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Perceptual Quantiser (PQ) to Hybrid Log-Gamma (HLG) Transcoding

BBC_HDRTV_PQ_HLG_Transcode

Display Light Scene

Display Light Scene Light Transcode PQ to HLG for a Common Display Brightness System Overview The transcode process outlined in ITU-R BT.2390 is as follows: PQ Signal Display Light PQ EOTF HLG EOTF -1 HLG Signal Figure 1: PQ TO HLG High Level Overview where the HLG EOTF -1 consists of an inverse Opto-Optical Transfer Function (OOTF -1 ) followed by the HLG OETF: PQ Signal PQ EOTF Display Light OOTF -1 Scene Light HLG OETF HLG Signal Peak Mastering Level, Colour Space Figure 2: PQ To HLG Overview The OOTF -1 process can be broken down as follows: Peak Mastering Level Colour Space RGB to Y Y Y 1/γ Gamma -1 Y (1-γ)/γ Ratio Figure 3: OOTF -1 procedure X X X {R,G,B}· Y (1-γ)/γ The PQ EOTF and HLG OETF are defined by BT.2100-0.

Step by Step Guide Below is a fully explained procedure for the transcode process. This process is appropriate for implementation in a 3D look up table (LUT). Note, the range of the values at each stage is between 0.0 and 1.0. 1. Apply the PQ EOTF to the signal, as per Table 4 in BT. 2100-0. Note the multiplication by 10000 is not required. This is now display light in the range of 0.0 to 1.0, where 1.0 represents 10000 cd/m 2 . Y max c 1 m 1 m 1 2 E c ,0 2 c E 3 1 1 m2 2. Normalise the display signal using the peak mastering level. The mastering level can be obtained from analysing the signal, or alternatively, may be available as metadata, e.g. by knowing which mastering monitor was used. For a PQ signal graded on a 1000 cd/m 2 display, there should be no data above 1000 cd/m 2 as this would have been clipped by the display. Therefore, the signal should be normalised so that 1.0 now corresponds to 1000 cd/m 2 . Similarly, for PQ content graded at 4000 cd/m 2 , there should be no data above 4000 cd/m 2 as this would have been clipped by the display. Therefore, the signal should be normalised so that 1.0 now corresponds to 4000 cd/m 2 . F d Y *10000 min , 1. 0 Lw where Y is the {R,G,B} signal, and L w is the peak brightness of mastering display in cd/m 2 . This signal is display referred linear. 3. The OOTF -1 is now applied to convert the display linear signal to a scene linear signal. First the display luminance of the display is calculated. Note that the coefficients in the conversion must correspond to the correct colour primaries of the signal. BT.2020 primaries have been used for the conversion below. Y 0.2627 R 0.6780G 0. 0593B d d d d