From 1f2d5b4fcfe59e36b8c0a9fec1551b42432397bb Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Thu, 4 Sep 2025 18:41:08 -0700
Subject: Refactor coordinate computation

---
 src/gfx2d.c | 51 +++++++++++++++++++++++++++++----------------------
 1 file changed, 29 insertions(+), 22 deletions(-)

diff --git a/src/gfx2d.c b/src/gfx2d.c
index 9767308..1c3cc39 100644
--- a/src/gfx2d.c
+++ b/src/gfx2d.c
@@ -85,6 +85,14 @@ static inline vec2 vec2_add(vec2 a, vec2 b) {
 
 static inline vec2 ivec2_to_vec2(ivec2 a) { return (vec2){a.x, a.y}; }
 
+/// Map map coordinates to screen coordinates, both Cartesian.
+static ivec2 map2screen(
+    ivec2 camera, int tile_width, int tile_height, int map_x, int map_y) {
+  return ivec2_add(
+      ivec2_neg(camera),
+      (ivec2){.x = map_x * tile_width, .y = map_y * tile_height});
+}
+
 // 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) {
@@ -116,9 +124,10 @@ 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 =
+  const ivec2 origin_world_space =
       ivec2_add(iso_space.o, ivec2_add(vx_offset, vy_offset));
-  const ivec2 origin_view_space = ivec2_add(screen_origin, ivec2_neg(camera));
+  const ivec2 origin_view_space =
+      ivec2_add(origin_world_space, ivec2_neg(camera));
   return origin_view_space;
 }
 
@@ -575,17 +584,23 @@ static void draw_rect(
 }
 
 /// Draw a tile.
-///
-/// 'screen_origin' is the screen coordinates of the top diamond-corner of the
-/// tile (the base tile for super tiles).
-///     World (0, 0) -> (screen_width / 2, 0).
-static void draw_tile(IsoGfx* iso, ivec2 screen_origin, Tile tile) {
+static void draw_tile(IsoGfx* iso, Tile tile, int iso_x, int iso_y) {
   assert(iso);
   assert(iso->tileset);
+  assert(iso_x >= 0);
+  assert(iso_y >= 0);
+  assert(iso_x < iso->map->world_width);
+  assert(iso_y < iso->map->world_height);
 
   const Ts_Tile* pTile  = ts_tileset_get_tile(iso->tileset, tile);
   const Pixel*   pixels = ts_tileset_get_tile_pixels(iso->tileset, tile);
 
+  // Compute the screen coordinates of the top diamond-corner of the tile (the
+  // base tile for super tiles).
+  //   World (0, 0) -> (screen_width / 2, 0).
+  const ivec2 screen_origin =
+      iso2cart(iso->iso_space, iso->camera, iso_x, iso_y);
+
   // Move from the top diamond-corner to the top-left corner of the tile image.
   // For regular tiles, tile height == base tile height, so the y offset is 0.
   // For super tiles, move as high up as the height of the tile.
@@ -609,15 +624,14 @@ static void draw_map(IsoGfx* iso) {
   const Tm_Layer* layer = tm_map_get_layer(iso->map, 0);
 
   // TODO: Culling.
-  // Ex: map the screen corners to tile space to cull.
-  // Ex: walk in screen space and fetch the tile.
-  // The tile-centric approach might be more cache-friendly since the
-  // screen-centric approach would juggle multiple tiles throughout the scan.
+  //  Ex: map the screen corners to tile space to cull.
+  //  Ex: walk in screen space and fetch the tile.
+  //  The tile-centric approach might be more cache-friendly since the
+  //  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 = iso2cart(iso->iso_space, iso->camera, wx, wy);
-      draw_tile(iso, screen_origin, tile);
+      const Tile tile = tm_layer_get_tile(iso->map, layer, wx, wy);
+      draw_tile(iso, tile, wx, wy);
     }
   }
 }
@@ -669,14 +683,7 @@ void isogfx_render(IsoGfx* iso) {
 }
 
 void isogfx_draw_tile(IsoGfx* iso, int x, int y, Tile tile) {
-  assert(iso);
-  assert(x >= 0);
-  assert(y >= 0);
-  assert(x < iso->map->world_width);
-  assert(y < iso->map->world_height);
-
-  const ivec2 screen_origin = iso2cart(iso->iso_space, iso->camera, x, y);
-  draw_tile(iso, screen_origin, tile);
+  draw_tile(iso, tile, x, y);
 }
 
 void isogfx_get_screen_size(const IsoGfx* iso, int* width, int* height) {
-- 
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