Banding is a "jump" in color or brightness that occurs because not enough information is present about the intervening colors/brightness to present a smooth transition. The considerations for the number of bits needed is twofold: The more bits you have, the less banding you perceive. Therefore, X number of bits per color gives you Y amount of information (a “bit” being a digital 1 or 0). The information about color and brightness is, in digital terms, stored in binary and output per color. Putting It To UseĪll of this extra information in brightness and color needs to be stored for output to your display. Therefore, the range of brightness representable by HDR is double that of SDR, so far as our perception of it is concerned. HDR meanwhile, ideally, covers 13.5 stops (more on that later). For reference, SDR covers 1-100 nits, or around six-and-a-half stops of brightness. digital to analog function.įor comparison, the reference for BT.709 caps out at 100 nits, meaning on our scale, HDR has six-and-a-half more stops of range than SDR. The EOTF is the mathematical function for transferring an electronic signal into the desired optical signal-i.e. This is why the common Electro-Optical-Transfer-Function, or EOTF, that’s used by all HDR standards caps out at 10K nits of brightness. Dolby, the first company to start testing for a standard for HDR, found 10,000 nits to be the sweet spot for its test setup. So how does all this relate to HDR? First, the goal for HDR was simple: to produce on a display an approximation of an extremely bright scene. A "stop", remember, is simply a point on that log2 scale, which is to say humans can simultaneously perceive in a scene something a million times brighter than the darkest part of that scene. Humans can perceive all of this at once in fact, humans can "perceive" up to twenty stops of brightness in a single scene. Īlthough the paper under a bright sun hits 40K nits, an office interior right next to the paper could be a mere 500 nits. Meanwhile, a piece of white paper under a bright sun can be around 40,000 nits, or rather 40,000 times brighter than a candle. ![]() A candela is as bright as a lit wax candle, so 1 candle = 1 nit. One nit is equal to one candela per square meter. You can describe the brightness of this theoretical light and paper using a standardized measurement: nits. For example, when looking at a scene with a light and a piece of paper, the light could be shooting 32 times more photons into your eye than the paper under it, but to you the light would appear five times brighter than the paper. This means that every time brightness (or the amount of photons hitting your eye) doubles, you perceive it as going up only one point on a linear scale (or going up one “stop”). The next thing to understand is how humans perceive brightness, which humans do on a log2 scale. And this is only half of HDR the other half involves bright highlights and dark. As a bonus, it’s also the same color gamut that has been used by movies for quite some time, meaning there's already equipment and content out there that supports and uses it.Īt some point in the future, though, the plan is that BT.2020 should be adopted by everyone, but that will have to wait until common display technologies can actually support it. Although quite a bit smaller than the full BT.2020 gamut, it still contains significantly more colors than the SDR gamut. Today's HDR-equipped displays instead settle for the smaller DCI-P3 gamut, which can be reproduced by OLEDs and LCDs as well as projectors. That means expensive laser projectors are the only display devices currently capable of showing all the colors involved -and only certain laser projectors with lasers at the right wavelengths at that. The ultimate goal here is the triangle dubbed “BT.2020.” The problem is that currently, the only way to produce the red, green, and blue colors required for this gamut is to use lasers (which is cool, we know, but impractical). One of the two concepts behind HDR is to expand that triangle of red, green, and blue from the current small one dubbed "BT.709" to something closer to filling the entire range of colors that humans can see (see the chart above).
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