Direct3d/C# - Blur area under a rectangle drawn on a sprite

Question

Any idea how to do it? I am drawing a rectangle that is supposed to be a half-transparent window. I managed to do the transparency by drawing a half-transparent texture, but I also want to blur whatever is under the window.

Normally (eg. using GDI) I would create a bitmap of the area, blur it and paint it as the background of my window. With Direct3D I don't even know how to get the area with whatever is already rendered on it. Or even there can be a different approach, can't it. Please help.

Answer 1

The D3D way is to use a pixel shader to "blur" the area underneath your rect.

This link shows you how to use a pixel shader in C#.
And this link has a guassian blur pixel shader.

Answer 2

It DOES require having your backbuffer as a texture. You can then render the whole thing to a NEW texture and blur the relevant part before putting your semi-trans window over the new texture.

Edit: AFAIK you can't use the Draw function inside a shader. You will need to write your own sprite renderer. The Begin and Draw set up a whole load of states that will break your usage of a vertex shader.