Added psnr from mse

Cargo.lock

@@ -182,6 +182,7 @@ name = "psnr"
 version = "0.1.0"
 dependencies = [
  "image 0.17.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "lazy_static 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]

Cargo.toml

@@ -5,6 +5,7 @@ authors = ["Thomas Forgione <thomas@tforgione.fr>"]
 
 [dependencies]
 image = "*"
+lazy_static = "*"
 
 [[bin]]
 name = "psnr"

src/lib.rs

@@ -1,7 +1,14 @@
+#[macro_use]
+extern crate lazy_static;
 extern crate image;
 
 use image::{Pixel, GenericImage, ImageResult, Rgba};
 
+lazy_static! {
+    /// This values is 20 * log10(255)
+    pub static ref TWENTY_LOG10_MAX: f64 = 20.0 * (std::u8::MAX as f64).log10();
+}
+
 /// Compute a MSE between an image and its color.
 ///
 /// Will panic an error if the two images don't have the same size.
@@ -58,9 +65,15 @@ pub fn mse<P: Pixel<Subpixel=u8>, T: GenericImage<Pixel=P>>(img1: &T, img2: &T)
     mse / (img1.dimensions().0 * img1.dimensions().1 * 3) as f64
 }
 
+/// Compute the PSNR from the MSE.
+/// Returns 10 * log10(255^2) - 10 * log10(mse)
+pub fn psnr_from_mse(mse: f64) -> f64 {
+    *TWENTY_LOG10_MAX - 10.0 * mse.log10()
+
+}
+
 pub fn psnr<P: Pixel<Subpixel=u8>, T: GenericImage<Pixel=P>>(img1: &T, img2: &T) -> f64 {
-    let twenty_log10_max: f64 = 20.0 * (std::u8::MAX as f64).log10();
-    twenty_log10_max - 10.0 * mse(img1, img2).log10()
+    *TWENTY_LOG10_MAX - 10.0 * mse(img1, img2).log10()
 }
 
 pub fn psnr_files(path1: &str, path2: &str) -> ImageResult<f64> {