Simple post processing material with just SceneColor (insures there is no other manipulation going on):
With a desaturate node:
The first image desaturated in photoshop:
As you can see they don’t match even though it is both the same desaturation, here it is better to see, top is UE4 desaturation and bottom photoshop:
It is like UE4 strips gray scale information away. Looks like the dark ball has no more shading.
Hi Davision -
The biggest difference is that the Desaturation node in the Material Editor functions BEFORE lighting gets applied, so while Scene Color in Post Process does eliminate any post process trickery, the material is still desaturating the material before lighting is applied and then lighting is applied (the Deferred Rendering Pipeline), once the Material is applied it does not have any shading. In photoshop you are actually desaturating the Material after lighting is applied to it and thus you are still getting variations in color.
If you are wanting to desaturate the whole scene like that I would recommend using the Post Process Volume (or Camera Post Processing) Saturation settings, this way the desaturation will occur after the lighting has been applied to the material.
Thank You
Eric Ketchum
Ok, then it makes sense yet it is a bit confusing. Is that behaviour then specific to the Desaturation node? Or are there more things that get applied before lighting?
Is there then some better way to make RGB a Greyscale inside a post processing material?
I stumbled upon this while trying to get this to work: Lighting in post process material - Rendering - Unreal Engine Forums
Ideally you would use the Desaturation node on the SceneTexture:PostProcess0 which treats the scene as is (with all lighting applied). Doing this will yield a result similar to the desaturation in Photoshop.
