Now, let's walk through each of the layers in this screen effect, break down why each of the lines of code is working the way it is, and get more insight about how we can add more to this screen effect.
Now that our old film screen effect is working, let's step through the lines of code in our frag() function, as all the other code should be pretty self-explanatory at this point in the book.
Just like our Photoshop layers, our shader is processing each layer and then compositing them together, so while we go through each layer, try to imagine how the layers in Photoshop work. Keeping this concept in mind always helps when developing new screen effects.
Here, we have the first set of lines of code in our frag() function: