Synology + HDR Content

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However, I felt that the movie did not appear to actually be playing in HDR. Being on an iPad I have no real way to verify other than my eyes, but that got me looking into how HDR works on Jellyfin. (Also, not sure if Dunkirk is just a bad HDR movie).

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This is where my first question comes into play, if I uploaded HDR content to Jellyfin and play it on an HDR compatible device like an iPad does it do any transcoding? While watching on the iPad is nice, I plan to do most of my movie viewing on a Windows computer. This computer has an RTX 4080 and an OLED ultrawide from Dell so HDR should be fully supported. (I know it is when playing games).

But in either of these cases would the Jellyfin server actually be transcoding anything for the end devices or are the end devices handling the HDR? (I read something about offloading the HDR transcoding to the end device in the documentation but didn't fully understand it) 

Or is some form of HDR support on the device the Jellyfin server is running on necessary for HDR content to be viewed at all?

To add to what TDP said, most clients will have an option to view playback information. This should tell you if the media is a) being transcoded, b) the bitrate, and c) additional information such as codec, bit-depth, dynamic range, etc... Depending on your client (the app you're using for playback), how to access this will differ. If you care to share, somebody might be able to help you find that information.

Additionally, I just want to point out that only your server will ever transcode. You cannot offload transcoding to your client. If you're watching on a beastly PC and your server is an rPi, it won't make a difference. Your client requests the media when you click a thumbnail and tells the server, "Hey, this is the type of client that I am." The server looks up the corresponding client device profile, decides whether transcoding and/or tonemapping is required, executes neither one, or both based on the profile, and streams the result to the client. All transcoding and tonemapping happens server-side. If direct play is chosen, interpretation of HDR metadata (which is separate from the media, technically) is done by the client. But that's not transcoding.

The server itself does NOT need to support HDR at all in order to direct stream anything. If you are transcoding, whatever you are using to transcode (CPU-only or hardware acceleration device) must support the codec(s) being transcoded. Most of the time HDR doesn't come in to play, with a few exceptions generally only reserved for DolbyVision, which you won't see if you're limiting yourself to UHD BR rips.