scp-fs2open · MjnMixael · May 6, 2026 · May 7, 2026
diff --git a/code/mission/mission_flags.h b/code/mission/mission_flags.h
@@ -36,6 +36,7 @@ namespace Mission {
 		Neb2_fog_color_override,	// Whether to use explicit fog colors instead of checking the palette - Goober5000
 		Fullneb_background_bitmaps, // Show background bitmaps despite fullneb
 		Preload_subspace,			// Preload the subspace tunnel for both the sexp and specs checkbox (for scripts) - MjnMixael
+		Large_ships_no_collide_by_default,	// Automatically puts all large ships in a shared collision group
 
 		NUM_VALUES
 	};

diff --git a/code/mission/missionparse.cpp b/code/mission/missionparse.cpp
@@ -182,6 +182,8 @@ SCP_unordered_set<int> Fred_migrated_immobile_ships;
 int Num_path_restrictions;
 path_restriction_t Path_restrictions[MAX_PATH_RESTRICTIONS];
 
+constexpr int DEFAULT_LARGE_SHIP_NO_COLLIDE_COLLISION_GROUP = 0;
+
 extern int debrief_find_persona_index();
 
 static bool mission_has_layer_name(const mission* pm, const SCP_string& layerName) {
@@ -397,7 +399,8 @@ flag_def_list_new<Mission::Mission_Flags> Parse_mission_flags[] = {
 	{"Toggle Starting in Chase View",             Mission::Mission_Flags::Toggle_start_chase_view,    true, false},
 	{"Nebula Fog Color Override",                 Mission::Mission_Flags::Neb2_fog_color_override,    true, true},
 	{"Full Nebula Background Bitmaps",            Mission::Mission_Flags::Fullneb_background_bitmaps, true, true},
-	{"Preload Subspace Tunnel",                   Mission::Mission_Flags::Preload_subspace,           true, false}
+	{"Preload Subspace Tunnel",                   Mission::Mission_Flags::Preload_subspace,           true, false},
+	{"Large Ships Do Not Collide By Default",    Mission::Mission_Flags::Large_ships_no_collide_by_default, true, false}
 };
 
 parse_object_flag_description<Mission::Mission_Flags> Parse_mission_flag_descriptions[] = {
@@ -431,6 +434,7 @@ parse_object_flag_description<Mission::Mission_Flags> Parse_mission_flag_descrip
 	{Mission::Mission_Flags::Neb2_fog_color_override,    "Whether to use explicit fog colors instead of checking the palette"},
 	{Mission::Mission_Flags::Fullneb_background_bitmaps, "Show background bitmaps despite full nebula"},
 	{Mission::Mission_Flags::Preload_subspace,         "Preload the subspace tunnel for both the sexp and specs checkbox"},
+	{Mission::Mission_Flags::Large_ships_no_collide_by_default, "Automatically places all large ships in the configured collision group, preventing large ships from colliding with each other"},
 };
 
 const size_t Num_parse_mission_flags = sizeof(Parse_mission_flags) / sizeof(flag_def_list_new<Mission::Mission_Flags>);
@@ -844,6 +848,18 @@ void parse_mission_info(mission *pm, bool basic = false)
 		stuff_int(&pm->contrail_threshold);
 	}
 
+	if (optional_string("+Large Ship Collision Group:")) {
+		stuff_int(&pm->large_ship_no_collide_collision_group);
+
+		if (pm->large_ship_no_collide_collision_group < 0 || pm->large_ship_no_collide_collision_group > 31) {
+			WarningEx(LOCATION,
+				"Invalid large ship collision group id %d specified. Valid IDs range from 0 to 31. Using group %d instead.\n",
+				pm->large_ship_no_collide_collision_group,
+				DEFAULT_LARGE_SHIP_NO_COLLIDE_COLLISION_GROUP);
+			pm->large_ship_no_collide_collision_group = DEFAULT_LARGE_SHIP_NO_COLLIDE_COLLISION_GROUP;
+		}
+	}
+
 	if (optional_string("+Volumetric Nebula:")) {
 		pm->volumetrics.emplace().parse_volumetric_nebula();
 	}
@@ -2238,6 +2254,11 @@ int parse_create_object_sub(p_object *p_objp, bool standalone_ship)
 	// Goober5000 - set the collision group if one was provided
 	Objects[objnum].collision_group_id = p_objp->collision_group_id;
 
+	// Mission-level performance helper: large ships may be grouped so they skip mutual collision checks.
+	if (The_mission.flags[Mission::Mission_Flags::Large_ships_no_collide_by_default] && sip->is_big_or_huge()) {
+		Objects[objnum].collision_group_id |= (1 << The_mission.large_ship_no_collide_collision_group);
+	}
+
 	// Goober5000 - set some fields that the mission log might need (if logged via parse_bring_in_docked_wing just below)
 	shipp->display_name = p_objp->display_name;
 	shipp->alt_type_index = p_objp->alt_type_index;
@@ -7178,6 +7199,7 @@ void mission::Reset()
 
 	envmap_name[ 0 ] = '\0';
 	contrail_threshold = CONTRAIL_THRESHOLD_DEFAULT;
+	large_ship_no_collide_collision_group = DEFAULT_LARGE_SHIP_NO_COLLIDE_COLLISION_GROUP;
 	ambient_light_level = DEFAULT_AMBIENT_LIGHT_LEVEL;
 	sound_environment.id = -1;
 
@@ -9557,6 +9579,9 @@ bool check_for_24_3_data()
 
 bool check_for_25_1_data()
 {
+	if (The_mission.flags[Mission::Mission_Flags::Large_ships_no_collide_by_default])
+		return true;
+
 	if (count_items_with_value(Props) > 0)
 		return true;
 

diff --git a/code/mission/missionparse.h b/code/mission/missionparse.h
@@ -207,6 +207,7 @@ typedef struct mission {
 	char	envmap_name[MAX_FILENAME_LEN];
 	int		skybox_flags;
 	int		contrail_threshold;
+	int		large_ship_no_collide_collision_group;
 	int		ambient_light_level;
 	std::optional<volumetric_nebula> volumetrics;
 	sound_env	sound_environment;

diff --git a/code/missioneditor/missionsave.cpp b/code/missioneditor/missionsave.cpp
@@ -2568,6 +2568,10 @@ int Fred_mission_save::save_mission_info()
 		if (The_mission.contrail_threshold != CONTRAIL_THRESHOLD_DEFAULT) {
 			fout("\n$Contrail Speed Threshold: %d\n", The_mission.contrail_threshold);
 		}
+
+		if (The_mission.flags[Mission::Mission_Flags::Large_ships_no_collide_by_default]) {
+			fout("\n+Large Ship Collision Group: %d\n", The_mission.large_ship_no_collide_collision_group);
+		}
 	}
 
 	{

diff --git a/qtfred/help-src/doc/dialogs/MissionSpecsDialog.html b/qtfred/help-src/doc/dialogs/MissionSpecsDialog.html
@@ -140,5 +140,15 @@ <h2>Flags</h2>
 box to find a specific flag by name, or use <strong>Select All</strong> /
 <strong>Select None</strong> to bulk-toggle. Hover over any flag label for a tooltip
 description.</p>
+<p>Some flags reveal an additional control when enabled:</p>
+<table>
+    <tr><th>Flag</th><th>Control</th><th>Description</th></tr>
+    <tr><td>Large Ships Do Not Collide By Default</td>
+        <td>Large Ship Collision Group</td>
+        <td>Collision group ID (0–31) that large and huge ships are automatically
+        placed into when the mission loads. Ships sharing the same group skip
+        collision checks against each other, reducing physics overhead in
+        missions with many capital ships. Default is 0.</td></tr>
+</table>
 </body>
 </html>
diff --git a/qtfred/src/mission/dialogs/MissionSpecDialogModel.cpp b/qtfred/src/mission/dialogs/MissionSpecDialogModel.cpp
@@ -70,6 +70,7 @@ void MissionSpecDialogModel::initializeData() {
 
 	_m_contrail_threshold = The_mission.contrail_threshold;
 	_m_contrail_threshold_flag = (_m_contrail_threshold != CONTRAIL_THRESHOLD_DEFAULT);
+	_m_large_ship_no_collide_collision_group = The_mission.large_ship_no_collide_collision_group;
 
 	_m_custom_data = The_mission.custom_data;
 	_m_custom_strings = The_mission.custom_strings;
@@ -125,6 +126,7 @@ bool MissionSpecDialogModel::apply() {
 	The_mission.support_ships.max_support_ships = (_m_disallow_support) ? 0 : -1;
 	The_mission.support_ships.max_hull_repair_val = _m_max_hull_repair_val;
 	The_mission.support_ships.max_subsys_repair_val = _m_max_subsys_repair_val;
+	The_mission.large_ship_no_collide_collision_group = _m_large_ship_no_collide_collision_group;
 
 	// Copy mission flags
 	The_mission.flags = _m_flags;
@@ -418,6 +420,22 @@ void MissionSpecDialogModel::setMissionFlagDirect(Mission::Mission_Flags flag, b
 	}
 }
 
+void MissionSpecDialogModel::setLargeShipNoCollideCollisionGroup(int group)
+{
+	if (group < 0) {
+		group = 0;
+	} else if (group > 31) {
+		group = 31;
+	}
+
+	modify(_m_large_ship_no_collide_collision_group, group);
+}
+
+int MissionSpecDialogModel::getLargeShipNoCollideCollisionGroup() const
+{
+	return _m_large_ship_no_collide_collision_group;
+}
+
 bool MissionSpecDialogModel::getMissionFlag(Mission::Mission_Flags flag) const {
 	return _m_flags[flag];
 }

diff --git a/qtfred/src/mission/dialogs/MissionSpecDialogModel.h b/qtfred/src/mission/dialogs/MissionSpecDialogModel.h
@@ -42,6 +42,7 @@ class MissionSpecDialogModel : public AbstractDialogModel {
 	float		_m_max_subsys_repair_val;
 	bool		_m_contrail_threshold_flag;
 	int			_m_contrail_threshold;
+	int			_m_large_ship_no_collide_collision_group;
 	SCP_map<SCP_string, SCP_string> _m_custom_data;
 	SCP_vector<custom_string> _m_custom_strings;
 	sound_env	_m_sound_env;
@@ -119,6 +120,8 @@ class MissionSpecDialogModel : public AbstractDialogModel {
 
 	void setMissionFlag(const SCP_string& flag_name, bool enabled);
 	void setMissionFlagDirect(Mission::Mission_Flags flag, bool enabled);
+	void setLargeShipNoCollideCollisionGroup(int group);
+	int getLargeShipNoCollideCollisionGroup() const;
 	bool getMissionFlag(Mission::Mission_Flags flag) const;
 	const SCP_vector<std::pair<SCP_string, bool>>& getMissionFlagsList();
 	static SCP_vector<std::pair<SCP_string, SCP_string>> getMissionFlagDescriptions();

diff --git a/qtfred/src/ui/dialogs/MissionSpecDialog.cpp b/qtfred/src/ui/dialogs/MissionSpecDialog.cpp
@@ -134,6 +134,7 @@ void MissionSpecDialog::initFlagList()
 	// per flag immediate apply to the model
 	connect(ui->flagList, &fso::fred::FlagListWidget::flagToggled, this, [this](const QString& name, bool checked) {
 		_model->setMissionFlag(name.toUtf8().constData(), checked);
+		updateLargeShipCollisionGroup();
 	});
 }
 
@@ -149,6 +150,15 @@ void MissionSpecDialog::updateFlags()
 	}
 
 	ui->flagList->setFlags(toWidget);
+	updateLargeShipCollisionGroup();
+}
+
+void MissionSpecDialog::updateLargeShipCollisionGroup()
+{
+	const auto enabled = _model->getMissionFlag(Mission::Mission_Flags::Large_ships_no_collide_by_default);
+	ui->largeShipCollisionGroupLabel->setVisible(enabled);
+	ui->largeShipCollisionGroup->setVisible(enabled);
+	ui->largeShipCollisionGroup->setValue(_model->getLargeShipNoCollideCollisionGroup());
 }
 
 void MissionSpecDialog::updateMissionType() {
@@ -433,6 +443,11 @@ void MissionSpecDialog::on_musicPackCombo_currentIndexChanged(int index) {
 	_model->setSubEventMusic(subMusic);
 }
 
+void MissionSpecDialog::on_largeShipCollisionGroup_valueChanged(int value)
+{
+	_model->setLargeShipNoCollideCollisionGroup(value);
+}
+
 void MissionSpecDialog::on_aiProfileCombo_currentIndexChanged(int index)
 {
 	auto aipIndex = ui->aiProfileCombo->itemData(index).value<int>();

diff --git a/qtfred/src/ui/dialogs/MissionSpecDialog.h b/qtfred/src/ui/dialogs/MissionSpecDialog.h
@@ -64,6 +64,7 @@ private slots:
 
 	// Right column
 	// flags are dynamically generated and connected
+	void on_largeShipCollisionGroup_valueChanged(int value);
 	void on_aiProfileCombo_currentIndexChanged(int index);
 
 	// General
@@ -84,6 +85,7 @@ private slots:
 
 	void initFlagList();
 	void updateFlags();
+	void updateLargeShipCollisionGroup();
 
 	void updateMissionType();
 	void updateCmdMessage();

diff --git a/qtfred/ui/MissionSpecDialog.ui b/qtfred/ui/MissionSpecDialog.ui
@@ -822,6 +822,39 @@
           </property>
          </widget>
         </item>
+        <item>
+         <layout class="QFormLayout" name="largeShipCollisionGroupLayout">
+          <property name="leftMargin">
+           <number>0</number>
+          </property>
+          <property name="topMargin">
+           <number>0</number>
+          </property>
+          <property name="rightMargin">
+           <number>0</number>
+          </property>
+          <property name="bottomMargin">
+           <number>0</number>
+          </property>
+          <item row="0" column="0">
+           <widget class="QLabel" name="largeShipCollisionGroupLabel">
+            <property name="text">
+             <string>Large Ship Collision Group</string>
+            </property>
+           </widget>
+          </item>
+          <item row="0" column="1">
+           <widget class="QSpinBox" name="largeShipCollisionGroup">
+            <property name="minimum">
+             <number>0</number>
+            </property>
+            <property name="maximum">
+             <number>31</number>
+            </property>
+           </widget>
+          </item>
+         </layout>
+        </item>
        </layout>
       </widget>
      </item>