1 files changed, 17 insertions, 7 deletions

diff --git a/src/isogfx.c b/src/isogfx.c
index 0aaeb9d..5d23ae4 100644
--- a/src/isogfx.c
+++ b/src/isogfx.c
@@ -55,6 +55,7 @@ typedef struct SpriteInstance {
 typedef struct IsoGfx {
   Screen          screen;
   CoordSystem     iso_space;
+  ivec2           camera;
   double          last_animation_time;
   Tile            next_tile; // For procedurally-generated tiles.
   Tm_Map*         map;
@@ -76,6 +77,8 @@ static inline ivec2 ivec2_scale(ivec2 a, int s) {
   return (ivec2){.x = a.x * s, .y = a.y * s};
 }
 
+static inline ivec2 ivec2_neg(ivec2 a) { return (ivec2){.x = -a.x, .y = -a.y}; }
+
 static inline ivec2 iso2cart(ivec2 iso, int s, int t, int w) {
   return (ivec2){.x = (iso.x - iso.y) * (s / 2) + (w / 2),
                  .y = (iso.x + iso.y) * (t / 2)};
@@ -490,13 +493,13 @@ void isogfx_update(IsoGfx* iso, double t) {
 /// Get the screen position of the top diamond-corner of the tile at world
 /// (x,y).
 static ivec2 GetTileScreenOrigin(
-    const CoordSystem iso_space, int world_x, int world_y) {
+    const CoordSystem iso_space, ivec2 camera, int world_x, int world_y) {
   const ivec2 vx_offset = ivec2_scale(iso_space.x, world_x);
   const ivec2 vy_offset = ivec2_scale(iso_space.y, world_y);
   const ivec2 screen_origin =
       ivec2_add(iso_space.o, ivec2_add(vx_offset, vy_offset));
-
-  return screen_origin;
+  const ivec2 origin_view_space = ivec2_add(screen_origin, ivec2_neg(camera));
+  return origin_view_space;
 }
 
 static Pixel alpha_blend(Pixel src, Pixel dst) {
@@ -598,8 +601,9 @@ static void draw_world(IsoGfx* iso) {
   // screen-centric approach would juggle multiple tiles throughout the scan.
   for (int wy = 0; wy < iso->map->world_height; ++wy) {
     for (int wx = 0; wx < iso->map->world_width; ++wx) {
-      const Tile  tile          = tm_layer_get_tile(iso->map, layer, wx, wy);
-      const ivec2 screen_origin = GetTileScreenOrigin(iso->iso_space, wx, wy);
+      const Tile  tile = tm_layer_get_tile(iso->map, layer, wx, wy);
+      const ivec2 screen_origin =
+          GetTileScreenOrigin(iso->iso_space, iso->camera, wx, wy);
       draw_tile(iso, screen_origin, tile);
     }
   }
@@ -631,11 +635,16 @@ static void draw_sprites(IsoGfx* iso) {
     assert(sheet);
 
     const ivec2 screen_origin = GetTileScreenOrigin(
-        iso->iso_space, sprite->position.x, sprite->position.y);
+        iso->iso_space, iso->camera, sprite->position.x, sprite->position.y);
     draw_sprite(iso, screen_origin, sprite, sheet);
   }
 }
 
+void isogfx_set_camera(IsoGfx* iso, int x, int y) {
+  assert(iso);
+  iso->camera = (ivec2){x, y};
+}
+
 void isogfx_render(IsoGfx* iso) {
   assert(iso);
   draw_world(iso);
@@ -649,7 +658,8 @@ void isogfx_draw_tile(IsoGfx* iso, int x, int y, Tile tile) {
   assert(x < iso->map->world_width);
   assert(y < iso->map->world_height);
 
-  const ivec2 screen_origin = GetTileScreenOrigin(iso->iso_space, x, y);
+  const ivec2 screen_origin =
+      GetTileScreenOrigin(iso->iso_space, iso->camera, x, y);
   draw_tile(iso, screen_origin, tile);
 }