Part1. Bring the Cinema to Your Home – HDR Technology & OLED Display

What does HDR (High Dynamic Range) technology, which is supported even by low-budget TVs nowadays, mean? In this column, we will discuss what HDR technology is and how OLED and HDR are related.

The concept of HDR technology is very familiar to us. Most of the smartphone cameras available in the market these days support HDR features. For sure, the HDR technology for display devices is not the same as the HDR technology for smartphone cameras, but the fundamental purpose is the same. Both kinds of HDR technology aim to exactly represent the world that we see through the human visual system on display devices.

For those of you who are yet unfamiliar with the HDR technology, let’s take HDR for cameras as an example. Imagine that we are sitting in a dark room. Human eyes can capture both the bright scenery outside the window and an object in a dark room at the same time. However, as shown in Figure 2, when you take a photo of a scene outside the window in a dark room with a regular camera sensor with narrower dynamic range, inside the room is captured in the photo, but the scene outside the window is blown out in white. Or, the scenery outside the window is captured in the picture while inside the room is crushed in black, which makes it difficult to distinguish the objects in the room. This depends on whether the focus of the camera’s sensor is on the room or the scenery outside the window. And probably many of you have experienced this phenomenon. However, thanks to the broader dynamic range of the camera sensors and advanced image processing technologies, the latest smartphones and cameras are designed to provide a more extensive dynamic range as you can see from the picture on the right side in Figure 2.

Then, what is the difference between HDR for TVs/other displays and HDR for camera? Basically, the images we see through various types of display such as TVs are the result of a camera converting the light that came in through the lens into an electrical signal, which is saved as a digital file and the display converting that again into the light. In fact, there are many different ways for cameras to digitize light, the analog signal. To have an accurate output from what the camera has taken initially, the way of converting the analog-to-digital signals must match. If the analog signals are converted into digital signals with one protocol and digital signals are converted into analog signals with another, the display will show completely different brightness and color, not the intended image by the original photography.

 

To prevent such confusion, most images we now see on digital devices are produced according to ITU-Rec. 709, a standard established by the International Telecommunication Union (ITU), which is nearly identical to the sRGB format. With the standard, we can see mostly the same type of images across different displays even if the image quality varies from one display to another. However, the standard was made a few decades ago, when there were no fully fledged camera and display technologies. So, it has been pointed out that it neither satisfies the dynamic range of the human visual system nor properly reflects the capabilities of modern camera and display technologies. Therefore, a new standard, ITU-Rec. 2100, otherwise known as the HDR standard, was established to accommodate an extended range of light and color. Although it is still controversial as to how wide of a dynamic range the real HDR display should cover, it is widely understood that an HDR display indicates a display which renders images based on this standard.

The dynamic range of the human visual system is large enough to recognize even from faint starlight in the night sky to the bright sunlight, but the dynamic range of modern display is not equivalent to the human visual system yet.

 

Among many methods proposed by various companies and organizations, the current HDR standard, ITU-Rec.2100, is recommended by HDR10 and Hybrid Log Gamma (HLG), for mastering HDR contents. Although not an international HDR standard, Dolby Vision, reportedly, which aims for better picture quality, is also available mainly on online-based video streaming services such as Netflix. Because ITU’s HDR standard was established in 2016 and hence HDR videos are not regularly broadcasted, prominent broadcasters such as NHK and BBC have been already conducting HDR test broadcasting or providing HDR content through their paid channels. HDR videos are gradually expanding its boundaries. For example, Netflix and other various streaming services in North America such as Hulu and Amazon Prime services are already providing various TV shows and movies in HDR. In addition, YouTube also supports upload and playback of HDR videos. At present, to watch HDR content through these streaming services, you need to have an HDR supporting display, sign up as a paid member of an HDR content provider, and use the streaming application of the content provider. Or, you can purchase and watch 4K/HDR content by connecting a Blu-ray player that supports HDR/4K to an HDR TV.

Technically, HDR videos are designed to have a physical brightness level of up to 10,000 cd/m2, which is 30 to 50 times brighter than normal monitors or TVs (200 ~ 300 cd/m2), but this does not mean that overall brightness of an entire video is 30 to 50 times brighter than the current level. (If it does, people will go blind watching these videos.) Because it is not practical for consumer displays to deliver up to 10,000 cd/m2 now, most guidelines for mastering HDR contents suggest that only a small region in the image is allowed to exceed 1,000 cd/m2. According to a recent study of investigating the feature of HDR contents from 41 HDR titles [4], the luminance of most pixels is equal to or higher than 1000 cd/m2 for only 3.8% of frames and the average frame light level (FALL) of 98.6% of frames is 170 cd/m2. This means that the average brightness required for HDR contents are like that for the contents we see now. Therefore, as shown in Figure 6, highlighted regions such as flame in the brazier and welding sparks are equal to or higher than 1000 cd/m2, and other regions, for example, dark regions should appear dark or even darker than SDR contents. One interesting fact is that the size of highlight regions in the HDR contents is small. Some articles [5, 6] reported that the size of highlight regions is usually less than 2% of the image size, and the sizes of the flame and sparks in the image in Figure 6 are less than 0.1% of the image size. It is noticeable that these highlights are often surrounded by deepest dark regions to make an image more dramatic.

(This article continues in Part.2)

References

[1] https://www.43rumors.com/whats-black-magic-on-the-new-pocket-cinema-camera/

[2] Timo Kunkel and Erik Reinhard. 2010. “A reassessment of the simultaneous dynamic range of the human visual system”. In Proceedings of the 7th Symposium on Applied Perception in Graphics and Visualization (APGV ’10). ACM, New York, NY, USA, 17-24. DOI: https://doi.org/10.1145/1836248.1836251

[3] Jan Froehlich, Stefan Grandinetti, Bernd Eberhardt, Simon Walter; Andreas Schilling and Harald Brendel. 2014. “Creating cinematic wide gamut HDR-video for the evaluation of tone mapping operators and HDR-displays”. Proceedings Volume 9023, Digital Photography X; 90230X (2014) https://doi.org/10.1117/12.2040003. 

[4] R. Boitard, M. Smith, M. Zink, G. Damberg and A. Ballestad, “Predicting HDR Cinema Requirements from HDR Home Master Statistics,” in SMPTE Motion Imaging Journal, vol. 128, no. 3, pp. 1-12, April 2019. doi: 10.5594/JMI.2019.2895703

keywords: {Digital cinema;high dynamic range (HDR);mastering},

[5] HDR REAL SCENE PEAK BRIGHTNESS, https://www.rtings.com/tv/tests/picture-quality/peak-brightness

[6] Yongmin Park and Jang-Un Kwon. 2017. “62‐4: Invited Paper: Consideration of Display Metrology for HDR and WCG Standards Based on Real Content”. SID Symposium Digest of Technical Papers (2017) https://doi.org/10.1002/sdtp.11787