What happens when I convert between SDR & HDR?

During standards conversion of UHD, it may be necessary to map between different HDR standards, such as PQ to HLG or vice versa.  Typically in up or down conversion applications between HD and UHD, mapping between SDR and HDR is required.

Although there are cameras which provide simultaneous SDR and HDR output, mapping a specific SDR acquisition to HDR using one of the standardised curves may not actually look the same as the HDR acquired at the same time if gain has been applied to the SDR version at the camera head.  Similarly, an HDR to SDR conversion might have the same problem.

The processing to be applied depends on the intended use of the content.  If SDR content is to be integrated into an HDR programme, which may then be distributed in both SDR and HDR, a “round trip” approach should be taken where the SDR content is mapped unchanged into the HDR space.   If the programme will remain in HDR, then the SDR content may be contrast enhanced by expanding the luminance range.

When HDR content is mapped to SDR, the process is not reversible.  Essentially, the additional contrast ratio available in HDR has to be reduced to fit the SDR space, thereby losing the fine granularity of the brightness levels.  HDR mapping into SDR may include soft clipping, where a graduated mapping into the smaller dynamic range is made.