HIGH DYNAMIC RANGE
Why displays are set for a revolution, and what it means for you
Forget curved panels, frame synching, 4K, and high refresh rates. There’s a new technology that might just blow them all away for sheer visual pop. It’s all about searing brightness and even deeper blacks. It’s about dramatically increasing the numbers of colors a monitor can display. And it’s coming soon to a PC near you. Get ready for HDR, people.
The basic concept of HDR (high dynamic range) is simple. It means stretching out the extremes of display capability—delivering more, even when that means less. But it’s hard to point at any one feature and say, “This is HDR.” Nor is it easy to define in terms of numbers. There is no one metric that definitively determines what an HDR display is. Instead, there’s a number of standards that are competing to become the de facto definition of HDR.
This is going to cause confusion. Some monitor makers are likely to play a little fast and loose with how screens are marketed. Distinguishing between what you might call a fullHDR feature set and its constituent parts, such as wider color gamuts, is going to be a challenge both for marketeers and consumers. It’s even tricky to define in terms of where it lies in the display chain. Game developers have talked about HDR rendering for years. But no games have output HDR visuals, and there were no displays to support that.
Nevertheless, HDR technology is rapidly becoming the norm in the HDTV market, and it’s coming to the PC. So here’s all you need to know.
What, exactly, constitutes an HDR display? Or should that be an HDR-10 display? Or maybe UHD Premium? Hang on, what about Rec. 2020? And BT.2100, SMPTE 2084, 12bit color, and wide gamuts?
From the get go, HDR display technology presents a problem. It’s difficult to define. Already, armies of competing standards are attempting to occupy HDR’s high ground. Perhaps the best place to start, therefore, is to understand what HDR attempts to achieve. The aim is to simulate reality. Or, more specifically, to converge with the acuity or abilities of the human eye.
That’s because the human eye has limitations beyond which it is futile to aspire. There are things in the real world that humans can’t perceive, whether that’s brightness, or colors, or granular detail. So, there’s little point in trying to replicate them on a display. However, for the most part, what we can perceive still exceeds what displays are capable of. HDR, like several other technologies, aims to close that gap.
A handy example is Apple’s Retina displays. Pack the pixels in a display close enough together, and the photo receptor cells in the retina—more specifically, the fovea, the most densely populated area of the retina—can no longer distinguish them individually. You’ve matched that aspect of the eye’s capability. Adding
more pixels will not improve image quality as perceived by humans. Of course, in this context, much depends on the distance between the eye and the screen. The further away the viewing point, the more densely packed the pixels appear. Apple isn’t even consistent about what a Retina display is, and even the most detailed Retina display probably only has around one third the pixel density required to truly match the capability of the human eye. But the ambition to close the gap on the eye’s capabilities is what matters, and it’s what HDR is trying to do, too, only with different aspects of human vision.
The aspects relevant to HDR are broadly captured by the notions of brightness and color. HDR display technology aims to offer a broader range of both. HDR isn’t about adding ever more pixels. It’s about making each pixel work harder and look punchier. Better pixels, not more.
The problem is that HDR isn’t synonymous with a single metric. Color depth, contrast, and brightness are all in the mix. But no single aspect encompasses everything that makes for a brave new HDR display. Moreover, multiple HDR standards exist. Here’s an excerpt from the definition of one of them, Rec. 2100, that gives a flavor of the complexity involved: “Rec. 2100 defines the high dynamic range (HDR) formats. The HDR formats are Hybrid Log-Gamma (HLG), which was standardized as ARIB STD-B67, and the Perceptual Quantizer (PQ), which was standardized as SMPTE ST 2084. HDR10 uses PQ, a bit-depth of 10 bits, and the Rec. 2020 color space. UHD Phase A defines HLG10 as HLG, a bit-depth of 10-bits, and the Rec. 2020 color space, and defines PQ10 as PQ, a bit-depth of 10 bits, and the Rec. 2020 color space.”
See what we mean? Anyway, let’s dig into the meaning of HDR, starting with color. You may be familiar with the notion of color channels and, more specifically, the number of bits per channel—for instance, 6-bit or 8-bit. To cut a long story short,
For the most part, what we can perceive still exceeds what displays are capable of. HDR, like several other technologies, aims to close that gap.
colors in a display are created by combining three primary channels in the form of subpixels—red, green, and blue, and hence RGB—to give a final target color. The bits per channel refer to the range of intensities available for each primary color channel. By varying the intensities, a range of colors is created, which is a mathematical function of the combined three channels.
BILLION COLOR QUESTION
By way of example, 8-bit-per-channel color, which until recently has represented the high end of consumer display technology, enables 16 million colors. Increase the color depth to 10 bits per channel, and the result is a billion colors. Take it up another notch to 12-bit, and we’re talking 68 billion
colors. That’s a lot. So how does that map to the capabilities of the human eye?
The target here, or at least one target, is something known as Pointer’s Gamut. It’s a set of colors that includes every hue that can be reflected off a real-world surface, and seen by the human eye. How it is calculated probably doesn’t matter. Nor does the fact that there’s a fair bit of variance from one human to the next. What is notable is that it’s only reflected colors—not luminescent colors, which can’t be fully reflected off material surfaces. Hence, even if a display completely captures Pointer’s Gamut, it doesn’t cover everything the eye can see.
However, Pointer’s Gamut is far larger than the standard color spaces or gamuts of PC monitors. By way of example, the full UHD Premium specification (which is one of several HDR specs) includes a color space known as Rec. 2020. It very nearly covers 100 percent of Pointer’s Gamut. The most common color space PC monitors support is sRGB, which only covers a bit more than two thirds of the colors of Pointer’s Gamut.
But supporting sRGB isn’t the same as fully achieving sRGB. In other words, your current sRGB screen probably can’t achieve the full range of sRGB colors, which is a space that’s significantly smaller than Pointer’s Gamut, which in turn doesn’t encompass every color the human eye can perceive. Put simply, your screen may look nice, but odds are that it’s pretty crappy at creating colors by any objective metric.
To grasp that difference in numbers, simply recall those bits-per-channel. UHD Premium requires a minimum of 10 bits per channel, or a billion colors. Unless you have a high-end pro display with 10-bit color, an HDR screen means a massive jump from around 16 million to at least a billion colors.
The other major part of the HDR equation is, effectively, contrast. It’s a bit more complicated than that because true HDR capability goes beyond mere contrast. To understand why, consider a display that can fully switch off any given pixel. In other words, a display capable of rendering true black tones. Strictly speaking, this is virtually, though not absolutely, impossible for an LCD monitor—there is always some leakage of light through the liquid crystals. It’s theoretically possible to have an individual and active backlight for each, but that’s highly impractical. Instead, technologies where pixels create their own light are a far more efficient route to infinite contrast. Which is where OLED displays come in. But we digress, and you can read about OLED displays in the boxout over the page.
The point is that if you have true or nearly true blacks, almost any amount of light constitutes effectively infinite contrast by comparison. So even a really dim screen