PQ Hard Clipping LUTs v1.0
PQ Hard Clipping LUTs v1.0
The MBOX PQ Hard Clipping LUTs are a set of LUTs designed to clip the high or low brightness levels of a PQ image to fixed values.
There are forty nine LUTs in four categories:
14 x Simple 1D Mapping
7 x Simple 1D Mapping with a Crush to 0 Nits
14 x Advanced 3D Mapping based on the BT.709 Transfer Matrix
14 x Advanced 3D Mapping Based on the BT.2020NC Transfer Matrix.
As the names imply, these LUTs are designed to clip the brights or darks of an image, to the stated target brightness value. The Low Clip LUTs define the lowest value the LUT permits any detail retention. Any brightness values below the stated value are clipped and brought up to the target value, with the exception of the "Crush to 0" LUTs, which bring all brightness values below the target value to be exactly 0.
Similarly, the High Clip LUTs impose a maximum brightness value, preventing the brightness values from exceeding the stated target value.
The Low Clipping & Crush to 0 Values are:
The High Clipping values are:
The Simple 1D LUTs apply the same clipping to to all channels, with a very high precision. (12 bit) However, any 10 bit delivery format may find values between two brightness values, and as such may result in slight variations of brightness around the clipping point. Higher bit depth deliveries and masteries (16 bit) may have slight rolls into the clipping points because of the higher channel precision.
Because the 1D LUTs act on all channels equally, without waiting, and because absolute brightness is determined by the weighting of each of the RGB channels, there is a margin of error in the top and bottom clips, the clipping points are not absolute.
To mitigate this issue, we've prepared more advanced 3D LUTs, that give up some of the precision while calculating the clipping points based on either the BT.709 RGB -> YCbCr transfer matrix or the BT.2020 -> YCbCr Transfer Matrix. Use the 709 matrix when using color primaries from the BT.709 or P3 D65 Color Spaces, and the 20202 matrix when using color primaries from the BT.2020 Color Space.
Because of the lower contrast precision of the 33x33x33 3D Cubes, the advanced LUTs apply a slight roll-into the clipped value in the whites or the darks. This may actually be desirable since it gives a slight roll into the clip point, rather than an abrupt transition.
For a demonstration of how the LUTs work and the problems they solve, visit our blog post here.
These LUTs are licensed for use in a post-production setting for applying to assets prior to the assets distribution. If you are interested in incorporating these LUTs into HDR encoding, distribution, or exhibition technology, please contact us for more information on licensing.