Use input->screen in tutorial

TUTORIAL.md

@@ -111,20 +111,21 @@ Now, what about mobile devices? We may not have a keyboard, so let's create an `
     )
 ```
 
-In this case, the screen map will contain an `:input-x` and `:input-y` for the point on the screen that was touched. We can simply check to see what part of the screen this point is by using `game` to get the overall game's width and height.
+In this case, the screen map will contain an `:input-x` and `:input-y` for the point on the screen that was touched. These are input coordinates, so normally it is best to convert them into screen coordinates with [input->screen](http://oakes.github.io/play-clj/core.input_screen.html) so they're using the same coordinate system as the entities.
 
 ```clojure
   :on-touch-down
   (fn [screen entities]
-    (cond
-      (> (:input-y screen) (* (game :height) (/ 2 3)))
-      (println "down")
-      (< (:input-y screen) (/ (game :height) 3))
-      (println "up")
-      (> (:input-x screen) (* (game :width) (/ 2 3)))
-      (println "right")
-      (< (:input-x screen) (/ (game :width) 3))
-      (println "left")))
+    (let [pos (input->screen screen (:input-x screen) (:input-y screen))]
+      (cond
+        (> (:y pos) (* (game :height) (/ 2 3)))
+        (println "up")
+        (< (:y pos) (/ (game :height) 3))
+        (println "down")
+        (> (:x pos) (* (game :width) (/ 2 3)))
+        (println "right")
+        (< (:x pos) (/ (game :width) 3))
+        (println "left"))))
 ```
 
 Conveniently, the `:on-touch-down` function also runs when a mouse is clicked on the screen, so we are adding mouse support to the game as well!
@@ -160,15 +161,16 @@ Now we can update our `:on-key-down` and `:on-touch-down` functions to move the
       (move (first entities) :left)))
   :on-touch-down
   (fn [screen entities]
-    (cond
-      (> (:input-y screen) (* (game :height) (/ 2 3)))
-      (move (first entities) :down)
-      (< (:input-y screen) (/ (game :height) 3))
-      (move (first entities) :up)
-      (> (:input-x screen) (* (game :width) (/ 2 3)))
-      (move (first entities) :right)
-      (< (:input-x screen) (/ (game :width) 3))
-      (move (first entities) :left)))
+    (let [pos (input->screen screen (:input-x screen) (:input-y screen))]
+      (cond
+        (> (:y pos) (* (game :height) (/ 2 3)))
+        (move (first entities) :up)
+        (< (:y pos) (/ (game :height) 3))
+        (move (first entities) :down)
+        (> (:x pos) (* (game :width) (/ 2 3)))
+        (move (first entities) :right)
+        (< (:x pos) (/ (game :width) 3))
+        (move (first entities) :left))))
 ```
 
 ## Camera