[DragonFly]

Could you help me out, please

I'm running SPMC 17.6a2. Exactly what settings are needed to avoid picture upscaling by the Shield? For example, do the Shield and SPMC both need to be at 1920x1080 or just the Shield set at 3840x2160 and SMPC at 1920x1080? Are there any further specific SPMC settings needed?

There are numerous posts scattered through various fora and sites with contradictory advices. A good summary would be much appreciated.  

Read THIS. In theory, you apply those settings and you don't need to do anything else. Shield TV since SE 6.1 (if I'm not wrong), sends native output resolution to your TV.

OTOH, Have you tested MX Player or Archos Player?   

This is the recommended setting for a 4K TV and a 4K SMPC GUI (which has a sloooow RSS ticker in that high resolution), where the upscaling is actually done by SMPC using Lanczos3 - optimised. I'd like to avoid upscaling done by Kodi/SPMC and/or the Shield altogether. My TV, which can probably do a far superior job, needs to do the legwork.

I did use this SPMC wiki recommendation for a 1080P GUI, which should avoid upscaling, but I don't know if these settings are enough.

Is there any way to see (in a debug.log) if upscaling by Kodi is bypassed/not executed and is there any way to see if the Shield is not doing the same?

Did compare the TV's native media player, running on Android TV, with Kodi using a HEVC sample. The native player produced a MUCH nicer result (especially in dark areas and greyscale artifacts)