I think if you watch that 4k video with nearest neighbor downsampling (i.e. the player which drops 75% of pixels and only render 25% of them, one from each 2x2 quad), you’ll get worse quality.
Fortunately, that’s not how video players normally resize the videos they play. At the very least, players are using bilinear sampling. For the exact 50% downsampling of 4k into 1080p, this means the GPU averages 2x2 quad of the source video texture into one output pixel. This averaging step hides substantial amount of codec artifacts. That bilinear sampling is even borderline free on GPUs: they have tons of VRAM bandwidth, and texture samplers are dedicated fixed-function pieces of hardware.
If you’re using Windows and Media Player Classic, the player has a preference to switch resizing algorithm, Options, Playback, Output, “Resizer” combobox.
Fortunately, that’s not how video players normally resize the videos they play. At the very least, players are using bilinear sampling. For the exact 50% downsampling of 4k into 1080p, this means the GPU averages 2x2 quad of the source video texture into one output pixel. This averaging step hides substantial amount of codec artifacts. That bilinear sampling is even borderline free on GPUs: they have tons of VRAM bandwidth, and texture samplers are dedicated fixed-function pieces of hardware.
If you’re using Windows and Media Player Classic, the player has a preference to switch resizing algorithm, Options, Playback, Output, “Resizer” combobox.