It looks like Pixie is having the peak cast a shadow onto the roof, but that isn't how it actually works. You can test this yourself if you have one of those white square erasers because it's really easy to see the shade on them and bend them. Even if you tilt them so that one side is in extreme shadow the entire side is of the same value. I tried it out and the only way to get the peak to throw a shadow onto the roof was to bend the eraser and make it a non flat surface. Indeed the only way I was able to cast a shadow in that manner actually had the opposite effect from what Pixie is drawing With the leading edge (closet to the light) in shadow and the trailing edge brighter.