#version 450 core
struct PosCol { vec4 pospos; vec4 col; };
layout (std140, binding = 0) uniform TheMat { mat4 theMat; } ;
layout (std430, binding = 1) readonly buffer Pos { PosCol data[]; };
out vec4 vCol;
out vec4 vPos;
out vec2 vScreenPos;
int indices[6] = 
    {0,3,1
    ,0,2,3};
// note reversed compared to walls
void main()
{
    vCol = data[gl_VertexID / 12].col;
    vec4 pp = data[gl_VertexID / 12].pospos;
    int j = indices[gl_VertexID % 6];
    bool t = gl_VertexID % 12 > 5.5;
    float z = (j > 1.5 
            ? (t ? 100 : 5000)
            : (t ?   0 :  100)
            );
    // we draw two quads: one which can be below clouds,
    // and one which is always above
    vec2 xy = (j % 2 > 0 ? pp.xy : pp.zw);
    vPos = vec4( xy, z, 1);
    gl_Position = theMat * vPos;
    vScreenPos = 0.5 * (vec2(1) + vec2(gl_Position.x / gl_Position.w
                    ,gl_Position.y / gl_Position.w));
}