diff --git a/src/image.rs b/src/image.rs
index d4ac604..f6fbd42 100644
--- a/src/image.rs
+++ b/src/image.rs
@@ -78,7 +78,7 @@ impl Extent {
     }
 }
 
-impl<P: Pixel> Image<P> {
+impl<P> Image<P> {
     pub fn load<F: Format<P>>(path: &str) -> Result<Image<P>, Error> {
         let file = std::fs::File::open(path)?;
         F::read(&mut std::io::BufReader::new(file))
@@ -89,16 +89,26 @@ impl<P: Pixel> Image<P> {
         F::write(&mut std::io::BufWriter::new(file), self)
     }
 
+	pub fn at_mut(&mut self, x: usize, y: usize) -> &mut P {
+        &mut self.pixels[y * self.extent.x + x]
+	}
+}
+
+impl<P: Clone + Default> Image<P> {
     pub fn with_extent(extent: Extent) -> Self {
         let mut pixels = Vec::with_capacity(extent.size());
         pixels.resize(extent.size(), P::default());
         Self { extent, pixels }
     }
+}
 
+impl<P: Copy> Image<P> {
     pub fn at(&self, x: usize, y: usize) -> P {
         self.pixels[y * self.extent.x + x]
     }
+}
 
+impl<P: Copy + Send + Sync> Image<P> {
     pub fn for_each<F>(&mut self, f: F)
     where
         F: Fn(usize, usize, &mut P) + Send + Sync,
@@ -118,7 +128,7 @@ impl<P: Pixel> Image<P> {
     pub fn map<F, Q>(&self, f: F) -> Image<Q>
     where
         F: Fn(P) -> Q + Send + Sync,
-        Q: Pixel,
+        Q: Copy + Default + Send + Sync,
     {
         let mut img = Image::with_extent(self.extent);
         img.for_each(|x, y, p| *p = f(self.at(x, y)));