I want to be able to downscale some videos, mainly SD footage which was recorded from a HD channel (upscaled)
Is there any good script that simply downscales the resolution without adding anything else (such as sharpening, edge enhancement, ect)
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IMHO Spline34 or Spline64 - personally prefer spline resizers over other (most other resizer is build on principle windowing Sinc or windowing function itself and ringing is unavoidable - you can simplify that they approach is objective bandwidth/frequency not vision).
I've heard good things about Spline. I was using BiCubicResizer but just entering the resolution i wanted it in and nothing else, but then again i don't know what else these resizers add when processing.
I've also just found out you can downscale/upscale in FFmpeg aswell (which i use to encode the .avs file) Don't suppose anyone knows if you get any artefacts that way aswell?
"Best" depends in part on your source and whether you are upscaling or downscaling. Sharper isn't always better.
When I'm looking for a sharpening resize I usually use Spline36Resize(). It gives sharp results with minimal oversharpening halos, aliasing and moire artifacts. But when I'm downscaling a very sharp source I usually use BilinearResize(). That gives a more natural look without creating or exacerbating oversharpening halos, and the least moire artifacts. When upscaling SD to HD I usually used nnedi3_rpow2(), sometimes preceded or followed with a mild aWarpSharp() and Sharpen(). That's best at retaining sharp edges without creating aliasing artifacts and oversharpening halos.
Avisynth on other way is very reliable on this as everything is explicitly controlled by you - no magic in behind (ffmpeg may call rescale silently and well it is written especially those older parts by people that don't care too much about accuracy of conversion - this is my very personal opinion about ffmpeg - unless you not doing research and you are not videophile purist then ffmpeg should be fine but if you need bitexact perfection then probably Avisynth seem to be correct choice).