This is a fairly straightforward example of how to intercept Steam Overlay
screenshot requests and do some processing before sending a screenshot back
to the overlay. This method uses a Texture2D to capture the screen and
sends back the pixel data to the overlay directly.
Simply place the SteamScreenshotCapturer.cs
script somewhere in your Assets folder and attach the script to a persistent
GameObject in your scene. If you are using a Canvas for your GUI, set the
target canvas on the script. If you are using something more complicated than
a Canvas, change the type of TargetCanvas in the script to GameObject or
whatever you need.
When the player presses the screen capture key configured in Steam Overlay (by
default F12), the overlay will send a screenshot request to your game, and this
script will receive that, disable TargetCanvas, generate the screenshot,
send it to the overlay, then reenable TargetCanvas. The end result will look
as if the overlay itself took the screenshot, but sans whatever objects you
don't want to appear.
If you'd like to do hi-res screenshots, have a look here for what you can change and adapt.
This project was made in response to a question Luke posted on the Steamworks Development discussions.