Attribute-less-rendering-based cube map filitering GLSL shader code template

fragment.glsl

// Copyright (C) 2018, Benjamin "BeRo" Rosseaux (benjamin@rosseaux.de) - License: CC0
#version 330
layout(location = 0) out vec4 oOutput; 
in vec2 vTexCoord;
flat in int vFaceIndex; 
vec3 getCubeMapDirection(in vec2 uv, 
                         in int faceIndex){
  vec3 zDir = vec3(ivec3((faceIndex <= 1) ? 1 : 0, 
                         (faceIndex & 2) >> 1, 
                         (faceIndex & 4) >> 2)) * 
             (((faceIndex & 1) == 1) ? -1.0 : 1.0),
       yDir = (faceIndex == 2) 
                ? vec3(0.0, 0.0, 1.0) 
                : ((faceIndex == 3) 
                     ? vec3(0.0, 0.0, -1.0) 
                     : vec3(0.0, -1.0, 0.0)),
       xDir = cross(zDir, yDir);
  return normalize((mix(-1.0, 1.0, uv.x) * xDir) + 
                   (mix(-1.0, 1.0, uv.y) * yDir) + 
                   zDir);
}
void main(){
  vec3 direction = getCubeMapDirection(vTexCoord, vFaceIndex);
  // put your stuff here
  oOutput = vec4(direction, 1.0); 
}

vertex.glsl

// Copyright (C) 2018, Benjamin "BeRo" Rosseaux (benjamin@rosseaux.de) - License: CC0
#version 330
#extension GL_AMD_vertex_shader_layer : enable
out vec2 vTexCoord;
flat out int vFaceIndex; 
void main(){
  // For 18 vertices (6x attribute-less-rendered "full-screen" triangles)
  int vertexID = int(gl_VertexID),
      vertexIndex = vertexID % 3,
      faceIndex = vertexID / 3;  
  vec2 uv = vec2(ivec2(vertexIndex >> 1, vertexIndex & 1));   
  vTexCoord = uv * 2.0;
  vFaceIndex = faceIndex;
  gl_Position = vec4(vec3((uv * 4.0) - vec2(1.0), 0.0), 1.0);
  gl_Layer = faceIndex; 
}