I dont understand for what i need the inverse of the perspective Matrix in Computer Graphics and how do i calculate it?Maybe someone has an explanation for me.
The inverse projection matrix can be used for mouse/ray picking (letting you click an object in the world)
An inverse projection matrix is used to invert coordinates from the projected space to either world or local space, i.e. You can use the inverse projection matrix to find out where a pixel is in relation to game objects.
This can be extremly useful for postprocessing effects like deferred rendering.
Calculating it can be a bit tricky and requires knowledge in linear algebra, if you have an API handy I suggest using it. Or google around for a while.