HDR on Projectors can be Tricky

You may have noticed that a lot of home theater and some ProAV projectors are now advertising HDR support.  That should mean that they are able to detect and HDR signal and switch into a mode that will properly display the images.  That may sound easy, but the devil is in the details.

First, there are multiple flavors of HDR: HLG, HDR10, HDR10+ and Dolby Vision.  HDR content must contain some sort of flag to identify which type of HDR content it is.  Such flags can be embedded in the HEVC stream, carried over HDMI as metadata, or via an OTT connection.  That means content arriving over HDMI from a Blu-ray player, playback device, set top box, or game console, or via wi-fi, a direct broadband LAN connection, or a USB port.  If the signal is also routed through a AV receiver or other components, will the HDR flags and metadata be lost?  Do all the interfaces support HDR? Unfortunately, it takes special gear to analyze the signal to even ensure that the projector is getting HDR content.

So here is a good idea for service providers and device makers: display details of the content being received with basic info like frame rate, resolution, color emcoding, bit rate and SDR/HDR info.  That way, I know the quality of the incoming signal and can better judge how the display device is handling this signal.  It is one thing to test and review displays using pristine test patterns, but real world signals are often not that great, so how does the display device perform with degraded signals is perhaps the more relevant question to answer.  But I digress.

Ok, so let’s assume an HDR signal gets to the projector.  It must properly identify the compression type, uncompress it, identify the right HDR type and configure the proper processing chain.  If HDR metadata  is absent, what does the projector do or assume?  Is HLG content displayed using the HLG  EOTF and the other flavors using the PQ EOTF?  There is no way to confirm this.

Projectors are like TVs that can only offer global dimming – there are no zones to modulate to create an improved HDR image – just the full image can be dimmed or brightened.  That makes the tone mapping algorithms simpler, but you are not going the get the wide intra-scene dynamic range that you can with higher performing flat panel displays.  And, unless you have a really dark room or a really good light rejecting screen, ambient light will compromise the contrast even further.

But perhaps the biggest challenge is determining how to tone map the image.  Tone mapping is needed when the peak luminance of the display is less than the content mastering device.  Most HDR content today is mastered on monitors with 1000 or 4000 nits of peak luminance.  If the display device has a peak luminance of say 500 nits, then you need to devote perhaps the top 100-200 lumens to display all the code values of the content from maybe 300 nits to 1000 or 4000 nits.  Simply rolling off the luminance is one way, but there are more sophisticated ways that can better preserve the colors and highlight details.

So how does that translate into a projector system that specifies peak lumens?  You have to take that peak lumen number and spread it out over the area of the screen to know the Cd/m2 of nit value of the projected image.  Make the screen nice and small and you can get some pretty bright images.

The issue here is that most projectors are not sold with a known screen solution.  So if you are the projector maker, how do you know what Cd/m2 will be on the screen in the application?  You don’t.  Therefore, I presume projector manufacturers make an assumption so you can adjust your tone mapping curve accordingly.  Did your projector make guess well?  There is no way to really know.

So here is my second industry suggestion.  Add a small sensor to the projector to detect the size of the screen or provide a data input capability that can add this information for helping to optimize the tone mapping of the projector.

As you can see, it is quite challenging to get projected images that are going to be able to match the HDR performance of higher-end flat panels.  Even with very bright projectors and ambient light rejecting screens and properly calibrated tone mapping curves, they are still only globally dimmed.  Nevertheless, I have seen some very fine HDR images on projectors, but they take a lot of engineering to perform well.  Good luck to the average installer or home theater enthusiast.

This article first appeared on www.displaydaily.com

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