How can any interactions between these two shaders work? I mean the vertex shader sends a vertex's color to the fragment shader, but that's only one fragment, what about all the other ones between the vertices?
Does the fragment shader get called a lot more than the vertex shader?
I mean the fragment-to-vertex ratio is pretty high in most games, given that a fragment is a pixel, right?
Vertex shaders gets called a lot less than fragment shaders. It's because the fragments get generated from the vertices with a process that is called rasterization. Rasterization basically generates fragments using the triangle's corners and give each fragment an interpolated value using the values in the vertices, produced by a vertex shader.
Let's say that you have a triangle ABC like this:
a = 0,0,0
b = 1,0,0
c = 1,1,0
Depending on the resolution, this generates some amount of fragments covering the whole triangle.
This will produce a triangle which covers half of the screen, which at 1920x1200 resolution will be 1920*1200/2 fragments. The amount of fragments increase with every triangle you draw.
A fragment is not a pixel. It is much more.
