As the title says.

At the moment, it is possible to antialias footage patches by creating a larger composition and scaling it down into a smaller one using bilinear or bicubic interpolation, but this is a slow method of doing this. For 2x SSA antialiasing, this requires 4x the perspective calculations, as well as the scaling method and extra composition as well.

Using Lanczos when sampling pixels from the "Source Layer" footage would remove hard edges and null the need for this kind of workaround in compositions where fine fidelity matters.

Log from file "Plugin Loading.log":

Loading C:\Program Files\Adobe\Adobe After Effects 2020\Support Files\Plug-ins\equirectangular_to_equirectangular.aex
Loading from disk...
PF_Cmd_GLOBAL_SETUP returned error -1610153865.
No loaders recognized this plugin, so the plugin is set to Ignore.

-DAFTEREFFECTS_SDK="<path to March 2021 version>/AfterEffectsSDK"
-DAFTEREFFECTS_LOCATION="C:/Program Files/Adobe/Adobe After Effects 2021"

Scanning dependencies of target perspective_to_perspective_resources
Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29915 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

PiPL.r
Microsoft (R) C/C++ Optimizing Compiler Version 19.28.29915 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

PiPL.rrc
CMakeFiles\perspective_to_perspective.dir\flags.make(6) : fatal error U1033: syntax error : '-Dperspective_to_perspective_EXPORTS' unexpected
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.28.29910\bin\HostX64\x64\nmake.exe"' : return code '0x2'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.28.29910\bin\HostX64\x64\nmake.exe"' : return code '0x2'
Stop.
[  0%] Built target perspective_to_perspective_resources

Which I believe is this line:

COMMAND cl.exe /I "${AFTEREFFECTS_SDK}/Examples/Headers" /EP "${CMAKE_SOURCE_DIR}/perspective_to_equirectangular/PiPL.r" > "${CMAKE_SOURCE_DIR}/perspective_to_equirectangular/PiPL.rr"

https://github.com/Gorialis/aftereffects_spatial_media_plugins/blob/2495315/CMakeLists.txt#L69

EDIT: In the meantime I cheated with https://github.com/pavelliavonau/cmakeconverter to create a CMakeLists.txt from the MSVC solution. Then I edited it to remove all the hardcoded paths and duplicate branches. Surprisingly it works.

At the moment, the edge of equirectangular patches sometimes get transparent "hems" on "Preserve layer" mode due to the Lanczos blending at the edge of the frame.

Specifically, due to the old method not blending at all, the "Preserve layer" mode only shows the original layer when the alpha channel is 0 (completely transparent), and won't blend on top of the original layer at any other value. This has gone unnoticed because when the alpha channel is 255 (fully opaque), no blending is required.

The correct solution to this would be to always blend with the source layer when "Preserve layer" is on, as this would also fix blending issues in cases where the source content is itself translucent.