One of the most common notes an operator will receive in virtual production is to address a note on the black point of the image - and only the black point of the image, the main reason is to help with the black point of the image being lifted when displayed on LED panels that are also not 100% absorbing the light reflecting off them.
In composting/color correction software, you have a standard set of color correction controls that Unreal also has such has gamma, gain, saturation, offset, but one of them is missing entirely from the engine… Lift.
When you want to change the white point of the image without affecting the black point, that’s what gain is for, the opposite of gain is lift.
Nuke:
Resolve:
You can find the math for lift here
http://www.designimage.co.uk/basic\-colour\-correction\-maths\-for\-compositors/
Output = ( input * ( 1 – lift ) ) + lift
In the absence of a lift control, you have to try some workarounds to address the note.
One way is to use the “shadows” section of the color controls, you go false color to see what unreal defines as shadows and then modify the shadows max parameter to dial in the area you want to correct. However, sometimes this fails because the value range you are trying to correct is not contrasted enough and therefore you can’t isolate the area you want.
Another way is to give more contrast, but this both lowers the black point and raises the white point at the same time, expanding the value range outward from the mid point of the image. You then have to gamma down to try and bring the white point back to where it was.
Another thing to consider is the The ASC-CDL standard Slope, offset, power (SOP), We’ve had DIT on set using Live Grade Studio using this method of color correction, and having this has an option can help bridge the gap if they are color correcting in this way and ask can we do the same.
https://pomfort.com/article/an\-in\-depth\-look\-at\-asc\-cdl\-based\-color\-controls/
- Slope multiplies the code values with a factor.
- Offset applies an offset (adds or subtracts) to all code values.
- Power is the power function of the results.