Fixed starfield tiling

src/system.rs

@@ -52,7 +52,7 @@ impl StarField {
 				q.x as i16,
 				q.y as i16,
 				(5.0 - (1.0 * wp.par)) as i16,
-				Color::RGBA(255, 255, 255, 100),
+				Color::RGB(100, 100, 100),
 			)?;
 		}
 		return Ok(());
@@ -64,35 +64,38 @@ impl StarField {
 		let ww = w / 2.0;
 		let hh = h / 2.0;
 
+		// Camera position relative to the center of this field.
 		let pos_in_field = Cartesian::new(
 			dc.camera.pos.x.signum() * (((dc.camera.pos.x.abs() + ww) % w) - ww),
 			dc.camera.pos.y.signum() * (((dc.camera.pos.y.abs() + hh) % h) - hh),
 		);
 
-		// Field center in world coordinates
+		// Center of this field, in world coordinates
 		let field_center = dc.camera.pos - pos_in_field;
 
-		// screen coordinates of top left position of starfield
-		let field_tl =
-			WorldPosition::new(field_center + Cartesian::new(-ww, hh), 3.0).screen_position(dc);
+		// Compute tile bounds.
+		// We use neighboring tiles' corners instead of this tile's corner to properly account for parallax.
+		// If we use the current tiles corners, we'll see stars appear when a tile is drawn--parallax moves them
+		// into view. The bounds below guarantee that no *other* tiles' stars will be drawn inside.
+		//
+		// bound_nw is the screen position of the bottom-right corner of north-west tile
+		// bound_se is the screen position of the top-right corner of south-east tile
+		let bound_nw =
+			WorldPosition::new(field_center + Cartesian::new(-ww, hh), 4.0).screen_position(dc);
+		let bound_se =
+			WorldPosition::new(field_center + Cartesian::new(ww, -hh), 4.0).screen_position(dc);
 
-		let field_br =
-			WorldPosition::new(field_center + Cartesian::new(ww, -hh), 3.0).screen_position(dc);
-
-		dc.canvas.aa_circle(
-			field_tl.x as i16,
-			field_tl.y as i16,
-			5,
-			Color::RGB(255, 0, 0),
-		)?;
-		dc.canvas.set_draw_color(Color::RGB(0, 0, 0));
-
-		let north = field_tl.y > 0.0;
-		let south = field_br.y < dc.window_size.y;
-		let east = field_br.x < dc.window_size.x;
-		let west = field_tl.x > 0.0;
+		// Naturally, we show tiles only if we can see their edges.
+		let north = bound_nw.y > 0.0;
+		let south = bound_se.y < dc.window_size.y;
+		let east = bound_se.x < dc.window_size.x;
+		let west = bound_nw.x > 0.0;
 
+		// Draw center tile
 		self.draw_with_offset(dc, pos_in_field, Cartesian::new(0.0, 0.0))?;
+
+		// Draw surrounding tiles
+		// (which are just offset clones of the main one)
 		if north {
 			self.draw_with_offset(dc, pos_in_field, Cartesian::new(0.0, h))?;
 		}
@@ -118,6 +121,7 @@ impl StarField {
 			self.draw_with_offset(dc, pos_in_field, Cartesian::new(-w, -h))?;
 		}
 
+		// draw_circle doesn't clean up the color it uses, so we do that here.
 		dc.canvas.set_draw_color(Color::RGB(0, 0, 0));
 		return Ok(());
 	}