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ABSTRACT

High dynamic range (HDR) allows us to capture an enormous range of luminance values within a still image or a sequence of video frames. But many consumers will not have the necessary displays to experience this in the near future.

To allow these ‘legacy’ users to benefit, an adaptation using global tone mapping would be a possible solution. But the results tend to suffer from low subjective contrast and can product large-area flicker.

To overcome these drawbacks, three enhancement steps are proposed. They are based on certain broadcast requirements as well as on viewer preferences, which were surveyed at the beginning of this study.

The basic idea is to analyse each luminance value for its relevance in the image and discard unimportant ones. This ‘virtual aperture’ will be processed across the whole image and on image sections. Finally the tone mapping result will be composed with the chrominance values by using a modified IPT colour space.

INTRODUCTION

One of the main goals of future television is to create a more immersive experience. The viewer should get the feeling that he or she is inside the action. One key component to achieve this is HDR. By preserving details in highlights and shadows simultaneously, a visual sensation is created close to viewing the real scene. For efficient coding, Hybrid-Log- Gamma and Perceptual Quantizer (PQ) are discussed.

This technology will produce a significant increase in subjective video quality as shown in several studies (e.g. 1, 2). But for the next few years we can expect that over 90% of viewers will still have legacy displays. So one of the main questions which comes with HDR is: ‘Is there a way to let the viewers with legacy displays also benefit from HDR?’ (Note that a faithful reproduction of the images in this paper is only available when viewed as a .pdf on a sRGB display.)

REQUIREMENTS AND GOAL

When we talk about showing HDR material on a standard dynamic range (SDR) display we have to think about how to handle the enormous dynamic range at capture compared to the small dynamic range of a particular display. The easiest way is to cut off all parts which are outside the range, but clipping would bring us back to the burned-out highlights and the undefined shadows.

A more promising way is to do a contrast compression also known as tone mapping. Our goal is to create a tone mapping system which is capable of live broadcast and does not suffer from the typical problems of tone mapping which are discussed later on.

At first we define the following requirements for live broadcast:

The most important point is to create a very pleasing image that matches the viewer’s preference. Even though a very pleasing image is not equivalent to the most realistic and natural image. In addition, the look should not be too different from the familiar look of television today - in order to gain the viewer’s acceptance.
The system needs to be very stable so that it can be used for live broadcast without any post production. At no time should tone mapping artefacts like flicker, ghosting or halos be produced.
Unlike an aperture which has discrete steps, a smooth adjustment to changing brightness during a scene should be performed. This transition should not take place if there is a scene change; in this case the exposure ought to adapt to the new situation immediately.
To utilize the capabilities of legacy displays, the system should be sensitive to the display and environmental brightness and should adapt to these parameters depending on the scene.
For live broadcast it is necessary that the system works in real-time.

RELATED WORK AND EVALUTATION

Before starting engineering, we ran a test letting 5 professional colourists grade 7 HDR sequences1 for SDR displays. In a viewing session, 20 subjects (mostly non-expert viewers) would decide which grading he or she liked best. Therefore every grading was compared one-to-one with every other, so the subjects only had to decide which one of the two was the more pleasing. The experiment showed that the viewers liked high contrast and high saturation.

This was confirmed in several previous studies (3,4,5). But moreover, sometimes excessive saturation was found and that high saturation does not necessarily lead to a pleasing image. It seemed rather important that all information within the image could be easily extracted.

Or, in other words: the viewer likes to see all the details in all parts of the image without having to pay too much attention. Saturation and contrast often enhance this aspect. Clipping too, is acceptable in this case, and was sometimes even missed when it was not there.

For example, to reproduce spectral highlights on SDR, clipping is an adequate process. Another outcome is that the white point is only of secondary importance. A warm grading was not preferred over a cold one, as one might expect.

So the white point is only a creative decision and does not need to be adjusted by the tone mapping itself.