If you have SweetFX working already, as GSalden has, then there's no worries - but I couldn't get any version of SweetFX to play nicely with P3D until I used ReShade. SweetFX is reasonably well known by the community and so you might have better luck with some recommended settings for P3D - for example, I'm using the ReShade with SweetFX 2.0 (the other download link), and all I have turned on is the SMAA (not even necessary really, if you use NI), curves and Cineon DPX colours.
ReShade, from what I gather, is just a method of injecting the HSHL shaders into P3D, it all depends on what shaders are being called for.
No, you don't HAVE to use SweetFX, but it does have a fair number of already written shaders which you can test. That's possible, it would probably be the default shaders that come with just ReShade and its settings that is doing that. Realistic Graphics mod by Frkn64to get ingame image as close to photorealistic as possible. Initially based on Real Road project these settings are intended to use with vanilla game as well as with. This is settings file for Reshade 2.0.3 + SweetFX 2.0. Hi Auto - I will continue this here as you have seen my post at ftx - do you have to have sweet loaded also maybe thats why I was only seeing half a screen - I only tried reshade standalone - thanks RESHADE V2.0.3 + SWEETFX V2.0 SETTINGS MOD.