pass image as reference

src/descriptors/dct.rs

@@ -101,7 +101,7 @@ impl DCT {
 }
 
 impl Descriptor for DCT {
-    fn describe(&self, img: image::DynamicImage) -> u64 {
+    fn describe(&self, img: &image::DynamicImage) -> u64 {
         let resized = self.resize(img);
         let mut dct_values = self.dct(&resized);
         debug!("DCT-coefficients:\n {}", print_matrix(dct_values));