diff --git a/main.cpp b/main.cpp
index 68fd61b..4d554d2 100644
--- a/main.cpp
+++ b/main.cpp
@@ -60,6 +60,22 @@ void makeRotationZ(float angle, float* m)
     m[3] =  0; m[7] =  0; m[11] = 0; m[15] = 1;
 }
 
+void computeLetterbox(int windowW, int windowH,
+                      int logicalW, int logicalH,
+                      int* x, int* y, int* w, int* h)
+{
+    float scaleX = (float)windowW / logicalW;
+    float scaleY = (float)windowH / logicalH;
+
+    float scale = (scaleX < scaleY) ? scaleX : scaleY;
+
+    *w = (int)(logicalW * scale);
+    *h = (int)(logicalH * scale);
+
+    *x = (windowW - *w) / 2;
+    *y = (windowH - *h) / 2;
+}
+
 extern "C" int APP_MAIN(int argc, char **argv) {
     SDL_SetAppMetadata("Example Renderer Primitives", "1.0", "com.example.renderer-primitives");
     SDL_SetHint(SDL_HINT_ORIENTATIONS, "LandscapeLeft LandscapeRight");
@@ -145,7 +161,15 @@ extern "C" int APP_MAIN(int argc, char **argv) {
     glBindBuffer(GL_ARRAY_BUFFER, 0);
     glBindVertexArray(0);
 
-    glViewport(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);
+    int drawableW, drawableH;
+    SDL_GetWindowSizeInPixels(window, &drawableW, &drawableH);
+
+    int vx, vy, vw, vh;
+    computeLetterbox(drawableW, drawableH,
+                     SCREEN_WIDTH, SCREEN_HEIGHT,
+                     &vx, &vy, &vw, &vh);
+
+    glViewport(vx, vy, vw, vh);
     glEnable(GL_DEPTH_TEST);
 
     float transform[16];