I am trying to calibrate my physical camera to an LED panel and I am getting absolutely unusable results.
I followed the workflow described here: https://docs.unrealengine.com/5.2/en-US/camera-color-calibration-for-in-camera-vfx-in-unreal-engine/
Workflow
I have done the following:
- created RGB patches using an Unlit material with absolute values, specifically (1.0, 0.0, 0.0) for red, and similar values for the other patches.
- created an OpenColorIO configuration that converts the color space from Unreal’s linear sRGB to my monitor’s sRGB.
- adjusted the EV setting so that a white patch is slightly below the maximum brightness of my monitor (I use a classic monitor for test purposes), measured with a Sekonic light meter and compared to the maximum in the spec sheet of the monitor
- everything was filmed with a Blackmagic Pocket 4K camera (as color profile film Gen 4 / LOG was used, 6500 Kelvin and Tint 0)
- this material was linearized in Davinci Resolve using a Color Space Transform Node, specifically Blackmagic Design Pocket 4K Film Gen 4 was transformed to Rec.709
- the values of the individual patches were read with an RGB picker in Davinci Resolve
- then the mathematics according to the documentation was applied to these values
a. once a Casio CAS was used and calculated by hand
b. after that I built an editor utility to calculate this automatically - the resulting matrix was added to the OpenColorIO configuration with Visual Studio Code and the config was adjusted so that another setting option appears that uses this matrix when converting to sRGB
- reloaded and displayed this OCIO configuration in Unreal, then filmed this again and checked the new RGB values in Davinci Resolve
- unfortunately the result is completely unusable and the color accuracy is worse than before. I expected the colors should also be absolute values in camera after calibration.
The mathematics, the OCIO configuration, the editor utility and further screenshots are attached below.
Result
These are the patch values before calibration:
The matrix in the editor utility:
The result of this matrix:
Used Methods
Math used:
or
https://docs.unrealengine.com/5.2/en-US/camera-color-calibration-for-in-camera-vfx-in-unreal-engine/#matrixcreationmath
Editor Utility:
https://blueprintue.com/blueprint/lf7db12m/
or
CalculateColorCorrectionMatrix.uasset (576.9 KB)
OCIO Config (added Indie LED Wall as display, made them active and placed the Matrix under looks):
or
https://pastebin.com/3HjZckcH
or
simple.config.ocio (6.7 KB)
Attempted solutions
- CAS vs UE Editor Utlitiy: the calculations produce different values for the matrices, probably because of different rounding of the values. However, both methods do not give usable results when looking at the colors again through the physical camera.
- I thought I misinterpreted the matrix in the documentation, for this reason I reversed the matrix (first RGB values of the red Patch were placed in the first row, then RGB values of green Patch in the second row etc.,
second / this time the red values of the red, green and blue patch were placed in the first column, after that the green Values of the red, green and blue Patch were place in the second column, etc).
However, both matrices do not give usable results when looking at the colors again through the physical camera (screenshots for that below)
Question
Unfortunately, I can’t say where the problem lies: is it an error in the workflow (linearization of the material in Davinci, camera color profile…), is it an error in the math or in the implementation of the math, is it an error within the OCIO Config, is it my monitor…
I’m also open to any form of other sources that can help with this calibration process, unfortunately I couldn’t find anything other than the actual documentation.
Thank you
First of all thanks for reading this long post this far and thanks for any support you have given if you commented.