Interesting. Personally, I’ve been using VMAF extensively in my imager.io project. With regards to only testing the luma channel, someone submitted a PR for such some time ago yet it’s never been merged; I was under the impression that the UV channel isn’t significant (in practice). Personally, I think VMAF is great for images. For me, the biggest downside of VMAF (implementation wise) is that it’s not currently thread-safe.