If you imagine extending the gray shaded region down to the right so that it has a single bottom right corner, then standing at that corner and looking at the texture space will give you the image of the screen space on the left.
Jordanwyli
I also initially had a hard time visualizing how this mapping makes sense. But if you were to imagine this was a table and the screen space was looking at it very closely, you can imagine the texture space to be looking at the table from top down, at a reasonably far distance.
If you imagine extending the gray shaded region down to the right so that it has a single bottom right corner, then standing at that corner and looking at the texture space will give you the image of the screen space on the left.
I also initially had a hard time visualizing how this mapping makes sense. But if you were to imagine this was a table and the screen space was looking at it very closely, you can imagine the texture space to be looking at the table from top down, at a reasonably far distance.