ffmpeg nearest-neighbor downscale not working as expected

Question

I'm trying to downscale a video with nearest-neighbour scaling, but the results are very poor.

To try and get to the bottom of it, I created this image which is a repeating pattern of various colours. The expected result is that the output image, when downscaled from 1080p to 360p, is solid green (#00ff00). Or possibly red, if it samples from the top-left instead of the centre.

Instead, the resulting image is green with vertical bars of #25e525 and #44c383, neither of which are present anywhere in the original image.

I've applied various other things to my command, as seen below, but it makes no difference.

ffmpeg -i 1080.png -vf scale=-1:360:flags=neighbor+accurate_rnd:sws_dither=none -pix_fmt rgb24 360.png

Upscaling works fine - it's just downscaling that goes wrong. Any ideas?

Answer 1

After some more back-and-forth with Bing, it seems zscale works far better than scale.

ffmpeg -i 1080.png -vf "zscale=-1:360:filter=point" 360.png

With video, add format=pix_fmts=rgb24, to the start of the vf chain.

Answer 2

It also seems that full_chroma_inp works with scale. Another option in case zscale isn't available.

ffmpeg -i 1080.png -vf scale=-1:360:flags=neighbor+full_chroma_inp 360.png