There is a discrepancy in that the docs are indicating color grading should cause contrast enhancing but you’re saying it can cause contrast lost. Which statement is the true one?
They are both true, in different areas. The doc is talking about the ‘toe’ at the dark end of the tonemapper, which increases contrast slightly in the darks. I’m talking about the generally more noticeable compression of bright colors which reduces contrast. Bright colors also get desaturated as they approach 1.
Additionally, changing lighting parameters will have no useful effect because the issue is on a flat plane in a heavily varied texture. The problem is relative color metric of the aerial image on a flat surface.
I see that now, and that is part of the problem. Material descriptions need to be surface properties, things like diffuse reflectance (this is what you plug into base color). You are plugging a final lit image (photograph) into Base Color, which is effectively tonemapping the image twice, since the camera tonemapped when it captured the photo (conversion from linear HDR lighting to gamma LDR). To reproduce the same result as the image, you would have to know the underlying diffuse colors of the materials captured in the photo.
So that’s the theory part of it. Obviously you want to make something work and you have an already tonemapped photo, not the raw diffuse colors of the scene that you would want. What you can do is try to undo the camera’s tonemapping with contrast and brightness operations in the BaseColor of your material.