// Draw "fancy" wireframe, displaying front-facing, back-facing and
// silhouette lines differently.
// Mike Erwin, April 2015

// After working with this shader a while, convinced we should make
// separate shaders for perpective & ortho. (Oct 2016)

// Due to perspective, the line segment's endpoints might disagree on
// whether the adjacent faces are front facing. We use a geometry
// shader to resolve this properly.

uniform mat4 ModelViewMatrix;
uniform mat3 NormalMatrix;

in vec3 pos;
in vec3 N1, N2; // normals of faces this edge joins (object coords)

out vec4 MV_pos;
out float edgeClass;

// TODO: in float angle; // [-pi .. +pi], + peak, 0 flat, - valley

bool front(vec3 N, vec3 eye)
{
	return dot(NormalMatrix * N, eye) > 0.0;
}

void main()
{
	MV_pos = ModelViewMatrix * vec4(pos, 1.0);

	vec3 eye = normalize(-MV_pos.xyz);

	bool face_1_front = front(N1, eye);
	bool face_2_front = front(N2, eye);

	if (face_1_front && face_2_front)
		edgeClass = 1.0; // front-facing edge
	else if (face_1_front || face_2_front)
		edgeClass = 0.0; // exactly one face is front-facing, silhouette edge
	else
		edgeClass = -1.0; // back-facing edge
}