Say that I had a 1-billion-byte (1 GB) MP4 file called original.mp4. Bytes from offset 0 to 500,000,000 are gone. I only have a 500MB file called f.mp4 which has bytes from offset 500,000,001 to 999,999,999. How would I play f.mp4?
I have messed with similar situations in GNU/Linux (in a CLI). MP4Box would say "invalid top-level box type" due to the resulting file having no header. Like the same thing happened with ffmpeg: "moov atom not found".
An MP4 is made of boxes, AKA atoms, so that plus headers factors into the solution. What if I had a 50 gigabyte .mp4 but lost the first 50 megabytes with only 49.9 gigabytes surviving? Then is the remaining data all just useless? With WebM data, this is less of a problem or isn't a problem (you can remove the first n bytes and the .webm will still be a playable video).
This question is different from this other question -- How to repair a corrupt MP4? -- because it's a specific situation. Additionally, this question does necessarily mean anything is corrupted. One could have simply split a file or took out megabytes starting at a certain non-zero byte offset (using dd or something).
