Accessing barycentric coordinates inside fragment shader

6
votes

In the fragment shader, values are naturally interpolated. For example, if I have three vertices, each with a color, red for the first vertex, green for the second and blue for the third. If I render a triangle with them, the expected result is the common triangle.

Obviously, OpenGL calculates the interpolation coefficients (a, b, c) for each point inside the triangle. Is there any way to explicitly access these values or would I need to calculate the fragment coordinates of the three vertices and find the barycentric coordinates of the point myself? I know this is perfectly feasible, but I thought OpenGL could have provided something.

2
openglglslfragment-shader
GLSL Spec 4.5, Ch7, Builtins does not state anything about builtins having barycentric coordinates. Perhaps you could misuse Tesselation, however I cannot comment on whether this is feasible. – Stefan Hanke

2 Answers

10
votes

I'm not aware of any built-in for getting the barycentric coordinates. But you should't need any calculations in the fragment shader.

You can pass the barycentric coordinates of the triangle vertices as attributes into the vertex shader. The attribute values for the 3 vertices are simply (1, 0, 0), (0, 1, 0), and (0, 0, 1). Then pass the attribute value through to the fragment shader (using a varying variable in legacy OpenGL, out in vertex shader and in in fragment shader in core OpenGL). Then value of the variable received by the fragment shader are the barycentric coordinates of the fragment.

This is very similar to the way you would commonly pass texture coordinates into the vertex shader, and them pass them through to the fragment shader, which receives the interpolated values.