Just wondering whether this is possible so you could create a fake depth of field for aiming down the sights of a gun or something.
Currently, there’s not. You can use a GLSL shader on a 2D plane and pass in a render of the screen texture with the bge.texture Python module, though.
Is there any tutorials on doing so? i don’t exactly know how to use python properly yet
Since samplers are just passed in as integer uniforms, you could probably set an int property on the object that sets up the 2D filter (properties on an object setting up a 2D filter get passed as uniforms to the 2D filter), and then setup the texture yourself using bgl/PyOpenGL and assign the generated OpenGL texture id to the object property. This would probably be best done in a pre-draw callback.
Edit: Forgot you don’t know Python yet. This may be useful after you learn (I believe SolarLune has some tutorials on his blog), or to someone else with a similar question if they know python and OpenGL.
There is a blend file here that uses two planes with GLSL shaders for effects.
Also lensflaresusing the same technique.