Fixes #10710

code/modules/lighting/light_source.dm

@@ -1,3 +1,5 @@
+//So much copypasta in this file, supposedly in the name of performance. If you change anything make sure to consider other places where the code may have been copied.
+
 /datum/light_source
 	var/atom/top_atom
 	var/atom/source_atom
@@ -11,9 +13,10 @@
 	var/lum_g
 	var/lum_b
 
-	var/tmp/old_lum_r
-	var/tmp/old_lum_g
-	var/tmp/old_lum_b
+	//hold onto the actual applied lum values so we can undo them when the lighting changes
+	var/tmp/applied_lum_r
+	var/tmp/applied_lum_g
+	var/tmp/applied_lum_b
 
 	var/list/effect_str
 	var/list/effect_turf
@@ -107,11 +110,6 @@
 	if(light_range && light_power && !applied)
 		. = 1
 
-	if(. || source_atom.light_color != light_color)//Save the old lumcounts if we need to update, if the colour changed DO IT BEFORE we parse the colour and LOSE the old lumcounts!
-		old_lum_r = lum_r
-		old_lum_g = lum_g
-		old_lum_b = lum_b
-
 	if(source_atom.light_color != light_color)
 		light_color = source_atom.light_color
 		parse_light_color()
@@ -149,6 +147,12 @@
 
 /datum/light_source/proc/apply_lum()
 	applied = 1
+
+	//Keep track of the last applied lum values so that the lighting can be reversed
+	applied_lum_r = lum_r
+	applied_lum_g = lum_g
+	applied_lum_b = lum_b
+
 	if(istype(source_turf))
 		FOR_DVIEW(var/turf/T, light_range, source_turf, INVISIBILITY_LIGHTING)
 			if(T.lighting_overlay)
@@ -164,9 +168,9 @@
 				effect_str += strength
 
 				T.lighting_overlay.update_lumcount(
-					lum_r * strength,
-					lum_g * strength,
-					lum_b * strength
+					applied_lum_r * strength,
+					applied_lum_g * strength,
+					applied_lum_b * strength
 				)
 
 			else
@@ -188,7 +192,11 @@
 
 		if(T.lighting_overlay)
 			var/str = effect_str[i]
-			T.lighting_overlay.update_lumcount(-str * old_lum_r, -str * old_lum_g, -str * old_lum_b)
+			T.lighting_overlay.update_lumcount(
+				-str * applied_lum_r, 
+				-str * applied_lum_g, 
+				-str * applied_lum_b
+			)
 
 		i++
 
@@ -218,9 +226,9 @@
 			effect_str += .
 
 			T.lighting_overlay.update_lumcount(
-				lum_r * .,
-				lum_g * .,
-				lum_b * .
+				applied_lum_r * .,
+				applied_lum_g * .,
+				applied_lum_b * .
 			)
 
 		else
@@ -241,7 +249,7 @@
 
 		if(T.lighting_overlay)
 			var/str = effect_str[idx]
-			T.lighting_overlay.update_lumcount(-str * lum_r, -str * lum_g, -str * lum_b)
+			T.lighting_overlay.update_lumcount(-str * applied_lum_r, -str * applied_lum_g, -str * applied_lum_b)
 
 		effect_turf.Cut(idx, idx + 1)
 		effect_str.Cut(idx, idx + 1)
@@ -279,10 +287,12 @@
 
 		effect_str[idx] = .
 
+		//Since the applied_lum values are what are (later) removed by remove_lum.
+		//Anything we apply to the lighting overlays HAS to match what remove_lum uses.
 		T.lighting_overlay.update_lumcount(
-			lum_r * .,
-			lum_g * .,
-			lum_b * .
+			applied_lum_r * .,
+			applied_lum_g * .,
+			applied_lum_b * .
 		)
 
 #undef LUM_FALLOFF