An Introduction to High Dynamic Range (HDR)
The industry has been constantly working towards improving the user experience in terms of video content consumption. The efforts have been multi-pronged and one of the key areas so far has been to focus on higher resolutions. While ten years ago, HD was a big thing, 4K has now become a common resolution in many production workflows. 4K is also delivered to consumers in many scenarios though it needs to penetrate better especially on the broadcast side.
Industry is now at a stage where it needs to decide what will bring the next significant improvement in user experience. Some of the key contenders are:
- Higher resolution. Going to 8K?
- Higher frame rate. Use more of 50 fps or 100 fps?
- Wider color range
- HDR (High Dynamic Range)
Tests have been conducted by many organizations including IRT, EBU etc. and they conclude that HDR probably offers a higher improvement in quality of experience compared to other technological improvements.
Benefits of High Dynamic Range (HDR)
The holy grail of quality is reproducing a video experience on user’s display device as close as possible to what a human being will perceive when they watch the same scene in nature with their eyes. While this still is an ambitious goal, HDR brings us closer to the reality by offering a wider brightness range (very close to human perception range) and thereby a more realistic experience.
It is known that human visual system (HVS) is more sensitive towards brightness than colors and for the same reason we have color spaces like YUV420, YUV422 that does subsampling for color but retain the brightness information for all the pixels.
Regular SDR (Standard Dynamic Range) monitors available in the market have a range of 1-400 Nits (cd/m2) while HDR allows the representation range of 0-10,000 Nits, which is a significantly wider range of brightness than offered by SDR devices. Currently, available HDR TVs have a range of up to approximately 2,000 Nits. Wider brightness range in HDR simply means that the brightness of each pixel can be more accurately represented rather than being transformed with a higher quantization factor resulting into inaccurate pixel representation (poor quality). The quality improvement with HDR is usually more visible in plain areas with gradients where minor degradation is easily perceived by human eyes.
In essence – HDR means more accurate pixels in terms of their brightness!
Read the next article on transfer functions and how they help in representing a wider brightness range for HDR
cd/m2 – The candela (cd) is the base unit of luminous intensity in the International System of Units (SI); that is, luminous power per unit solid angle emitted by a point light source in a particular direction. A common wax candle emits light with a luminous intensity of roughly one candela.
Nits – A non-SI unit used to describe the luminance. 1 Nit = 1 cd/m2.
HDR – High Dynamic range. It is a technology that improves the brightness & contrast range in an image (up to 10,000 cd/m2)
SDR – Standard Dynamic range. It refers to the brightness/contrast range that is usually available in regular, non-HDR televisions usually with range of up to 100 cd/m2. This term came into existence after HDR was introduced