What is HDR?
High Dynamic Range (HDR) refers to a technique used to make your media look more realistic through more accurate colour reproduction across a wider range of available colours. The technique also enhances the contrast between the brightest whites and the darkest blacks within an image on your screen. When reproducing colour on a display, the two most important factors are:
- The accuracy of the colour reproduction - how close to real-life the colour is displayed, or closer to how the director intends the image to appear to the viewer;
- The contrast ratio - how bright and dark the pixels on the display can emit light or colour
If you were to put two displays next to each other, one with a more accurate colour reproduction and a better contrast ratio, and the second with a higher resolution (i.e. 4K vs 1080p) but with no colour / contrast enhancement, the image with the enhanced colour reproduction will appeal to more viewers as the image will look more natural and much more "real", regardless of the reduced detail than the higher resolution display.
Increased Colour Range
HDR features an increased Bit-Depth over Standard Dynamic Range's 8-bits to either a 10-bit or 12-bit colour depth. The Bit-Depth refers to the number of individual colours each RGB pixel can individually create on a display. For 8-bit, this is 256 different shades from each Red, Green & Blue part of the pixel, which can represent a combination of over 16 million different colours that can be created by mixing the different intensities of light from each. For 10-bit colour, this is increased to 1,024 different shades, and over 1 billion colour combinations in total; and then for 12-bit colour, this expands to 4,096 different shades and over 68 billion individual colours.
Below right: an increase in colour depth allows for a screen to more accurately represent a flow of one colour to a different shade of the same colour without seeing 'banding' between the shades. With more colours available to be represented, the flow of a spectrum will appear more blended than lower colour depths.
Wide Colour Gamut (or Chroma Subsampling) is also increased over SDR’s Rec.709 colour gamut which was introduced with HDTV's back in 1990, which could only represent a mere 35.9% of what the human eye is capable of perceiving. With newer UHD screens, the ability of colour reproduction increased to the newer Rec.2020 (or BT.2020) colour gamut that covers 75.8% of the human eyes visible colour spectrum can now be fully reproduced by using different HDR formats.
Below: a graphical representation of the Rec.709 standard versus the increased colour reproduction of the Rec.2020 standard across the spectrum of colour a human eye can perceive.
Increased Contrast Ratio
For a HDR panel to give much brighter brights, and darker darks, the display needs to be able to emit more light than SDR panels, but also be able to reproduce colours at very low levels of light intensity. We measure the light output of a display in Nits. One Nit is equal to the amount of light emitted from one candela per square meter - a standardised measurement of luminous intensity (cd/m2).
Typical SDR displays provide a peak brightness output between 300-500 Nits. The HDR specification requires a minimum of 1,000 Nits of peak luminance, while subsequently allowing dark scenes to a level of only 0.05 Nits of light emission to accurately recreate the darker HDR content. All of this together gives greatly enhanced detailing within shadows as well as on bright, explosive scenes.
Different HDR Formats
There are several different formats of HDR available on the market from different manufacturers. The standard HDR format is HDR10 which is supported in every HDR display. However, Dolby Vision HDR, HDR10+ and HLG (Hybrid Log Gamma) are advanced HDR formats that will not necessarily be supported by every display.
HDR / HDR10
- Colour bit depth: 10-bit signal
- Colour gamut: BT.2020 / DCI-P3
- Peak brightness of 1,000 Nits and less than 0.05 nits of black
- HDR / HDR10 carries a piece of static metadata that describes the colour and contrast properties of the entire media content (such as maximum and average light levels) for the entire film or show
- Colour bit depth: 12-bit signal
- Colour gamut: BT.2020
- Peak brightness target of 4,000 Nits, however it will support up to 10,000 Nits in the future
- Dolby Vision's approach to HDR includes a piece of dynamic metadata that accompanies each and every frame of the video signal being sent, enhancing the consumer experience by optimising the the way a screen displays the media on a frame-by-frame basis
- Colour bit depth: 10-bit signal
- Colour gamut: BT.2020
- Peak brightness of 4,000 Nits
- Adds dynamic metadata to standard HDR for frame-by-frame dynamic range adjustments (similar to Dolby Vision, but this will be an 'open standard' for any manufacturer to adopt)
Below: the dynamic metadata in both Dolby Vision HDR and HDR10+ allows for the HDR properties to be applied dynamically, on a frame-by-frame basis so that the image is optimised consistently throughout a piece of media. With HDR, the same HDR metadata is applied throughout the film or show as a constant.
Hybrid Log Gamma
- The aim of Hybrid Log Gamma (HLG) is to recreate the HDR10 standard for broadcast, rather than streaming or optical disc media. HLG is designed not to be dependent on metadata, and as a result allows backward compatibility to a display that does not support HDR. Displays receiving an HLG signal that do not support HDR will receive a ‘washed out’ picture however will still be able to see the content.