High Dynamic Range (HDR) is a term that has been bandied about in the tech world for a long time now. We've seen it as a camera option that increases the depth of blacks and makes pictures look more vibrant, we've seen HDR video capture too, and now we have the latest generation of smartphone displays touting HDR viewing capabilities. This doesn't have anything to do with resolution for a change, such as 1080p or 4K, instead we're talking about color quality, specifically: a color spectrum with high dynamic range.
At its most basic level, dynamic range, when we talk about imaging and video, refers to the range of colors that we can see (or that can be displayed) between our darkest (black) and lightest (white) tones. This isn't the same as saying that a picture is bright or dark. Instead, an image with a good dynamic range can pick out a wider range of tones, such as subtle greys that aren't quite black, or colored highlights that still managed to stand apart from a bright light source. A picture, video, or display with poor dynamic range might look washed out, display notable banding between similar colors, or present darker areas that merge into one giant black blob.
A crude example of the typical differences between SDR (left) and HDR (right). Differences may be subtle, but zoom in to spot superior gradients and colors in the HDR image.
Nits, contrast ratio and color gamut
The term High Dynamic Range comes into play when talk turns to boosting the dynamic range over "traditional" methods, be that in photography or display technology. There are a number of ways that this could be done. For example, on the image capture side this usually involves capturing a number of different exposures of the same image and sticking them together using tone mapping.
Printing or viewing this improved image on a display requires some new solutions too. If our media can display more colors, then our display panel needs to keep up. This is where display luminosity, contrast, and color gamut become a factor. The former determines how bright the panel is at peak brightness (measured in nits), but we're also interested in the range of luminance between peak brightness and black (off).
Traditional TVs offer brightness in the range of 100-500 nits and contrast of around 4000:1. HDR panels bump this up to around 1000 nits or more with a black level less than 0.05 nits. This gives us a very high contrast ratio (brightness luminance/darkest luminance) of 20000:1.
In a nutshell, HDR displays will appear brighter and more colorful when playing back supported media.
HDR panels are able to provide a gamut that exceeds the Rec. 709 color space of SDR panels. Source: AvsForum
All of this combined means that a HDR panel will be able to display a wider range of colors than your typical display. This can be demonstrated with support for new color space formats, which encompass a wider range of colors that come closer than ever to matching the limits of human vision.
HDR panels are designed to move past the old Rec. 709 color space targeted by most standard dynamic range panels, aiming to meet the DCI-P3 standard, the color space currently used by the film industry for cinema, and heading towards the much larger Rec. 2020 space that covers 76 percent of the visible spectrum.
No HDR panels produced yet can show the full Rec. 2020 color space, but panels have come close to meeting around 90 percent of the DCI-P3 standard. This means that they're currently only showing 60 percent or so for the Rec. 2020 wide color space, but this will continue to improve with new panel innovations.
Color depth: 8-bit vs 10-bit
In order to facilitate a common standard for HDR content, there have been a number of groups vying for manufacturer and content support. The HDR10 open standard is the most popular format, which increases the color depth of media formats to 10-bit RGB for HDR content, up from the 8-bits used in standard dynamic range TVs.
Color depth refers to the amount of information that we can store and recreate for each of the red, green, and blue primary colors. Currently, 8-bits for RBG gives us 0 to 255 different options for each RBG color, for a total of 16.78 million different shades of color when combined together. Increasing this to 10-bits provides 1024 options for each primary color, leading to a massive boost up to 1.07 billion total shades. This greatly increases the range of colors a panel can display and reduces the occurrence of banding between similar colors, resulting in smoother gradients.
Not only does HDR boast a wider color gamut, but the move to 10-bit color data means that there is a greater number of steps in between each old 8-bit color value too.
Dolby Vision, a rival to HDR10, proposes a leap to 12-bit color and up to 4000 nits of luminance, but has been judged by many as not worth the smaller improvement to image quality at this stage. Displays can struggle to make the most of the color data and there's an added cost to implementing Dolby's proprietary standard. HDR10 looks to be the winner for now, but Dolby Vision's advantages could become more cost effective in the future. As a reminder, the LG G6 makes use of Dolby Vision HDR and is also compatible with HDR10.
Importantly, the playback device has to support HDR10 or some other high color depth format, such as a media player or your smartphone. Qualcomm has recently introduced 10-bit color support natively inside its Snapdragon 835, but early HDR smartphones, such as the LG G6 and the ill-fated Galaxy Note 7, have already implemented their own solutions.
All of these components have to come together in order to view HDR content at its best. The source media has to be captured with an HDR technique and with a camera with low enough noise to make use of this extra resolution. It then has to be encoded in a 10- or 12-bit format using a video codec that supports the extra resolutions, including H.264 or H.265. This is then played back through a device with HDR video output support, before finally being pushed to a display that has the necessary luminance and color gamut to show the media off in its full glory. Simple, right?
Will HDR make my screen look better?
With all of that being said, we know that playing back SDR content through a HDR display won't necessarily make it look any better. HDR displays are not a magic solution to automatically create better looking images and video, the media content itself has to step up. Therefore, we're going to have to wait until more HDR content is widely distributed before making the most of this new technology on a regular basis.
The HDR tag is no guarantee of quality, as no two displays will be exactly the same. Luminance, contrast, and gamut capabilities are not set in stone, so it's always worth comparing two HDR displays.
It's also worth noting that HDR is not a definitive standard and different implementations and panels can produce different results. We've already discussed two HDR standards and there are several others vying for attention too. Furthermore, no two HDR displays will be exactly the same, as luminance, contrast, and gamut capabilities are not set in stone. Therefore, it's always worth comparing two displays, even if they both boast HDR support, to see which is best.
That being said, HDR displays should still perform better than non-labelled displays, ensuring that even standard dynamic range content is viewed looking at its best.
The Galaxy Note 7 was the first smartphone to feature an High Dynamic Range capable display.
LCD vs AMOLED
If display technology wasn't already complicated enough, the rollout of high dynamic range displays throws another layer of complication on the long running LCD vs OLED debate. Importantly, HDR doesn't require any radically new display technology inside the actual panel. Cutting edge LCD and OLED panels can meet the luminance and color criteria to make the most of this content.
While both technologies are capable of producing displays which meet the criteria, the way that these displays accomplish this remains different and each has their own pros and cons. We won't rehash the differences today, but OLED's high contrast ratio and typically wide color gamut lends itself nicely to this type of content. Meanwhile LCD's bright backlighting and new color developments, such as quantum dot technology, make it a competitive choice in small form factors as well.
Wrap Up
High Dynamic Range displays are a welcome development in the mobile space that will offer superior looking media while on the go. However, we should understand that HDR in a mobile form factor is not going to come without some compromises. Mobile display panels typically don't reach the 1000 nits of luminance required to match top of the line TVs and boosting display brightness is going to sap our limited batteries quicker than ever.
There are also trade-offs with larger media files and limited streaming bandwidths that may put off many consumers from making the most of their cutting edge mobile displays. Still, our photos and videos will look better than ever and this year's smartphones will be ready for the arrival of more HDR content in the coming years.
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