#version 430 core
layout (triangles) in;
layout (triangle_strip, max_vertices = 3) out;
layout (std140, binding = 0) uniform TheMat { mat4 theMat; } ;
uniform vec3 lightPos;
// this code is duplicated in lineShadow.geom, should not be changed on its own
vec4 projNear (vec4 pos)
{
    // note we project to a specific height
    // this is quite brittle, not ideal
    vec3 dir = pos.xyz - lightPos ;
    float a = (140 - pos.z) / dir.z ;
    vec2 xy = (pos.xyz + a * dir).xy ;
    return vec4 ( xy, 140 , 1) ;
}
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( p0.xyz - vec3 (0,0,1), cross( p0.xyz - p1.xyz, p1.xyz - p2.xyz)) < 0
           // && 
            ( p0.z - lightPos.z > 0 )
            && ( p1.z - lightPos.z > 0 )
            && ( p2.z - lightPos.z > 0 )
            )
    {
        // front cap
        vec4 v1 = vec4 (0,0,1,0) ;
        gl_Position = theMat * projNear(p0); EmitVertex();
        gl_Position = theMat * projNear(p1); EmitVertex();
        gl_Position = theMat * projNear(p2); EmitVertex();
        EndPrimitive();
        // if light points downwards onto surface, draw back cap too
        if (dot ( p0.xyz - lightPos, vec3 ( 0,0,1) ) > 0)
        {
        }
        else {}
    }
    else {}
}