diff --git a/smb.lua b/smb.lua
index dbda609..71137ef 100644
--- a/smb.lua
+++ b/smb.lua
@@ -88,21 +88,29 @@ local function set_timer(time)
 end
 
 local function mark_sprite(x, y, t)
+    if t == 0 then
+        -- place unused/unseen sprites
+        x = -0x08 -- just off the left side of the screen
+        y = 0xC8 -- at ground level.
+    end
+
+    local cx = 2 * (x - 0x80) -- relative to center of screen.
+    local cy = 2 * (y - 0x88) -- relative to standing on 4th block from floor.
+
     if x < 0 or x >= 256 or y < 0 or y > 224 then
         sprite_input[#sprite_input+1] = 0
         sprite_input[#sprite_input+1] = 0
         sprite_input[#sprite_input+1] = 0
     else
-        sprite_input[#sprite_input+1] = x
-        sprite_input[#sprite_input+1] = y
+        sprite_input[#sprite_input+1] = cx
+        sprite_input[#sprite_input+1] = cy
         sprite_input[#sprite_input+1] = t
     end
-    if t == 0 then return end
-    if overlay then
+
+    if overlay and t ~= 0 then
         gui.box(x-4, y-4, x+4, y+4)
-        --gui.text(x-2, y-3, tostring(i), '#FFFFFF', '#00000000')
         gui.text(x-13, y-3-9, ("%+04i"):format(t), '#FFFFFF', '#0000003F')
-        --gui.text(x-5, y-3+9, ("%02X"):format(x), '#FFFFFF', '#0000003F')
+        gui.text(x-13, y-3+9, ("%+04i"):format(cx), '#FFFFFF', '#0000003F')
     end
 end