ColorBox In-Line Color and HDR/SDR Transform v1.0 34 www.aja.com
in the best way possible. It depends very much on the camera manufacturer
as some have creative tools independent for HDR and SDR outputs. Matching
configuration settings to the HDR converted signal “look” for the SDR output
must be found and locked.
HDR and SDR Reference Points
This controls the HDR and SDR “anchor points”. At the default settings of 75 and
100, for example for an HLG to SDR conversion, if no compression or expansion
is applied, 75% HLG will be mapped to exactly 100% SDR (direct mapping). If any
compression is applied on the pre-compression, then the absolute colorimetric
conversion to SDR is performed, scaled based on the selected HLG Ref and
SDR Ref. However, if these values are above the pre-compression and/or post-
compression points, the correspondence will be shifted.
HDR Peak
The peak luminance (referred to as LW in ITU-R BT.2100) in Nits of the ‘virtual HLG
display` used in conversion calculations. This is normally left at the default value
of 1000, but could, for example, be reduced to 392 which is the value for an HLG
display which produces 100 nits at 75% IRE.
This does not affect the relative scaling of HDR and SDR, as that is controlled by
the HDR Ref and SDR Ref above. But it does control the gamma of the HLG OOTF
as defined in ITU-R BT.2100.
NOTE: This value is not used in scene-light conversions.
Post-compression/Post-expansion
Knee -
This slider controls the threshold at which compression (for down-
conversion) or expansion (for up-conversion) begins. Example: A value of 50
places this at 50% SDR IRE in a down-conversion. The start of the compression
will be seen at this value on a waveform monitor because this compression is
the last step of the process.
Amount -
This controls the amount of compression in down-conversion and
expansion in up-conversion. When compressing, a value of zero applies no
compression (at which point the value of Knee is irrelevant) and a value of 1
will compress completely, flattening everything above the value of Knee to
that value.
Output
Clamping
Internally the result of the conversion is unclamped float, meaning that even HDR
values that cannot be represented on an SDR display after conversion, will be
converted to SDR values outside the displayable range. When outputting over
SDI, it is necessary to limit the range of the output. This control applies a clamp to
the output R'G'B' values.
Select from:
• Unclamped (clamping will still occur in the hardware output)
• Clip sub-blacks
• -7 to 109 IRE (SDI permissible range)
• -5 to 105 IRE (EBU R 103 preferred range)
• 0 to 100 IRE
