Request: allow custom AviSynth DLL path

[stax76]

I would like to suggest adding support to define the AviSynth DLL location via command line.

If this is too much work then support via environment variable would be sufficient.

Or better support for both env var and CLI were CLI has higher priority.

Why is it important?

In staxrip we want to support installed avisynth and also portable avisynth. We want to have the possibility to use portable mode even when avisynth is installed. This is currently not possible because avisynth is installed in system32. Only a soft link workaround is possible but this requires admin rights or developer mode and that is very unfortunate for a portable app.

Currently, staxrip uses about ten avisynth reader tools and ffmpeg is the only one that blocks default DLL loading by the OS, this is extremely unfortunate because portable mode will not work even when avisynth is not installed.

Sample code found in the frameserver.dll of staxrip:

WCHAR* dllPath = _wgetenv(L"AviSynthDLL");
HMODULE dll;

if (FileExists(dllPath))
    dll = LoadLibrary(dllPath);
else
    dll = LoadLibrary(L"AviSynth.dll");

[stax76]

I missed that only CLI without env var would not be sufficient.

env var would be necessary because of tools that use lavf like players and encoders.

[Zafer Kantar]

I support this request. Staxrip provides a very useful interface for FFMPEG among other tools, and having a portable solution is very handy.

[Patman86]

I also support this request. I build many tools for Staxrip (also ffmpeg) and I want to support the project in every way. ffmpeg offer many possibilities in combination with the available libraries.

[Stupido]

I support this request, too. Would be wonderful if this parameter could be added.

[Kyle JK]

I strongly believe this request should be implemented in ffmpeg.
Making AviSynth portable and having ffmpeg accommodate it will give a lot of advantage and freedom to the big group of AviSynth users.