The official guide to swresample for confused developers.
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Current (simplified) Architecture:
---------------------------------
                        Input
                          v
       __________________/|\___________
      /                   |            \
     /    input sample format convert   v
    /                     | ___________/
    |                     |/
    |                     v
    |         ___________/|\___________              _____________
    |        /            |            \            |             |
    |   Rematrix          |          resample <---->|   Buffers   |
    |        \___________ | ___________/            |_____________|
    v                    \|/
Special Converter         v
    v         ___________/|\___________              _____________
    |        /            |            \            |             |
    |   Rematrix          |          resample <---->|   Buffers   |
    |        \___________ | ___________/            |_____________|
    |                    \|/
    |                     v
    |                     |\___________
    \                     |            \
     \   output sample format convert   v
      \_________________  | ___________/
                         \|/
                          v
                        Output

Planar/Packed convertion is done when needed during sample format convertion
Every step can be skiped without memcpy when its not needed.
Either Resampling and Rematrixing can be performed first depending on which
way its faster.
The Buffers are needed for resampling due to resamplng being a process that
requires future and past data, it thus also introduces inevitably a delay when
used.
Internally 32bit float and 16bit int is supported currently, other formats can
easily be added
Externally all sample formats in packed and planar configuration are supported
Its also trivial to add special converters for common cases