diff --git a/src/gfx.cpp b/src/gfx.cpp
index ef9f854..fa3bed1 100644
--- a/src/gfx.cpp
+++ b/src/gfx.cpp
@@ -7,9 +7,7 @@
 
 #include "io.hpp"
 
-const int WIDTH = 640;
-const int HEIGHT = 480;
-const float ASPECT = (float)WIDTH / (float)HEIGHT;
+float aspect_;
 
 // Initialize GLFW, OpenGL and GLEW, open a window and make it the current context.
 // Returns 0 for success, and -1 for any failure.
@@ -25,8 +23,10 @@ int initGraphics(GLFWwindow** window, const std::string& title)
     glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
     glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
     glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
+    glfwWindowHint(GLFW_RESIZABLE, GLFW_TRUE);
+
+    *window = glfwCreateWindow(640, 480, title.c_str(), NULL, NULL);
 
-    *window = glfwCreateWindow(WIDTH, HEIGHT, title.c_str(), NULL, NULL);
     if (!window)
     {
         glfwTerminate();
@@ -34,6 +34,8 @@ int initGraphics(GLFWwindow** window, const std::string& title)
         return -1;
     }
 
+    glfwSetWindowSizeCallback(*window, windowSizeCallback);
+
     glfwMakeContextCurrent(*window);
 
     glewExperimental = GL_TRUE;
@@ -48,6 +50,12 @@ int initGraphics(GLFWwindow** window, const std::string& title)
     return 0;
 }
 
+void windowSizeCallback(GLFWwindow* window, int width, int height)
+{
+    aspect_ = (float)width / (float)height;
+    glViewport(0, 0, width, height);
+}
+
 GLuint compileShader(const std::string& shaderPath, GLenum shaderType)
 {
     GLuint shader;
@@ -102,7 +110,7 @@ GLuint compileShaderProgram(const std::string& fragShaderPath)
 
 void updateProjectionMatrix(GLuint shaderProgram)
 {
-    float left = -ASPECT, right = ASPECT, bottom = -1.0, top = 1.0, near = -1.0, far = 1.0;
+    float left = -aspect_, right = aspect_, bottom = -1.0, top = 1.0, near = -1.0, far = 1.0;
     glm::mat4 projection = glm::ortho(left, right, bottom, top, near, far);
     GLint projectionLocation = getShaderUniformLocation(shaderProgram, "_Projection");
     glUniformMatrix4fv(projectionLocation, 1, GL_FALSE, &projection[0][0]);
diff --git a/src/gfx.hpp b/src/gfx.hpp
index f15bbda..1de9fac 100644
--- a/src/gfx.hpp
+++ b/src/gfx.hpp
@@ -5,11 +5,8 @@
 #include "GL/glew.h"
 #include <GLFW/glfw3.h>
 
-extern const int WIDTH;
-extern const int HEIGHT;
-extern const float ASPECT;
-
 int initGraphics(GLFWwindow** window, const std::string& title);
+void windowSizeCallback(GLFWwindow* window, int width, int height);
 
 GLuint compileShaderProgram(const std::string& fragShaderPath);
 GLuint compileShader(const std::string& shaderPath, GLenum shaderType);
@@ -18,4 +15,4 @@ GLint getShaderUniformLocation(GLuint shaderProgram, const std::string& uniformN
 void updateProjectionMatrix(GLuint shaderProgram);
 void updateModelMatrix(GLuint shaderProgram, float time);
 void updateViewMatrix(GLuint shaderProgram);
-void updateModelViewProjectionMatrix(GLuint shaderProgram, float time);
\ No newline at end of file
+void updateModelViewProjectionMatrix(GLuint shaderProgram, float time);