feat(enhancement): Allow ships to define their center of rotation (en…

…dless-sky#9423)

source/Body.cpp

@@ -20,6 +20,7 @@ this program. If not, see <https://www.gnu.org/licenses/>.
 #include "GameData.h"
 #include "Mask.h"
 #include "MaskManager.h"
+#include "pi.h"
 #include "Random.h"
 #include "Sprite.h"
 #include "SpriteSet.h"
@@ -147,6 +148,13 @@ const Point &Body::Velocity() const
 
 
 
+const Point Body::Center() const
+{
+	return -rotatedCenter + position;
+}
+
+
+
 // Direction this Body is facing in.
 const Angle &Body::Facing() const
 {
@@ -231,6 +239,8 @@ void Body::LoadSprite(const DataNode &node)
 		}
 		else if(child.Token(0) == "rewind")
 			rewind = true;
+		else if(child.Token(0) == "center" && child.Size() >= 3)
+			center = Point(child.Value(1), child.Value(2));
 		else
 			child.PrintTrace("Skipping unrecognized attribute:");
 	}
@@ -262,6 +272,8 @@ void Body::SaveSprite(DataWriter &out, const string &tag) const
 			out.Write("no repeat");
 		if(rewind)
 			out.Write("rewind");
+		if(center)
+			out.Write("center", center.X(), center.Y());
 	}
 	out.EndChild();
 }
@@ -332,6 +344,39 @@ void Body::UnmarkForRemoval()
 
 
 
+// Turn this object around its center of rotation.
+void Body::Turn(double amount)
+{
+	angle += amount;
+	if(!center)
+		return;
+
+	auto RotatePointAroundOrigin = [](Point &toRotate, double radians) -> Point {
+		float si = sin(radians);
+		float co = cos(radians);
+		float newX = toRotate.X() * co - toRotate.Y() * si;
+		float newY = toRotate.X() * si + toRotate.Y() * co;
+		return Point(newX, newY);
+	};
+
+	rotatedCenter = -RotatePointAroundOrigin(center, (angle - amount).Degrees() * TO_RAD);
+
+	position -= rotatedCenter;
+
+	rotatedCenter = RotatePointAroundOrigin(rotatedCenter, Angle(amount).Degrees() * TO_RAD);
+
+	position += rotatedCenter;
+}
+
+
+
+void Body::Turn(const Angle &amount)
+{
+	Turn(amount.Degrees());
+}
+
+
+
 // Set the current time step.
 void Body::SetStep(int step) const
 {

source/Body.h

@@ -57,6 +57,7 @@ class Body {
 	// Positional attributes.
 	const Point &Position() const;
 	const Point &Velocity() const;
+	const Point Center() const;
 	const Angle &Facing() const;
 	Point Unit() const;
 	double Zoom() const;
@@ -88,13 +89,18 @@ class Body {
 	void MarkForRemoval();
 	// Mark that this object should not be removed (e.g. a launched fighter).
 	void UnmarkForRemoval();
+	// Turn this object around its center of rotation.
+	void Turn(double amount);
+	void Turn(const Angle &amount);
 
 
 protected:
 	// Basic positional attributes.
 	Point position;
 	Point velocity;
 	Angle angle;
+	Point center;
+	Point rotatedCenter;
 	// A zoom of 1 means the sprite should be drawn at half size. For objects
 	// whose sprites should be full size, use zoom = 2.
 	float zoom = 1.f;

source/Engine.cpp

@@ -505,7 +505,7 @@ void Engine::Step(bool isActive)
 	}
 	else if(flagship)
 	{
-		center = flagship->Position();
+		center = flagship->Center();
 		centerVelocity = flagship->Velocity();
 		Preferences::ExtendedJumpEffects jumpEffectState = Preferences::GetExtendedJumpEffects();
 		if(flagship->IsHyperspacing() && jumpEffectState != Preferences::ExtendedJumpEffects::OFF)
@@ -1587,7 +1587,7 @@ void Engine::CalculateStep()
 	Point newCenterVelocity;
 	if(flagship)
 	{
-		newCenter = flagship->Position();
+		newCenter = flagship->Center();
 		newCenterVelocity = flagship->Velocity();
 	}
 	draw[calcTickTock].SetCenter(newCenter, newCenterVelocity);

source/Ship.cpp

@@ -1306,7 +1306,7 @@ void Ship::Place(Point position, Point velocity, Angle angle, bool isDeparting)
 {
 	this->position = position;
 	this->velocity = velocity;
-	this->angle = angle;
+	this->Turn(angle);
 
 	// If landed, place the ship right above the planet.
 	// Escorts should take off a bit behind their flagships.
@@ -4267,7 +4267,7 @@ void Ship::DoMovement(bool &isUsingAfterburner)
 				slowness += scale * attributes.Get("turning slowing");
 				disruption += scale * attributes.Get("turning disruption");
 
-				angle += commands.Turn() * TurnRate() * slowMultiplier;
+				Turn(commands.Turn() * TurnRate() * slowMultiplier);
 			}
 		}
 		double thrustCommand = commands.Has(Command::FORWARD) - commands.Has(Command::BACK);