Well I have a 10bit "version" of Avengers infinity War that I obtained through the legal ways obviously as piracy is a crime and should be stamped out...
And it does now switch to HDR when played in both mx player or the Samsung video player.
I've also tested MX Player with a HDR video, it works on both Stock and Mx player as you've said. I will say this though, there are still minor gradient issues with the darker to light colours but that's for nornal non-HDR videos that I've noticed it for. E.g. when watching Unbox Therapy, the background Lewis (if that's his name) uses.
His backgrounds have always had the gradient, I looked back on a few LCD devices and an S6, the gradient was there too I think a lot of YouTube gradients do because the YouTube bitrate is garbage
I am still seeing this issue.
On webpages in all display modes, greyscale 0 through 7 are crushed to black. This isn't a very big deal since 0-7 are very dark in the first place, but it is still a bit disappointing
In video apps (YouTube/Amazon) when using adaptive display mode, greyscale values 0-15 are crushed to black resulting in excessively dark video.
For example, in the above video, squares 0-16 are crushed to black in adaptive mode.
Squares 7-16 light up as soon as the contrast boost stops being applied by using the recent apps button.
I've purchased the S9+ and received it on Friday (09/10/2018) as I've pre-ordered. I've noticed that HDR videos that I have on either the Micro SD card or copied to internal storage DO NOT playback the videos in HDR via the native video player. I have the S8 too and it is able to do so.
Obviously 3rd party apps like Youtube may need to release an update before HDR works there so that's understandable, but for the S9+ not to have this out of the box for the native video player is disappointing.
These clips are short 30 seconds recording on the Xbox One X and are full 4K HDR (Tomb Raider, Horizon 3 or Destinty 2 clips), and they play as they should on the S8 with 'HDR' showing when playing. However, on the S9+ it shows 'HDR' in bold near the title when the video plays, without the actual HDR enabled.
I've contacted Samsung, they've taken my details, serial number and IMEI number. They said they'll get back to me this week.
Just a heads up to everyone. I have the UK version, Exynos.
[Edit: 27/03/2018 - I do have colour grading issues, I was watching a HDR video at 1440p by Digital Foundry on YouTube and I can definitely see a sort of grainy presentation of dark and black areas of the screen. Black Crush is definitely present. Plenty of stuff on reddit and android central]
[Edit: I have chased Samsung and they called me back today (14/03/2018). They have escalated to their Product Support team as their Chat Team was unable to help and their other frontline support. They mentioned that they don't know when that team will get back to me as it's the first time anyone has raised it. I guess many people can't tell the difference and think HDR is working or it's actually working for them out of the box.]