diff --git a/bitplane-thingie.p8 b/bitplane-thingie.p8
index 3cb643f..2ea83a4 100644
--- a/bitplane-thingie.p8
+++ b/bitplane-thingie.p8
@@ -137,6 +137,34 @@ function handle_ui_click(x, y)
   return
  end
 
+ -- checkboxes: qualify
+ if (y < 113 or y > 120 or x < 32) return
+
+ if x <= 67 then
+  x -= 32
+  if (x % 9 == 8) return 
+  local box = x \ 9
+  --check range
+  if (box < 0 or box > 3) return
+  toggle_mask_bit(box)
+  return
+ end
+ 
+ x -= 90
+ if (x % 9 == 8) return 
+ local box = x \ 9
+ --check range
+ if (box < 0 or box > 3) return
+ toggle_mask_bit(box + 4)
+ return
+end
+
+function toggle_mask_bit(n)
+ local sel = 1 << n
+ local b = plane_mask & sel
+ plane_mask = plane_mask & ~sel
+ if (b > 0) return
+ plane_mask = plane_mask | sel
 end
 -->8
 -- draw_cursor