4K Ultra HD is about more than just increased resolution, it also means a wider colour space and increased dynamic range.
In simple terms, HDR is a way of displaying the wider dynamic range inherent in the original capture of content. A film camera or a modern digital camera is capable of capturing a higher level of brightness (luminance) and perceivably deeper blacks than a current video display is capable of showing. However if a transfer can be created that retains all this luminance detail and a display is capable of showing it, then the resulting image will be a better representation of what the human eye is capable of resolving.

It’s important to realise that HDR isn’t simply about increasing the brightness of an image, although that is a common misconception. The idea is to increase the overall dynamic range between black and white by making the dark parts darker and the bright parts brighter, whilst retaining detail when both are in the frame. So for example if the image was of the inside of a room you could see details in the dark shadows but also make out details through the brightly-lit window.

However it isn’t just about the difference between the darkest and brightest elements in an image, HDR also calls for greater expression and detail within colours too. So HDR will use a wider colour space than the current standard of Rec.709. Exactly which colour space will be used hasn’t actually been announced yet but probably the DCI standard that is used in the cinema, although it could theoretically go as wide as Rec.2020.



If you look at the example above, where the bell shape represents the total visible spectrum of the human eye, the smallest triangle within it represents the current limitations of the Rec.709 standard. The larger triangle represents the new Rec.2020 standard for colour, which is the maximum colour space proposed for the new 4K Ultra HD standards, and it's obviously much wider in scope than what we have been seeing up until now.

The current 8-bit video standard allows for a maximum of 256 shades of any one primary colour to be displayed but under the new 4K Ultra HD standards, displays will need to be capable of handling colours at a depth of 10-bits, allowing for 1,024 shades of every primary colour. The combination of a wider colour space and greater bit depth will result in a more natural looking and detailed images than can be obtained at present.

Although HDR and wider colour spaces are technically separate aspects of image reproduction, the two are becoming linked by standards bodies into what is often referred to as colour volume. This is essentially a three-dimension version of the graph above with colour and luminance combined. The bigger the luminance range and the wider the colour space, the larger the overall colour volume and thus the greater the impact on the perceived image.
The combination of increased dynamic range and a wider colour space results in a larger colour volume and a better perceived image.
How does HDR work?
Until recently the standard for creating video transfers has largely remained unchanged for decades and it is based upon the capabilities off the old CRT (cathode ray tube) monitors that were used in the mastering studios. As a result of the historical limitations of the display technology, all high definition transfers are created using an 8-bit video depth, the Rec.709 colour space and at a peak brightness of 100 Nits (which is a measure of luminance).

This has been frustrating for colourists working in the film industry because the actual colour and luminance detail in the original footage captured (be it on film or digitally) is far greater than the current standards are able to convey. In addition display technology has moved on and is now able to handle standards far in excess of what is currently being used. Therefore as part of the move towards 4K Ultra HD, the standards are being changed to reflect the capabilities of modern displays.

What this essentially means is that new 4K Ultra HD transfers are now being created using a 10-bit video depth and a wider colour space. Although there hasn’t been confirmation of the exact colour space to be used, since films use the DCI colour space for theatrical presentation at the cinema, that would seem the obvious choice. The final aspect of this new standard is the addition of HDR, which means using a much higher peak luminance than 100 Nits when creating new transfers.

By combining all these elements, film colourists will be able to create transfers that can take full advantage of the luminance and colour detail inherent in the original content. In particular, HDR will allow them to distinguish bright details in highlights that are often compressed in traditional video transfers as well as more perceptible details in shadows. In addition there will be greater separation of colour details in diffuse near-white colours and in strongly saturated parts of the image.

Unfortunately this is where it gets confusing because there are actually competing versions of HDR. Whilst they all essentially aim to deliver a similar experience, they take slightly different approaches to how HDR content is mastered and delivered to the consumer. The concern is that these different versions of HDR might not be compatible and as a result SMPTE (Society of Motion Picture and Television Engineers) is trying to develop a single HDR ecosystem.