diff --git a/src/model/mod.rs b/src/model/mod.rs
index 0d28d08..99a184e 100644
--- a/src/model/mod.rs
+++ b/src/model/mod.rs
@@ -5,11 +5,12 @@ pub mod material;
 
 use std::collections::HashMap;
 use std::collections::hash_map::Entry;
+use std::rc::Rc;
 
 use glium::VertexBuffer;
 use glium::texture::SrgbTexture2d;
 
-use parser::{parse, ParserError};
+use parser::{parse_file, ParserError};
 
 use model::face::{FaceVertex, Face};
 use model::material::Material;
@@ -38,11 +39,14 @@ pub struct Part {
     pub material_name: Option<String>,
 
     /// List of all the faces in the part.
-    pub faces: Vec<Face>,
+    faces: Vec<Face>,
 
     /// VertexBuffer for rendering.
     vertex_buffer: Option<VertexBuffer<Vertex>>,
 
+    /// True if the faces have been changed since the last time the buffer was built.
+    needs_update: bool,
+
 }
 
 impl Part {
@@ -53,6 +57,7 @@ impl Part {
             material_name: material_name,
             faces: vec![],
             vertex_buffer: None,
+            needs_update: true,
         }
     }
 
@@ -63,6 +68,7 @@ impl Part {
         let mut f = f;
         f.material_name = self.material_name.clone();
         self.faces.push(f);
+        self.needs_update = true;
     }
 
     /// Returns a reference to the vertex buffer.
@@ -70,6 +76,11 @@ impl Part {
         &self.vertex_buffer
     }
 
+    /// Returns a reference to the faces.
+    pub fn faces(&self) -> &Vec<Face> {
+        &self.faces
+    }
+
 
 }
 
@@ -91,7 +102,7 @@ pub struct Model {
     pub materials: HashMap<String, Material>,
 
     /// Map associating the name of a texture and the real texture.
-    pub textures: HashMap<String, SrgbTexture2d>,
+    pub textures: HashMap<String, Rc<SrgbTexture2d>>,
 
     /// The list of parts of the model.
     pub parts: Vec<Part>,
@@ -127,10 +138,57 @@ impl Model {
         }
     }
 
-    /// Merges the model passed as parameter into the current model.
-    pub fn merge(&mut self, other: Model) {
+    /// Copies the materials and textures from the other model into self.
+    pub fn merge_material_and_textures(&mut self, other: &Model) {
+        // Merge the materials
+        for (name, material) in &other.materials {
+            let entry = self.materials.entry(name.clone());
+            if let Entry::Occupied(_) = entry {
+                eprintln!("Warning: materials merged but sharing the same name");
+                // Return error
+            } else {
+                entry.or_insert(material.clone());
+            }
+        }
+
+        // Merge the textures
+        for (name, texture) in &other.textures {
+            let entry = self.textures.entry(name.clone());
+            if let Entry::Occupied(_) = entry {
+                eprintln!("Warning: textures merged but sharing the same name");
+                // return Error;
+            } else {
+                entry.or_insert(texture.clone());
+            }
+        }
+    }
+
+    /// Merges the model passed as parameter into new parts of the current model.
+    pub fn merge_in_new_parts(&mut self, other: Model) {
         let mut other = other;
 
+        // Merge the materials
+        for (name, material) in other.materials {
+            let entry = self.materials.entry(name);
+            if let Entry::Occupied(_) = entry {
+                eprintln!("Warning: materials merged but sharing the same name");
+                // Return error
+            } else {
+                entry.or_insert(material);
+            }
+        }
+
+        // Merge the textures
+        for (name, texture) in other.textures {
+            let entry = self.textures.entry(name);
+            if let Entry::Occupied(_) = entry {
+                eprintln!("Warning: textures merged but sharing the same name");
+                // return Error;
+            } else {
+                entry.or_insert(texture);
+            }
+        }
+
         let vertex_offset = self.vertices.len();
         let tex_coord_offset = self.texture_coordinates.len();
         let normal_offset = self.normals.len();
@@ -161,9 +219,17 @@ impl Model {
                 new_part.faces.push(face.clone());
                 self.faces.push(face.clone());
             }
+            new_part.needs_update = true;
             self.parts.push(new_part);
         }
 
+
+    }
+
+    /// Merges the model passed as parameter into existing parts of the current model.
+    pub fn merge_in_existing_parts(&mut self, other: Model) {
+        let mut other = other;
+
         // Merge the materials
         for (name, material) in other.materials {
             let entry = self.materials.entry(name);
@@ -185,11 +251,51 @@ impl Model {
                 entry.or_insert(texture);
             }
         }
+
+        let vertex_offset = self.vertices.len();
+        let tex_coord_offset = self.texture_coordinates.len();
+        let normal_offset = self.normals.len();
+        let face_offset = self.faces.len();
+
+        // Merge the elements
+        self.vertices.append(&mut other.vertices);
+        self.texture_coordinates.append(&mut other.texture_coordinates);
+        self.normals.append(&mut other.normals);
+
+        // Merge the parts and faces
+        for part in &mut other.parts {
+            self.change_part(part.material_name.clone());
+            let mut new_faces = vec![];
+            {
+                let new_part = self.current_part_mut().unwrap();
+                for face in &mut part.faces {
+                    for fv in &mut [&mut face.a, &mut face.b, &mut face.c] {
+                        fv.vertex += vertex_offset;
+                        if let Some(tex_coord) = fv.texture_coordinate {
+                            fv.texture_coordinate = Some(tex_coord + tex_coord_offset);
+                        }
+                        if let Some(normal) = fv.normal {
+                            fv.normal = Some(normal + normal_offset);
+                        }
+                    }
+                    if let Some(id) = face.id {
+                        face.id = Some(id + face_offset);
+                    }
+                    face.material_name = new_part.material_name.clone();
+                    new_part.faces.push(face.clone());
+                    new_faces.push(face);
+                }
+            }
+
+            for face in new_faces {
+                self.faces.push(face.clone());
+            }
+        }
     }
 
     /// Creates a model from a file.
     pub fn from(path: &str) -> Result<Model, ParserError> {
-        parse(path)
+        parse_file(path)
     }
 
     /// Computes the area of a 3D face of the model.
@@ -380,7 +486,7 @@ impl Model {
     /// Creates only non-existant vertex buffers, doesn't update the old ones.
     pub fn build_new_vertex_buffers(&mut self, renderer: &Renderer) {
         for part in &mut self.parts {
-            if part.vertex_buffer.is_none() {
+            if part.vertex_buffer.is_none() || part.needs_update {
                 Model::build_vertex_buffer_for_part(
                     &self.vertices,
                     &self.texture_coordinates,
@@ -400,7 +506,7 @@ impl Model {
         renderer: &Renderer) {
 
         let mut vertex_buffer = vec![];
-        for face in &part.faces {
+        for face in part.faces() {
             for &&v in &[&face.a, &face.b, &face.c] {
                 let vertex = vertices[v.vertex].into();
                 let tex_coord = if let Some(tex_index) = v.texture_coordinate {
@@ -424,6 +530,7 @@ impl Model {
         }
 
         part.vertex_buffer = Some(renderer.build_vertex_buffer(&vertex_buffer));
+        part.needs_update = false;
     }
 
     /// Builds the textures of all materials.
@@ -438,17 +545,22 @@ impl Model {
         if let Some(path) = material.textures.get("map_Kd") {
             let texture = renderer.make_texture(path);
             // Don't need to insert multiple times the same texture
-            self.textures.entry(path.to_owned()).or_insert(texture);
+            self.textures.entry(path.to_owned()).or_insert(Rc::new(texture));
         };
     }
 
     /// Returns the rendering texture.
-    pub fn get_texture_by_name(&self, name: &str) -> Option<&SrgbTexture2d> {
+    pub fn get_texture_by_name(&self, name: &str) -> Option<&Rc<SrgbTexture2d>> {
         self.textures.get(name)
     }
 
+    /// Returns the material.
+    pub fn get_material_by_name(&self, name: &str) -> Option<&Material> {
+        self.materials.get(name)
+    }
+
     /// Returns a ref mut to the table of textures.
-    pub fn textures_mut(&mut self) -> &mut HashMap<String, SrgbTexture2d> {
+    pub fn textures_mut(&mut self) -> &mut HashMap<String, Rc<SrgbTexture2d>> {
         &mut self.textures
     }
 }
diff --git a/src/parser/mod.rs b/src/parser/mod.rs
index ed5d659..2021ba7 100644
--- a/src/parser/mod.rs
+++ b/src/parser/mod.rs
@@ -6,7 +6,7 @@ pub mod mtl;
 use std::fmt;
 use std::path::{Path, PathBuf};
 use std::fs::File;
-use std::io::{BufReader, Error};
+use std::io::{BufReader, Error, Read};
 use math::vector::{Vector2, Vector3};
 use model::face::Face;
 use model::material::Material;
@@ -112,31 +112,46 @@ pub enum ParserError {
     OtherError(String),
 }
 
+/// Parses a content from a reader, guess the file type from path, and return the model.
+pub fn parse<R: Read>(reader: R, path: &str) -> Result<Model, ParserError> {
+    let mut model = Model::new();
+    parse_into_model(reader, path, &mut model)?;
+    Ok(model)
+}
+
+/// Parses a file and append the result to the model passed as parameter.
+pub fn parse_file_into_model(path: &str, model: &mut Model) -> Result<(), ParserError> {
+    let file = match File::open(path) {
+        Ok(file) => file,
+        Err(e) => return Err(ParserError::IoError(path.to_owned(), e)),
+    };
+
+    let bufreader = BufReader::new(file);
+    parse_into_model(bufreader, path, model)?;
+
+    Ok(())
+}
+
 /// Detects the format from path name and parses it into the model.
-pub fn parse_into_model(path: &str, model: &mut Model) -> Result<(), ParserError> {
+pub fn parse_into_model<R: Read>(reader: R, path: &str, model: &mut Model) -> Result<(), ParserError> {
     let extension = Path::new(path).extension().unwrap().to_str().unwrap();
 
     use self::obj::ObjParser;
     use self::mtl::MtlParser;
 
-    let mut parser: Option<Box<Parser>> = match extension {
-        "obj" => Some(Box::new(ObjParser::new())),
-        "mtl" => Some(Box::new(MtlParser::new())),
-        _ => None,
-    };
-
-    if let Some(parser) = parser.as_mut() {
-        parser.parse_into_model(model, path)?;
-        Ok(())
-    } else {
-        Err(ParserError::UnknownFormat(path.to_owned()))
+    match extension {
+        "obj" => ObjParser::new().parse_into_model(model, reader, path)?,
+        "mtl" => MtlParser::new().parse_into_model(model, reader, path)?,
+        _ => return Err(ParserError::UnknownFormat(path.to_owned())),
     }
+
+    Ok(())
 }
 
 /// Detects the format from path name and parses it into a new model.
-pub fn parse(path: &str) -> Result<Model, ParserError> {
+pub fn parse_file(path: &str) -> Result<Model, ParserError> {
     let mut model = Model::new();
-    parse_into_model(path, &mut model)?;
+    parse_file_into_model(path, &mut model)?;
     Ok(model)
 }
 
@@ -171,22 +186,22 @@ impl fmt::Display for ParserError {
 pub trait Parser {
 
     /// Parses a file and adds its content into an empty model and returns the model.
-    fn parse(&mut self, path: &str) -> Result<Model, ParserError> {
+    fn parse<R: Read>(&mut self, reader: R, path: &str) -> Result<Model, ParserError> {
         let mut model = Model::new();
-        self.parse_into_model(&mut model, path)?;
+        self.parse_into_model(&mut model, reader, path)?;
         Ok(model)
     }
 
     /// Parses a file and adds its content to an already existing model.
-    fn parse_into_model(&mut self, model: &mut Model, path: &str) -> Result<(), ParserError> {
+    fn parse_into_model<R: Read>(&mut self, model: &mut Model, reader: R, path: &str) -> Result<(), ParserError> {
         logln!("Parsing {}...", path);
-        self.priv_parse_into_model(model, path)
+        self.priv_parse_into_model(model, reader, path)
     }
 
     /// Parses a file and adds its content to an already existing model.
     ///
     /// This is the method you should implement for you format parser.
-    fn priv_parse_into_model(&mut self, model: &mut Model, path: &str) -> Result<(), ParserError>;
+    fn priv_parse_into_model<R: Read>(&mut self, model: &mut Model, reader: R, path: &str) -> Result<(), ParserError>;
 }
 
 /// Anything that can parse a 3D file line by line.
@@ -200,16 +215,9 @@ pub trait LineParser {
 
 impl<LP: LineParser> Parser for LP {
 
-    fn priv_parse_into_model(&mut self, model: &mut Model, path: &str) -> Result<(), ParserError> {
+    fn priv_parse_into_model<R: Read>(&mut self, model: &mut Model, reader: R, path: &str) -> Result<(), ParserError> {
 
-        let file = File::open(path);
-
-        if file.is_err() {
-            return Err(ParserError::IoError(path.to_owned(), file.err().unwrap()));
-        }
-
-        let file = file.unwrap();
-        let file = BufReader::new(&file);
+        let file = BufReader::new(reader);
 
         let mut current_material_name = None;
 
@@ -218,11 +226,10 @@ impl<LP: LineParser> Parser for LP {
 
             let num = num + 1;
 
-            if line.is_err() {
-                return Err(ParserError::OtherError(path.to_owned()));
-            }
-
-            let line = line.unwrap();
+            let line = match line {
+                Ok(x) => x,
+                Err(e) => return Err(ParserError::IoError(path.to_owned(), e)),
+            };
 
             let element = self.parse_line(num, &line, &path)?;
 
@@ -238,7 +245,7 @@ impl<LP: LineParser> Parser for LP {
                     let mut path = PathBuf::from(path);
                     path.pop();
                     path.push(material_path);
-                    if let Err(err) = parse_into_model(path.to_str().unwrap(), model) {
+                    if let Err(err) = parse_file_into_model(path.to_str().unwrap(), model) {
                         return Err(err);
                     }
                 },
diff --git a/src/programs/viewer.rs b/src/programs/viewer.rs
index 6011e37..4c4fa48 100644
--- a/src/programs/viewer.rs
+++ b/src/programs/viewer.rs
@@ -20,7 +20,7 @@ use glium::glutin::VirtualKeyCode;
 use model_converter::scene::Scene;
 use model_converter::math::bounding_box::BoundingBox3;
 use model_converter::math::vector::Vector3;
-use model_converter::parser::parse;
+use model_converter::parser::parse_file;
 use model_converter::renderer::Renderer;
 use model_converter::controls::{OrbitControls, FirstPersonControls};
 use model_converter::camera::Camera;
@@ -65,7 +65,7 @@ fn main() {
     let mut models = vec![];
 
     for input in matches.values_of("input").unwrap() {
-        match parse(&input) {
+        match parse_file(&input) {
             Ok(model) => {
                 if model.vertices.len() > 0 {
                     bbox = bbox.union(&model.bounding_box());
diff --git a/src/scene.rs b/src/scene.rs
index 38a9e09..abd1d65 100644
--- a/src/scene.rs
+++ b/src/scene.rs
@@ -1,6 +1,6 @@
 //! This module contains everything related to scenes.
 
-use std::slice::Iter;
+use std::slice::{Iter, IterMut};
 use std::vec::IntoIter;
 use std::rc::Rc;
 use std::cell::RefCell;
@@ -33,6 +33,11 @@ impl Scene {
     pub fn iter(&self) -> Iter<Rc<RefCell<Model>>> {
         self.models.iter()
     }
+
+    /// Returns a mutable iterator to the model of the scene.
+    pub fn iter_mut(&mut self) -> IterMut<Rc<RefCell<Model>>> {
+        self.models.iter_mut()
+    }
 }
 
 impl IntoIterator for Scene {