Brightness, luminance, luma, NITS and candela. What are the differences between these similar terms? If you’ve not been working closely with displays and video, you may not know but as HDR grows in adoption, it pays to have at least a passing understanding of the terms in use.
Date: Thursday January 23rd – 11am ET / 16:00 GMT
Last week, The Broadcast Knowledge covered the difference between Luma and Luminance in this video from YouTube channel DisplacedGamers. It’s a wide ranging video which explains many of the related fundamentals of human vision and analogue video much of which is relevant in this webinar.
To explain the detail of not only what these mean, but also how we use them to set up our displays, the IABM have asked Norm Hurst from SRI, often known as Sarnoff, to come in and discuss his work researching test patterns. SRI make many test patterns which show up how your display is/isn’t working and also expose some of the processing that the signal has gone through on its journey before it even got to the display. In many cases these test patterns tell their story without electronic meters or analysers, but when brightness is concerned, there can still be place for photometers, colour analysers and other associated meters.
HDR and its associated Wide Colour Gamut (WCG) bring extra complexity in ensuring your monitor is set up correctly particularly as many monitors can’t show some brightness levels and have to do their best to accommodate these requests from the incoming signal. Being able to operationally and academically assess and understand how the display is performing and affecting the video is of prime importance. Similarly colours, as ever, a prone to shifting as they are processed, attenuated and/or clipped.
This free webinar from the IABM is led by CTO Stan Moote.
The second most popular video of 2019 looked at HDR. A long promised format which routinely wows spectators at conferences and shops a like is increasingly seen, albeit tentatively, in the wild. For instance, this Christmas UK viewers were able to watch HDR Premiership football in HDR with Amazon Prime, but only a third of the matches benefitted from the format. Whilst there are many reasons for this, many of them due to commercial and practical reasons rather than technical reasons, this is an important part of the story.
Brian Alvarez from Amazon Prime Video goes into detail on the background and practicalities of HDR in this talk given at the Video Tech Seattle meet up in August, part of the world-wide movement of streaming video engineers who meet to openly swap ideas and experiences in making streaming work. We are left with a not only understanding HDR better, but with a great insight into the state of the consumer market – who can watch HDR and in what format – as well as who’s transmitting HDR.
The Broadcast Knowledge exists to help individuals up-skill whatever your starting point. Videos like this are far too rare giving an introduction to a large number of topics. For those starting out or who need to revise a topic, this really hits the mark particularly as there are many new topics.
John Mailhot takes the lead on SMPTE 2110 explaining that it’s built on separate media (essence) flows. He covers how synchronisation is maintained and also gives an overview of the many parts of the SMPTE ST 2110 suite. He talks in more detail about the audio and metadata parts of the standard suite.
Eric Gsell discusses digital archiving and the considerations which come with deciding what formats to use. He explains colour space, the CIE model and the colour spaces we use such as 709, 2100 and P3 before turning to file formats. With the advent of HDR video and displays which can show bright video, Eric takes some time to explain why this could represent a problem for visual health as we don’t fully understand how the displays and the eye interact with this type of material. He finishes off by explaining the different ways of measuring the light output of displays and their standardisation.
Yvonne Thomas talks about the cloud starting by explaining the different between platform as a service (PaaS), infrastructure as a service (IaaS) and similar cloud terms. As cloud migrations are forecast to grow significantly, Yvonne looks at the drivers behind this and the benefits that it can bring when used in the right way. Using the cloud, Yvonne shows, can be an opportunity for improving workflows and adding more feedback and iterative refinement into your products and infrastructure.
Looking at video deployments in the cloud, Yvonne introduces video codecs AV1 and VVC both, in their own way, successors to HEVC/h.265 as well as the two transport protocols SRT and RIST which exist to reliably send video with low latency over lossy networks such as the internet. To learn more about these protocols, check out this popular talk on RIST by Merrick Ackermans and this SRT Overview.
Rounding off the primer is Linda Gedemer from Source Sound VR who introduces immersive audio, measuring sound output (SPL) from speakers and looking at the interesting problem of forward speakers in cinemas. The have long been behind the screen which has meant the screens have to be perforated to let the sound through which interferes with the sound itself. Now that cinema screens are changing to be solid screens, not completely dissimilar to large outdoor video displays, the speakers are having to move but now with them out of the line of sight, how can we keep the sound in the right place for the audience?
This video is a great summary of many of the key challenges in the industry and works well for beginners and those who just need to keep up.
HDR has long being heralded as a highly compelling and effective technology as high dynamic range can improve video of any resolution and much better mimics the natural world. HDR continues its relatively slow growth into real-world use, but continues to show progress.
HDR is so compelling because it can feed our senses more light and it’s no secret that TV shops know we like nice, bright pictures on our TV sets. But the reality of production in HDR is that you have to contend with human eyes which have a great ability to see dark and bright images – but not at the same time. The total ability of the eye to simultaneously distinguish brightness is about 12 stops, which is only two thirds of its non-simultaneous total range.
The fact that our eyes constantly adapt and, let’s face it, interpret what they see, makes understanding brightness in videos tricky. There are dependencies on overall brightness of a picture at any one moment, the previous recent brightness, the brightness of local adjacent parts of the image, the ambient background and much more to consider.
Selios Ploumis steps into this world of varying brightness to creat a ways of quantitatively evaluating brightness for HDR. The starting place is the Average Picture Level (APL) which is what the SDR world uses to indicate brightness. With the greater dynamic range in HDR and the way this is implemented, it’s not clear that APL is up to the job.
Stelios explains his work in analysing APL in SDR and HDR and shows the times that simply taking the average of a picture can trick you into seeing two images as practically the same, whereas the brain clearly sees one as more ‘bright’ than the other. On the same track, he also explains ways in which we can work to differentiate signals better, for instance taking in to account the spread of the brightness values as opposed to APL’s normalised average of all pixels’ values.
The talk wraps up with a description of how the testing was carried out and a summary of the proposals to improve the quantitive analysis of HDR video.