world position from depth buffer value

gistfile1.c

// Reconstruct worldspace depth value from z/w depth buffer
float depth = -(z_near * z_far) / (zbuffer_depth * (z_far - z_near) - z_far);	

// Compute screenspace coordinate of pixel
float2 screenspace = (float2(pixel.xy) / float2(viewport_size.xy)) * 2.0f - 1.0f;

// Get direction of ray from camera through pixel	
float3 ray_direction = normalize(camera_forward + camera_right * screenspace.x - camera_up * screenspace.y);
	
// Reconstruct world position from depth: depth in z buffer is distance to picture plane, not camera
float distance_to_camera = depth / dot(ray_direction, camera_forward);
float3 world_position = camera_position + ray_direction * distance_to_camera;