light: add _light_twosided 1 key for forcing two-sided light receiving on opaque surfaces

common/bspfile.cc

@@ -1830,7 +1830,7 @@ static auto as_tuple(const surfflags_t &flags)
         flags.no_bounce, flags.no_minlight, flags.no_expand, flags.no_phong, flags.light_ignore,
         flags.surflight_rescale, flags.surflight_style, flags.surflight_color, flags.surflight_minlight_scale,
         flags.surflight_targetname, flags.phong_angle, flags.phong_angle_concave, flags.phong_group, flags.minlight,
-        flags.minlight_color, flags.light_alpha, flags.maxlight, flags.lightcolorscale, flags.surflight_group,
+        flags.minlight_color, flags.light_alpha, flags.light_twosided, flags.maxlight, flags.lightcolorscale, flags.surflight_group,
         flags.world_units_per_luxel, flags.object_channel_mask);
 }
 

include/common/bspfile.hh

@@ -184,6 +184,9 @@ struct surfflags_t
     // custom opacity
     std::optional<vec_t> light_alpha;
 
+    // two-sided lighting
+    std::optional<bool> light_twosided;
+
     // maxlight value for this face
     vec_t maxlight;
 

light/light.cc

@@ -1176,6 +1176,9 @@ static void LoadExtendedTexinfoFlags(const fs::path &sourcefilename, const mbsp_
         if (val.contains("light_alpha")) {
             flags.light_alpha = val.at("light_alpha").get<vec_t>();
         }
+        if (val.contains("light_twosided")) {
+            flags.light_twosided = val.at("light_twosided").get<bool>();
+        }
         if (val.contains("lightcolorscale")) {
             flags.lightcolorscale = val.at("lightcolorscale").get<vec_t>();
         }

light/ltface.cc

@@ -608,6 +608,11 @@ static std::unique_ptr<lightsurf_t> Lightsurf_Init(const modelinfo_t *modelinfo,
         const surfflags_t &extended_flags = extended_texinfo_flags[face->texinfo];
         lightsurf->curved = extended_flags.phong_angle != 0 || Q2_FacePhongValue(bsp, face);
 
+        // override the autodetected twosided setting?
+        if (extended_flags.light_twosided) {
+            lightsurf->twosided = *extended_flags.light_twosided;
+        }
+
         // nodirt
         if (modelinfo->dirt.is_changed()) {
             lightsurf->nodirt = (modelinfo->dirt.value() == -1);

qbsp/map.cc

@@ -821,6 +821,11 @@ static surfflags_t SurfFlagsForEntity(
         flags.light_alpha = std::clamp(lightalpha, 0.0, 1.0);
     }
 
+    // handle "_light_twosided"
+    if (entity.epairs.has("_light_twosided")) {
+        flags.light_twosided = entity.epairs.get_int("_light_twosided");
+    }
+
     return flags;
 }
 

qbsp/writebsp.cc

@@ -417,6 +417,9 @@ static void WriteExtendedTexinfoFlags(void)
         if (tx.flags.light_alpha) {
             t["light_alpha"] = *tx.flags.light_alpha;
         }
+        if (tx.flags.light_twosided) {
+            t["light_twosided"] = *tx.flags.light_twosided;
+        }
         if (tx.flags.lightcolorscale != 1.0) {
             t["lightcolorscale"] = tx.flags.lightcolorscale;
         }