You need to install 2 things.
1) The sofware.
2) The specific preset you like. This is a configuration file. You can get different config files from
http://sfx.thelazy.net I tried KPutt's config, because I used his GTAV preset, and I liked it.
SweetFX nowadays works together with other free opensource software called ReShade.
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Go to this website:
http://reshade.me/
Click on "download" in the top menu bar.
You'll see three big blue buttons.
Click on "ReShade + SweetFX".
Download the file Reshade 0.18.4. It's a zipfile (7z).
Put the zipfile on your desktop for now.
Now go to this website:
http://sfx.thelazy.net/games/preset/3691/
At the bottom of the webpage, just above the screenshots, there
are 2 buttons. The first button is: "download preset".
Click that button and download the preset.
Filename: SweetFX_Settings_The Witcher 3_ Wild Hunt_K-putt'e Config 1.1.txt.
Put it also on your desktop for now.
Open the folder where the Witcher3.exe executable is.
That's in <somewhere>\The Witcher 3 Wild Hunt\bin\x64
The witcher3.exe is here.
Copy from the first zipfile (Reshade 0.18.4) the following files
to the folder where witcher3.exe is:
1) the complete SweetFX folder
2) ReShade.fx
3) Sweet.fx
4) ReShade64.dll
Rename the "ReShade64.dll" file to "dxgi.dll".
First half is done.
Now we need to install KPutt's preset.
Go into the SweetFX folder that you've just put in your Witcher's folder.
Rename "SweetFX_settings.txt" to "SweetFX_settings - orig.txt" or something like that.
Copy the file:
SweetFX_Settings_The Witcher 3_ Wild Hunt_K-putt'e Config 1.1.txt
that you had put on your desktop a few minutes ago, to the
"<somewhere>\The Witcher 3 Wild Hunt\bin\x64\SweetFX" folder.
Rename the file
SweetFX_Settings_The Witcher 3_ Wild Hunt_K-putt'e Config 1.1.txt
to
SweetFX_Settings.txt.
Done.
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If you want to disable SweetFX, it's simple.
Find the file "dxgi.dll" that you've installed yourself.
Rename it to something else. E.g. "dxgi.dll-disabled".
If you want to try it again, rename it back to dxgi.dll.
You can look inside SweetFX_Settings.txt in the SweetFX folder with a text editor.
Like Notepad.
After the comments at the top, you will see a section called:
// Choose Effects //
Here you can enable or disable certain features, like SMAA, FXAA, sharpening, bloom, etc.
Then in the bottom part are all the fine detail settings.
When you now start The Witcher (directly or via Galaxy), during the first 5-10 seconds, you'll see text by SweetFX. That's when you know it worked.
If you want to get rid of that text, you can do that by editing the Global_settings.txt file. You can also enable an fps-counter there, if you like.
Normally you should be able to toggle the effects on or off in-game, by pressing the ScrollLock key. But for some reason that doesn't work now.
If you use other software with on-screen overlays, like Fraps, Steam Overlay, Afterburner, etc, there might be a conflict, and it won't work. You can run only one of them at the same time. SweetFX has it's own fps-counter these days, so it's not that bad. Sometimes it works if you start another tool after you started the game. (E.g. start the game, SweetFX will start at the same time. Then alt-tab to the desktop, and start Fraps. Should work then).
Enjoy.