctly in FFmpeg, for reference
4. ffmpeg-20250724-121042.log - a log file produced with -report

​https://drive.google.com/drive/folders/1CLbc7th81_v7nd1wzGsjSlrLedJA1yVg