Getting the true z value from the depth buffer

Question

Sampling from a depth buffer in a shader returns values between 0 and 1, as expected. Given the near- and far- clip planes of the camera, how do I calculate the true z value

Answer 1

I ended up here trying to solve a similar problem when Nicol Bolas's comment on this page made me realize what I was doing wrong. If you want the distance to the camera and not the distance to the camera plane, you can compute it as follows (in GLSL):

float GetDistanceFromCamera(float depth, 
                            vec2 screen_pixel, 
                            vec2 resolution) {
  float fov = ... 
  float near = ...
  float far = ...
  float distance_to_plane = near / (far - depth * (far - near)) * far;


  vec2 center = resolution / 2.0f - 0.5;
  float focal_length = (resolution.y / 2.0f) / tan(fov / 2.0f);
  float diagonal = length(vec3(screen_pixel.x - center.x,
                               screen_pixel.y - center.y,
                               focal_length));

  return distance_to_plane * (diagonal / focal_length);
}

(source) Thanks to github user cassfalg: https://github.com/carla-simulator/carla/issues/2287