blender-archive/source/blender/draw/modes/shaders/object_outline_prepass_geom.glsl

layout(lines_adjacency) in;
layout(line_strip, max_vertices = 2) out;

uniform mat4 ProjectionMatrix;

in vec4 pPos[];
in vec3 vPos[];

void main()
{
	bool is_persp = (ProjectionMatrix[3][3] == 0.0);

	vec3 view_vec = (is_persp) ? normalize(vPos[1]) : vec3(0.0, 0.0, -1.0);

	vec3 v10 = vPos[0] - vPos[1];
	vec3 v12 = vPos[2] - vPos[1];
	vec3 v13 = vPos[3] - vPos[1];

	vec3 n0 = cross(v12, v10);
	vec3 n3 = cross(v13, v12);

	float fac0 = dot(view_vec, n0);
	float fac3 = dot(view_vec, n3);

	/* If both adjacent verts are facing the camera the same way,
	 * then it isn't an outline edge. */
	if (sign(fac0) == sign(fac3))
		return;

	/* Don't outline if concave edge. */
	/* That would hide a lot of non useful edge but it flickers badly.
	 * TODO revisit later... */
	// if (dot(n0, v13) > 0.01)
	// 	return;

	gl_Position = pPos[1]; EmitVertex();
	gl_Position = pPos[2]; EmitVertex();
	EndPrimitive();
}