Rep. ITU-R BT.2407-0 1
REPORT ITU-R BT.2407-0
Colour gamut conversion from Recommendation ITU-R BT.2020 to
Recommendation ITU-R BT.709
(2017)
1 Introduction
In wide-gamut UHDTV production, particularly under the assumption of simultaneous live UHDTV
and HDTV broadcast scenarios, real-time high-quality gamut mapping from UHDTV to HDTV is
essential. This Report addresses in general the following goals:
– To advise on the advantages and disadvantages of automatic conversion techniques and to
seek to identify optimum conversion mechanisms.
– To advise on any additional measures that may be appropriate in order to achieve optimum
colour gamut conversion and avoiding the introduction of subjectively disturbing artefacts.
– To investigate a possible method of colorimetry conversion from Recommendation ITU-R
BT.2020 (BT.2020) to Recommendation ITU-R BT.709 (BT.709) in terms of image quality
and feasibility that ideally satisfies the following requirements:
• Colours inside the BT.709 gamut should be unchanged.
• The conversion method should facilitate multiple conversions between BT.2020 and
BT.709.
• Perceived hue change must be as small as possible.
• No significant loss of spatial details.
• Will not introduce visible discontinuities in colour.
• The mapping method is mathematically definable.
There is no universal gamut mapping method which can achieve all of these requirements
simultaneously. In converting from a wider colour gamut to a smaller colour gamut, modification of
colours outside the BT.709 gamut is unavoidable. This conversion is necessarily a compromise
between different requirements which may vary depending on the application. Gamut mapping
algorithms are often motivated by aspects of artistic creation, human vision, technical constraints and
experience. Metrics for identifying colour gamut conversion performance have not yet been
developed along with an associated suite of tests.
2 Simple conversion from BT.2020 to BT.709 based on linear matrix transformation
Figure 1 shows the block diagram of the colour conversion from BT.2020 to BT.709 based on linear
matrix transformation. This is the exact inverse of the operation specified in Recommendation ITU-R
BT.2087 for conversion from BT.709 to BT.2020, except that the output signals are hard-clipped.
This method is the most straightforward and implementable in the least amount of hardware.
For standard dynamic range television (SDR TV).
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