Reduce grain, shimmer, and texture that changes from frame to frame to clean up the picture. Too much makes skin and grass look smeared like plastic.
Dark scenes, clips with visible compression artifacts, and before upscaling.
In AI clips the texture changes slightly from frame to frame and can even flicker. Start with a light temporal filter, hqdn3d=1.5:1.5:6:6 or atadenoise, and lower the strength if faces or the areas around text get smeared. If you will add film grain, go in the order denoise → grain (see Film Grain / Halation).
Calm flickering grain and texture that changes frame to frame with a gentle temporal denoise: ffmpeg -i in.mp4 -vf hqdn3d=1.5:1.5:6:6 -c:a copy out.mp4. Lower the values if skin or text gets waxy, and apply denoise before adding film grain.
Not verified