BCi Digital: What is HDR?


What exactly is HDR?

There has been a lot of talk about HDR as 4k TVs are rolled out. For me the prospect of HDR is far more appealing than the big push a few years back for 3D TV. For those who are not so sure what HDR is all about, it all started in photography. If you take a picture of a scene with large areas of both very bright and very dark image then this constitutes a large dynamic range. For example the amount of light coming into the camera from the bright area could easily be 256 times as much as from the dark. However with a single shot the camera has to be set with an appropriate aperture size (the hole that lets the light in) to let in only a certain amount of light. This aperture size is defined in terms of f-stop number.

In a camera each f-stop increment means doubling the amount of light, e.g. going from f-8 to f-5.6 doubles the light coming into the camera, so going from f-22 to f-1.4 is 8 stops, hence x256 more light is let in going from one extreme to the other.

F-1.4 may be suitable for the dark areas but not the light, and vice versa for f-22.

To capture the detail in the dark areas would need either the camera lens shutter to stay open a long time to capture more light, or for the shutter to have a much wider aperture thus letting more light in. (For our discussion we will assume the shutter speed is kept constant.

As we all know taking a picture of someone in front of a window means the person being photographed becomes very silhouetted against the bright background. The eye of course is very good at dynamically changing pupil size (equivalent of the camera aperture) and quickly adapts to the particular area of interest it’s looking at. The camera having only one shot at the picture can’t.

A High Dynamic Range (HDR) picture is then rather intuitively made by taking two pictures of the exact same scene, one shot with small aperture and one with large and combining them together to create one final image.

Of course more than two images can be combined in this way.

HDR_Amsterdam

This then is HDR for images. For video it is not restricted by the camera f-numbers. For TV’s the technology is moving ahead fast and with the 4K variants coming onto the market they offer a greater number of pixels, and equally importantly greater contrast. This technology can really show off the content produced by HDR cameras to give vibrant, high contrast, super realistic video with more detail than ever before.

Currently Fox are releasing the following movies in HDR in 4k format: ‘Kingsman: The Secret Service’, ‘Life of Pi’, ‘Exodus: Gods and Kings’ and ‘The Maze Runner’. If we’re not lucky enough to subscribe to a provider with HDR content then we’ll have to get 4k HDR Blu-ray players and get the movies on disc.

Mark Massel – BCi digital

BCi Digital provides a range of professional services and solutions for broadcasters of all types.

Please contact us to arrange an informal meeting to see how we can help you with  your needs – contact us