#version 460

layout(triangles, invocations=2) in;
in vec4 fca[]; 
layout(triangle_strip, max_vertices=9) out;
out vec4 fcolor;
uniform mat4 mvp;

float getavg(int idx){
	return 
		(gl_in[0].gl_Position[idx] + 
		gl_in[1].gl_Position[idx] + 
		gl_in[2].gl_Position[idx])
		/ 3;
}

void make_triangle(vec4 v1, vec4 c1, vec4 v2, vec4 c2, vec4 v3, vec4 c3){
	gl_Position = mvp * v1;
	fcolor = c1;
	EmitVertex();
	gl_Position = mvp * v2;
	fcolor = c2;
	EmitVertex();
	gl_Position = mvp * v3;
	fcolor = c3;
	EmitVertex();
	EndPrimitive();

}

void main(void) {

	if(gl_InvocationID == 1){
		make_triangle(
			gl_in[0].gl_Position, vec4(0, .6, .6, 1),
			gl_in[1].gl_Position, vec4(0, .6, .6, 1),
			gl_in[2].gl_Position, vec4(0, .6, .6, 1));
	}
	else {
		vec3 av = gl_in[2].gl_Position.xyz - gl_in[0].gl_Position.xyz;
		vec3 bv = gl_in[1].gl_Position.xyz - gl_in[0].gl_Position.xyz;
		vec4 normal = vec4(normalize(cross(av,bv)), 0);
	
		vec4 center = vec4(getavg(0), getavg(1), getavg(2), 1);
		vec4 top = center + normal;
		make_triangle(
			gl_in[0].gl_Position, fca[0],
			gl_in[1].gl_Position, fca[1],
			top, vec4(1, 1, 1, 1));
		make_triangle(
			gl_in[1].gl_Position, fca[1],
			gl_in[2].gl_Position, fca[2],
			top, vec4(1, 1, 1, 1));
		make_triangle(
			gl_in[2].gl_Position, fca[2],
			gl_in[0].gl_Position, fca[0],
			top, vec4(1, 1, 1, 1));
	}
	
}