#version 430 core
layout (triangles) in;
layout (triangle_strip, max_vertices = 3) out;
layout (std140, binding = 0) uniform TheMat { mat4 theMat; } ;
uniform vec3 lightPos;
void main()
{
    vec4 p0 = gl_in[0].gl_Position ;
    vec4 p1 = gl_in[1].gl_Position ;
    vec4 p2 = gl_in[2].gl_Position ;
    if ( dot( lightPos, cross( p0.xyz - p1.xyz, p1.xyz - p2.xyz)) < 0)
    {
        gl_Position = theMat * p0; EmitVertex();
        gl_Position = theMat * p1; EmitVertex();
        gl_Position = theMat * p2; EmitVertex();
        EndPrimitive();
    }
    else {}
}