OpenGL vertex shader behaviour

Question

I'm learning OpenGL. And I just learned about matching an output variable in a vertex shader to an input variable in a fragment shader by giving them the same name.

So I wrote this vertex shader:

#version 330 core
layout (location = 0) in vec3 aPos;
out vec3 vertexColor;

void main()
{
   gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);
   vertexColor = vec3(aPos.x, aPos.y, 1);
}

and wrote this fragment shader:

#version 330 core
in vec3 vertexColor;
out vec4 FragColor;
void main()
{
   FragColor = vec4(vertexColor.xyz, 1.0f);
}

This is what I got:

I thought that vertex shaders were run once per vertex, but the each triangle is colored by a gradient, which is very confusing, since it seems that the vertex shader is run for every pixel, which I don't think is the case here. So, what is going on?

Rabbid76 · Accepted Answer · 2022-08-09 20:03:23Z

3

The vertex shader is executed for each vertex. The fragment shader is executed for each fragment (pixel). The outputs of the vertex shader of a primitive are interpolated for the fragments covered by that primitive. The interpolated value is the input for the fragment shader. This is how you get the gradient, because the vertices of the triangle have different color attributes and these colors are interpolated over the triangle.

answered Aug 9, 2022 at 20:03

Rabbid76

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4 Comments

Stack Exchange Broke The Law Over a year ago

In fact, this is the reason vertex and fragment shaders are separate! Data goes through the vertex shader, then the built-in interpolation, then the fragment shader

Alessandro Di Cicco Over a year ago

I hypothesized that might be the case, thanks! Also, is there any way to override the default interpolation logic? i.e.: could I specify a formula/function to be used in place of the default interpolation?

Rabbid76 Over a year ago

You can't specify your own formula, however you can use an Interpolation qualifier (e.g. flat)

Stack Exchange Broke The Law Over a year ago

@AlessandroDiCicco you could interpolate something like 0 to 1 (the default way) and then the fragment shader could calculate the real thing you want to interpolate - sometimes

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