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50,000 Shades of Gray - An Exploration of HDR Pleasure and Pain!

50,000 Shades of Gray - An Exploration of HDR Pleasure and Pain!

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50,000 Shades of Gray - An Exploration of HDR Pleasure and Pain!
Posted: Thursday, February 26, 2015

OK, so technically High Dynamic Range Ultra High Definition TV (HDR UHDTV) might not quite reach 50,000 shades of gray (at least not just yet), but you've got to admit that you might not have read this far if I had put some math in the title! I am very excited about HDR. "Why" you ask? Read on...

The thing about having more pixels on the screen for UHDTV (think 4 or 8K) is that it only enhances the viewing experience if you can see them! In my personal case, I am short sighted and without my contact lenses or glasses the TV is just one HUGE blurred pixel making light on the wall. Even with my contact lenses, I cannot see the detail when I move away from the screen. High Dynamic Range, however, is not just about brighter screens; it's about seeing detail in the bright clouds while, at the same time, seeing detail in the deep blacks. It's about seeing rich, vibrant colors as well as subtle blues in the skies, instead of burned out whites. In fact, where today HD has approximately 250 different levels of brightness (“shades of gray”), UHD will have 1000 or even 2000 – or quite possibly, more.

If high dynamic range is so good, then why haven't we seen it before? Well, we're in an interesting place in history today. Semiconductor physics has reached the point where we are able to build sensors with a response similar to that of the human eye. LCD and light panel physics has reached the stage where we can build a 10mm-thick panel of more than 2m diagonal that can produce a luminance and color range similar to the response of the human eye. Wait. I know that over half the people reading this are saying, "Bruce, what about the eye's night vision accommodation or high-end accommodation?” Well, I'm sure that it is possible to build a TV that permanently burns the image into the eyeballs, or one that is so dim that we see it in only black and white, but I'm not sure that those 50 shades of eyeball pain would be a commercial success.

So we have conquered the capture and display barriers that physics has thrown up in front of us. Surely the "middle bit" must be easy to solve. In fact, everything you can think of can be solved with enough money and software, but we want to solve this particular high dynamic range problem at a cost that allows broadcasting, movies and OTT/IPTV to exist and grow compared to the market place today. This is where the techno-electro-commercial politics of the media industry are coming to bear.

We need to have a representation of color that is wider than today's BT.709 color space to match the eye's response. This is (sort of) decided with the ITU BT.2020 specification. We then need to ensure that the encoders and decoders that exist are modified to allow this broader color space with enough dynamic range to resolve the details in the whites at the same time as resolving the detail in the blacks. Although this sounds simple, we also have to ensure that the amazing efficiencies of HEVC and AVC are not broken by this act.

Finally we have to have some agreement between vendors on what the numerical values of pixels actually mean. This isn't just a case of matching the black value and the white value, but we have to ensure that any two devices that capture photons produce the same numeric output for a given number of RGB photons at the input. This is known as the OEFT - the Opto-Electrical Transfer Function. At the other end of the chain we have to match the numerical values to the number of photons emitted by a display. This is known as the EOTF - Electro-Optical Transfer Function. When you get down into the details you discover that there are many ways to do this, and there are currently several competing proposals in the standards committees vying for the position of "default transfer function." 

More critically for many of the devices processing video in the chain today, HDR requires more bit depth and more accurate filtering than standard dynamic range video. Anyone trying to cut corners will introduce artifacts into their images that will destroy the whole idea behind UHDTV – to give content a "WOW" factor so viewers want to own the UHDTV HDR experience.

If you missed our webinar on the basics of UHDTV that covers HDR, not to worry. I will be covering it in one of my Bruce's Shorts very soon. Be sure to sign up and you'll get an email filling you in on our latest hijinks. Oh – and there is a good chance that I'll repeat the UHD webinar if we get enough requests. There seems to be a lot of interest out there!

Posted by Bruce Devlin
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