Before any light related work can be accomplished, all the data stored in the GBuffer has to be unpacked back into a format that we can use with the lighting code presented in the forward lighting recipes.
You may have noticed one thing missing from our GBuffer and that is the world space position needed for lighting. Although we could store per-pixel world positions in our GBuffer, those positions are implicitly stored already in the depth buffer as non-linear depth. Together with the projection parameters, we are going to reconstruct the linear depth of each pixel (world distance from the camera) as part of the GBuffer unpacking process. The linear depth will later be used together with the inversed view matrix to reconstruct the world position of each pixel.