NEC PA311D - Hardware HDR mapping curves with BT.2390 tone mapping

As I've described in my previous review, NEC PA311D is an excellent monitor, with great out of the box performance.

One thing I found over time can be improved with proper tuning is the HDR handling.

By default, monitor accurately maps native color representation to HDR luminance according to appropriate input signal curve (normally this is PQ for Windows/HDR10 connection). Then luminance values are directly displayed according to current brightness setting. E.g. with panel luminance set to 320 nit, everything below 320 displayed 1:1, everything above is clipped to 320. Its also possible to "decouple" clipping value in PQ settings from actual luminance, e.g. you can set clipping to 1000 and have luminance at 300 - means that it will scale 0:1000 range to 0:300 range, making everything dimmer, but displaying wider range in accurate fashion.

Still above model is quite limited and often requires specific settings for specific content brightness ranges. Lot of "entertainment" content (video games, HDR movies) with incorrect settings will be either too dim or have "washout areas" where too bright spots are clipped to solid color.

Obviously, no consumer display can display whole 0-10000 nit range which HDR10/PQ signal can have. Lot of HDR monitors & TVs use tone mapping to "scale" that to monitor range, preserving "perceptual brightness" and trying to keep detail both in shadows and highlights.

PA311D has no tone mapping out of the box. Luckily, its reprogrammable 3D LUT can be set up to do any color transform we want - entirely in hardware too.

All that needed is a custom gamma transform. Such custom gamma can be easily uploaded from text file using "MultiProfiler" tool. Text file contains set of value which describe gamma curve. In our case gamma curve will have two transforms on top of each other:

• standard PQ mapping to transform HDR10 color values to NIT
• BT.2390 transform to shrink 0-10000 NIT values to the panel displayable range (0-100%)

I've shared the calc in this sheet - PQ - BT.2390 mapping - Google Sheets. You can use it as a template and customize some parameters:

• MaxEL (I2) - specifies maximum brightness monitor will display (NITs). I've set it to 400 (which is actually higher than actual panel max, so will make image somewhat darker, but will preserve more low-range detail more accurately).
• Lw (L2) - Brightness above which HDR colors will be clipped (below, tone mapped). Technically, this is 10000 as max possible NIT for HDR10 signal. However, most of HDR content does not go as high (or have very very small amount of pixels above, like super bright glints). Hence, its somewhat wasteful trying to tone map to 10000. I've set it to 3000, which in my trials gives best experience for HDR games and movies out there with very good brightness and fidelity.
• Note that actual "gamma curve values" for PA311D/MultiProfiler will be output in H5:H260. To load it in the monitor, you will need to export these numbers as plain text file.

I am also sharing some pre-built gamma curves:

• Tone map to 10000 NIT max (full PQ/HDR10 range)
  
• Tone map to 1500 NIT max, clip above
  
• (best one IMO) Tone map to 3000 NIT max, clip above
  

Rough steps to set up the monitor:

• Switch on the HDR in Windows HD Color Settings
• Monitor should switch to HDR profile (signal format in monitor menu bottom should have "HDR-PQ" in it)
• Launch NEC MultiProfiler and upload custom gamma
• I also recommend disabling adaptive brightness, uniformity compensation and setting luminance to maximum, to maximize panel native range
• Afterwards monitor will remember these settings, and you can switch back off Windows HDR. This will switch preset back to sRGB/SDR. Monitor will automatically return to custom gamma tone map preset when HDR switched back on

Note that afterwards you should just always send the native HDR signal to monitor. Tone-mapping by applications is not necessary anymore (since monitor already does tone mapping for us).

E.g. in madVR use "HDR passthrough", and do not enable madVR's own tone mapping. In video games which have "maximum HDR display brightness" parameter/slider in settings, you should use the maximum allowed (10000).

And here there are some comparison photos of display playing youtube HDR videos:

"Costa Rica HDR", default PQ

"Costa Rica HDR", tone mapped

"Iceland HDR", default PQ

"Iceland HDR", tone mapped

As you can see, without tone mapping its impossible to discern any details in cloud/bright sky, it all becomes uniformly clipped blob. While tone-mapped display preserves lot more detail and actually looking somewhat higher contrast overall.