Draw camera frustum in opengl (glsl)

[SkepticalFox]

Geometric shader:

#version 450 core

layout (points) in;
layout (line_strip, max_vertices = 24) out;

uniform mat4 invViewProj_to_draw;
uniform mat4 viewProj;

// in case of reversed-z it should be 0
#define NEAR_DEPTH -1

const vec4 f[8] = vec4[8](
    // near
    vec4(-1, -1, NEAR_DEPTH, 1),
    vec4(1,  -1, NEAR_DEPTH, 1),
    vec4(1,   1, NEAR_DEPTH, 1),
    vec4(-1,  1, NEAR_DEPTH, 1),
    // far
    vec4(-1, -1,  1, 1),
    vec4(1,  -1,  1, 1),
    vec4(1,   1,  1, 1),
    vec4(-1,  1,  1, 1)
);

void main(void)
{
    vec4 v[8];

    for (int i = 0; i < 8; i++) {
      vec4 ff = invViewProj_to_draw * f[i];
      v[i].xyz = ff.xyz / ff.w;
      v[i].w = 1.0f;
      v[i] = viewProj * v[i];
    }
  
    gl_Position = v[0];
    EmitVertex();
    gl_Position = v[1];
    EmitVertex();
    EndPrimitive(); // 1

    gl_Position = v[1];
    EmitVertex();
    gl_Position = v[2];
    EmitVertex();
    EndPrimitive(); // 2

    gl_Position = v[2];
    EmitVertex();
    gl_Position = v[3];
    EmitVertex();
    EndPrimitive(); // 3

    gl_Position = v[3];
    EmitVertex();
    gl_Position = v[0];
    EmitVertex();
    EndPrimitive(); // 4

    gl_Position = v[4];
    EmitVertex();
    gl_Position = v[5];
    EmitVertex();
    EndPrimitive(); // 5

    gl_Position = v[5];
    EmitVertex();
    gl_Position = v[6];
    EmitVertex();
    EndPrimitive(); // 6

    gl_Position = v[6];
    EmitVertex();
    gl_Position = v[7];
    EmitVertex();
    EndPrimitive(); // 7

    gl_Position = v[7];
    EmitVertex();
    gl_Position = v[4];
    EmitVertex();
    EndPrimitive(); // 8

    gl_Position = v[0];
    EmitVertex();
    gl_Position = v[4];
    EmitVertex();
    EndPrimitive(); // 9

    gl_Position = v[1];
    EmitVertex();
    gl_Position = v[5];
    EmitVertex();
    EndPrimitive(); // 10

    gl_Position = v[2];
    EmitVertex();
    gl_Position = v[6];
    EmitVertex();
    EndPrimitive(); // 11

    gl_Position = v[3];
    EmitVertex();
    gl_Position = v[7];
    EmitVertex();
    EndPrimitive(); // 12
}

Since the shader is useful, I post it here. I don’t remember I took it, or wrote it myself.