Extend support for ISO/IEC base media format #330
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MP4 files are often encoded using this format, the first 4 bytes are a box length and should not be part of the file identity magic.
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No objections but would be better to have @IanButterworth double-check it.
| Detect ISO/IEC 14496-12 ISO/IEC base media format files. These files start with | ||
| a 32-bit big-endian length, and then the string 'ftyp' which is followed by | ||
| details of the container and codec. Finding 'ftyp' is enough to know to dispatch | ||
| to VideoIO. |
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Finding 'ftyp' is enough to know to dispatch to VIdeoIO
I'm not an expert on video formats. Are there any references for this that can help me/others to clarify this?
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I'm not an expert either, I used these three references to come to this conclusion:
Wikipedia has a readable introduction: https://en.wikipedia.org/wiki/ISO/IEC_base_media_file_format#File_type_box
There is a publicly available version of the spec here: https://standards.iso.org/ittf/PubliclyAvailableStandards/c068960_ISO_IEC_14496-12_2015.zip
Section 4.3.1 is the relevant part I think.
As a check, I compared to the C++ code here: https://github.com/DigiDNA/ISOBMFF
MP4 files are often encoded using this format, the first 4 bytes are a
box length and should not be part of the file identity magic.