It can be useful to allow the user to read and modify the MPD manifest before downloading media segments from it. Use cases:
print additional diagnostics concerning the manifest
delete some Periods that the user is not interested in (based for example on their duration)
delete Representations that use undesired codecs
modify the BaseURL to include another CDN
delete all subtitle languages and formats except for the one the user is interested in (as a complement to the prefer_language functionality)
delete audio Representations whose language the user is not interested in
drop an audio AdaptationSet if the user only is interested in video (though this functionality is already built in to the library)
As of 2023-10-22 we have implemented support for XSLT stylesheets via xsltproc, which allows XML rewriting. This is a standards-based and easy to implement solution to filtering/rewriting, but xsltproc only supports XSLT v1.0 (v3.0 is more flexible and for example includes functions for manipulating xs:duration attributes) and XSLT is not the most widely adopted language.
Possible alternatives:
Saxon-HE, free Java software (MPL v2) which implements XSLT 3.0
A generic filter interface implemented as a pipe
A command API that takes two filename arguments
A WebAssembly-based interface that could be implemented in any programming language that can generate WASM bytecode
I will be thinking about implementing some of these in the next few months.
It can be useful to allow the user to read and modify the MPD manifest before downloading media segments from it. Use cases:
As of 2023-10-22 we have implemented support for XSLT stylesheets via xsltproc, which allows XML rewriting. This is a standards-based and easy to implement solution to filtering/rewriting, but xsltproc only supports XSLT v1.0 (v3.0 is more flexible and for example includes functions for manipulating xs:duration attributes) and XSLT is not the most widely adopted language.
Possible alternatives:
I will be thinking about implementing some of these in the next few months.