Offset sprites properlyHEADmain

2 files changed, 52 insertions, 44 deletions

diff --git a/demos/isomap/isomap.c b/demos/isomap/isomap.c
index b9b6568..bca27f6 100644
--- a/demos/isomap/isomap.c
+++ b/demos/isomap/isomap.c
@@ -59,7 +59,7 @@ static bool init(GfxApp* app, GfxAppState* state, int argc, const char** argv) {
   }
 
   state->stag = isogfx_make_sprite(iso, state->stag_sheet);
-  isogfx_set_sprite_position(iso, state->stag, 5, 4);
+  isogfx_set_sprite_position(iso, state->stag, 0, 0);
 
   if (!((state->backend = iso_backend_init(iso)))) {
     return false;
diff --git a/src/gfx2d.c b/src/gfx2d.c
index da265b0..9767308 100644
--- a/src/gfx2d.c
+++ b/src/gfx2d.c
@@ -79,17 +79,49 @@ static inline ivec2 ivec2_scale(ivec2 a, int 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)};
-}
-
 static inline vec2 vec2_add(vec2 a, vec2 b) {
   return (vec2){.x = a.x + b.x, .y = a.y + b.y};
 }
 
 static inline vec2 ivec2_to_vec2(ivec2 a) { return (vec2){a.x, a.y}; }
 
+// Not actually used because we pre-compute the two axis vectors instead.
+// See make_iso_coord_system() and the other definition of iso2cart() below.
+// 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)};
+// }
+
+/// Create the basis for the isometric coordinate system with origin and vectors
+/// expressed in the Cartesian system.
+static CoordSystem make_iso_coord_system(
+    const Tm_Map* const map, const Screen* const screen) {
+  assert(map);
+  assert(screen);
+  const ivec2 o = {screen->width / 2, 0};
+  const ivec2 x = {
+      .x = map->base_tile_width / 2, .y = map->base_tile_height / 2};
+  const ivec2 y = {
+      .x = -map->base_tile_width / 2, .y = map->base_tile_height / 2};
+  return (CoordSystem){o, x, y};
+}
+
+/// Map isometric coordinates to Cartesian coordinates.
+///
+/// For a tile, this gets the screen position of the top diamond-corner of the
+/// tile.
+///
+/// Takes the camera displacement into account.
+static ivec2 iso2cart(
+    const CoordSystem iso_space, ivec2 camera, int iso_x, int iso_y) {
+  const ivec2 vx_offset = ivec2_scale(iso_space.x, iso_x);
+  const ivec2 vy_offset = ivec2_scale(iso_space.y, iso_y);
+  const ivec2 screen_origin =
+      ivec2_add(iso_space.o, ivec2_add(vx_offset, vy_offset));
+  const ivec2 origin_view_space = ivec2_add(screen_origin, ivec2_neg(camera));
+  return origin_view_space;
+}
+
 // Method 1.
 // static inline vec2 cart2iso(vec2 cart, int s, int t, int w) {
 //  const double x    = cart.x - (double)(w / 2);
@@ -125,20 +157,6 @@ static inline Pixel* screen_xy_mut(Screen* screen, int x, int y) {
   return (Pixel*)screen_xy_const_ref(screen, x, y);
 }
 
-/// Create the basis for the isometric coordinate system with origin and vectors
-/// expressed in the Cartesian system.
-static CoordSystem make_iso_coord_system(
-    const Tm_Map* const map, const Screen* const screen) {
-  assert(map);
-  assert(screen);
-  const ivec2 o = {screen->width / 2, 0};
-  const ivec2 x = {
-      .x = map->base_tile_width / 2, .y = map->base_tile_height / 2};
-  const ivec2 y = {
-      .x = -map->base_tile_width / 2, .y = map->base_tile_height / 2};
-  return (CoordSystem){o, x, y};
-}
-
 // -----------------------------------------------------------------------------
 // Renderer, world and tile management.
 // -----------------------------------------------------------------------------
@@ -498,18 +516,6 @@ void isogfx_update(IsoGfx* iso, double t) {
 // Rendering and picking.
 // -----------------------------------------------------------------------------
 
-/// Get the screen position of the top diamond-corner of the tile at world
-/// (x,y).
-static ivec2 GetTileScreenOrigin(
-    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));
-  const ivec2 origin_view_space = ivec2_add(screen_origin, ivec2_neg(camera));
-  return origin_view_space;
-}
-
 static Pixel alpha_blend(Pixel src, Pixel dst) {
   if ((src.a == 255) || (dst.a == 0)) {
     return src;
@@ -609,17 +615,15 @@ static void draw_map(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, iso->camera, wx, wy);
+      const Tile  tile          = tm_layer_get_tile(iso->map, layer, wx, wy);
+      const ivec2 screen_origin = iso2cart(iso->iso_space, iso->camera, wx, wy);
       draw_tile(iso, screen_origin, tile);
     }
   }
 }
 
 static void draw_sprite(
-    IsoGfx* iso, ivec2 origin, const SpriteInstance* sprite,
-    const Ss_SpriteSheet* sheet) {
+    IsoGfx* iso, const SpriteInstance* sprite, const Ss_SpriteSheet* sheet) {
   assert(iso);
   assert(sprite);
   assert(sheet);
@@ -627,10 +631,18 @@ static void draw_sprite(
   assert(sprite->animation < sheet->num_rows);
   assert(sprite->frame >= 0);
 
+  // Apply an offset similarly to how we offset tiles. The sprite is offset by
+  // -base_tile_width/2 along the x-axis to align the sprite with the leftmost
+  // edge of the tile it is on.
+  const ivec2 screen_origin = iso2cart(
+      iso->iso_space, iso->camera, sprite->position.x, sprite->position.y);
+  const ivec2 offset   = {-(iso->map->base_tile_width / 2), 0};
+  const ivec2 top_left = ivec2_add(screen_origin, offset);
+
   const Ss_Row*  row   = ss_get_sprite_sheet_row(sheet, sprite->animation);
   const uint8_t* frame = ss_get_sprite_sheet_sprite(sheet, row, sprite->frame);
   draw_rect(
-      &iso->screen, origin, sheet->sprite_width, sheet->sprite_height,
+      &iso->screen, top_left, sheet->sprite_width, sheet->sprite_height,
       sheet->palette.colours, frame);
 }
 
@@ -641,10 +653,7 @@ static void draw_sprites(IsoGfx* iso) {
        sprite                       = sprite->next) {
     const Ss_SpriteSheet* sheet = sprite->sheet;
     assert(sheet);
-
-    const ivec2 screen_origin = GetTileScreenOrigin(
-        iso->iso_space, iso->camera, sprite->position.x, sprite->position.y);
-    draw_sprite(iso, screen_origin, sprite, sheet);
+    draw_sprite(iso, sprite, sheet);
   }
 }
 
@@ -666,8 +675,7 @@ 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, iso->camera, x, y);
+  const ivec2 screen_origin = iso2cart(iso->iso_space, iso->camera, x, y);
   draw_tile(iso, screen_origin, tile);
 }