diff --git a/scripts/gen_sprite_map.ml b/scripts/gen_sprite_map.ml
index 1689de8..c94f87a 100644
--- a/scripts/gen_sprite_map.ml
+++ b/scripts/gen_sprite_map.ml
@@ -16,11 +16,21 @@ let sexp_of_clip c =
     List.map Sexplib.Conv.sexp_of_int
       [c.x; c.y; c.w; c.h; c.ox; c.oy])
 
-let sexp_of_sprite_map clips =
-  Sexp.List (
-    Atom "map" ::
-    (List.sort (fun a b -> compare b.name a.name) clips |>
-     List.rev_map sexp_of_clip))
+type sprite_map = {
+  dpi : string;
+  clips : clip list;
+}
+
+let sexp_of_sprite_map s =
+  Sexp.List [
+    Atom "map";
+    List [Atom "dpi"; Atom s.dpi];
+    List (
+      Atom "frames" ::
+      (List.sort (fun a b -> compare b.name a.name) s.clips |>
+       List.rev_map sexp_of_clip)
+    )
+  ]
 
 let svg v = ("http://www.w3.org/2000/svg", v)
 let ink v = ("http://www.inkscape.org/namespaces/inkscape", v)
@@ -107,10 +117,12 @@ let extract_clips xml =
   s.clips
 
 let gen_sprite_map ic oc =
-  let sprite_map =
-    Xmlm.make_input (`Channel ic) |>
-    extract_clips
-  in
+  let sprite_map = {
+    dpi = Sys.argv.(1);
+    clips =
+      Xmlm.make_input (`Channel ic) |>
+      extract_clips
+  } in
   Format.kasprintf (output_string oc) "%a\n"
     Sexp.pp_hum (sexp_of_sprite_map sprite_map)
 
diff --git a/scripts/gen_sprites.sh b/scripts/gen_sprites.sh
index a1896ac..ef5e104 100755
--- a/scripts/gen_sprites.sh
+++ b/scripts/gen_sprites.sh
@@ -28,7 +28,7 @@ function gen() {
         ($svg_to_png $src -o $dst_png -d $dpi || exit 1)
 
     [[ "$src" -nt "$dst_map" ]] &&
-        ($gen_sprite_map < $src > $dst_map || exit 1)
+        ($gen_sprite_map $dpi < $src > $dst_map || exit 1)
 }
 
 gen blocks 192
diff --git a/src/main.ml b/src/main.ml
index 5cd5d36..a5da44b 100644
--- a/src/main.ml
+++ b/src/main.ml
@@ -12,7 +12,7 @@ let main () =
   info (fun m -> m "renderer initialized");
 
   let ctx = Scene.make_context () in
-  Scene.register_sprite_map ctx (Asset.load_sprite_map "blocks" ~dpi:192);
+  Scene.register_sprite_map ctx (Asset.load_sprite_map "blocks");
   Scene.register_sprite_map ctx (Asset.load_sprite_map "hud");
   Scene.register_font ctx (Asset.load_font "roman-md");
   Scene.register_font ctx (Asset.load_font "mono-sm");
diff --git a/src/s2/s2.mli b/src/s2/s2.mli
index 94d08a4..46f00af 100644
--- a/src/s2/s2.mli
+++ b/src/s2/s2.mli
@@ -55,6 +55,6 @@ module Asset : sig
   val load_string : string -> string
   val load_file : string -> (bigstring -> 'a) -> 'a
   val load_sexp_conv : string -> (Sexp.t -> 'a) -> 'a
-  val load_sprite_map : ?dpi:int -> string -> Sprite_map.t
+  val load_sprite_map : string -> Sprite_map.t
   val load_font : string -> Font.t
 end
diff --git a/src/s2/sprite_map.ml b/src/s2/sprite_map.ml
index ecfb420..56e79f5 100644
--- a/src/s2/sprite_map.ml
+++ b/src/s2/sprite_map.ml
@@ -66,23 +66,25 @@ let frame_of_sexp ~pdf = function
   | sexp ->
     Sexp_conv.of_sexp_error "invalid sprite" sexp
 
-let of_sexp ~name ~texture ?dpi = function
-  | Sexp.List (Atom "map" :: args) ->
-    let pdf = match dpi with
-      | Some dpi -> Float.of_int dpi /. 96.0
-      | None -> 1.0
-    in
-    let frames = List.to_seq args |> Seq.map (frame_of_sexp ~pdf) in
+let of_sexp ~name ~texture = function
+  | Sexp.List [
+      Atom "map";
+      List [Atom "dpi"; dpi];
+      List (Atom "frames" :: frames)
+    ] ->
+    let dpi = Sexp_conv.int_of_sexp dpi in
+    let pdf = Float.of_int dpi /. 96.0 in
+    let frames = List.to_seq frames |> Seq.map (frame_of_sexp ~pdf) in
     make ~name ~texture ~frames
   | sexp ->
     Sexp_conv.of_sexp_error "invalid sprite map" sexp
 
 module Asset = struct
-  let load_sprite_map ?dpi name =
+  let load_sprite_map name =
     let tex_path = Format.sprintf "sprites/%s.png" name in
     let map_path = Format.sprintf "sprites/%s.map" name in
     let texture = Texture.Asset.load_texture tex_path in
-    let spritemap = Asset.load_sexp_conv map_path (of_sexp ~name ~texture ?dpi) in
+    let spritemap = Asset.load_sexp_conv map_path (of_sexp ~name ~texture) in
     debug (fun m -> m "loaded sprite map %S" name);
     spritemap
 end