How often will a vertex shader be called?

Question

Just to clarify while learning this stuff:

If I have a scene with just one rectangle declared like this:

const Vertex Vertices[] = {
    {{1, -1, -7}, {1, 0, 0, 1}}, // X Y Z, R G B A
    {{1, 1, -7}, {0, 1, 0, 1}},
    {{-1, 1, -7}, {0, 0, 1, 1}},
    {{-1, -1, -7}, {0, 0, 0, 1}}
};

How often will the vertex shader be called to render one frame? I believe 4 times.

But there is some interpolation going on with varying variables in the vertex shader. The fragment shader gets called a lot more often (usually for each pixel once). Is this correct?

Answer 1

The vertex shader is called for each vertex in the primitive. The fragment shader is called for each fragment, in practice, this usually means for each pixel.