Source file Psnr.ml

(* Claude Code
 *
 * Copyright (C) 2026 Yoann Padioleau
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public License
 * (LGPL) as published by the Free Software Foundation; either version
 * 2 of the License, or (at your option) any later version.
 *)

(* See Psnr.mli *)

let mse (a : Rgba_image.t) (b : Rgba_image.t) : float =
  if a.width <> b.width || a.height <> b.height then invalid_arg "Psnr.mse: not the same size";
  let sum = ref 0 in
  for i = 0 to (a.width * a.height) - 1 do
    for c = 0 to 2 do
      let d = a.rgba.{(4 * i) + c} - b.rgba.{(4 * i) + c} in
      sum := !sum + (d * d)
    done
  done;
  float_of_int !sum /. float_of_int (3 * a.width * a.height)

let of_mse (e : float) : float = if e = 0. then infinity else 10. *. log10 (255. *. 255. /. e)
let psnr (a : Rgba_image.t) (b : Rgba_image.t) : float = of_mse (mse a b)