Now that we know how to decode the GBuffer, we can use it to calculate directional lighting. Unlike forward rendering, where we rendered the scene in order to trigger the pixel shader and do the light calculations, the whole point using a deferred approach is to avoid rendering the scene while doing the lighting. In order to trigger the pixel shader we will be rendering a mesh with a volume that encapsulates the GBuffer pixels affected by the light source. Directional light affects all the pixels in the scene, so we can render a full screen quad to trigger the pixel shader for every pixel in the GBuffer.
Back in the old days of Direct3D 9 you would need to prepare a vertex buffer to render a full screen quad. Fortunately, this is a thing of the past. With DirectX11 (as well as DirectX10) it is very easy to render a full screen quad with null buffers and sort out the values in the vertex shader. You will still need a constant buffer to store the same light information...