Changed particle spawning behaviour in ParticleSystems to spawn at an interpolated position between the location at the previous update and the current one. This results in much smoother behaviour when moving a ParticleSystem constantly via ParticleSystem:setPosition.

Also fixed particle spawning to better fill up the buffer.

src/modules/graphics/opengl/ParticleSystem.cpp

@@ -118,6 +118,7 @@ ParticleSystem::ParticleSystem(const ParticleSystem &p)
 	, emissionRate(p.emissionRate)
 	, emitCounter(0.0f)
 	, position(p.position)
+	, prevPosition(p.prevPosition)
 	, areaSpreadDistribution(p.areaSpreadDistribution)
 	, areaSpread(p.areaSpread)
 	, lifetime(p.lifetime)
@@ -205,14 +206,14 @@ uint32 ParticleSystem::getBufferSize() const
 	return maxParticles;
 }
 
-void ParticleSystem::addParticle()
+void ParticleSystem::addParticle(float t)
 {
 	if (isFull())
 		return;
 
 	// Gets a free particle and updates the allocation pointer.
 	particle *p = pFree++;
-	initParticle(p);
+	initParticle(p, t);
 
 	switch (insertMode)
 	{
@@ -231,10 +232,13 @@ void ParticleSystem::addParticle()
 	activeParticles++;
 }
 
-void ParticleSystem::initParticle(particle *p)
+void ParticleSystem::initParticle(particle *p, float t)
 {
 	float min,max;
 
+	// Linearly interpolate between the previous and current emitter position.
+	love::Vector pos = prevPosition + (position - prevPosition) * t;
+
 	min = particleLifeMin;
 	max = particleLifeMax;
 	if (min == max)
@@ -243,8 +247,8 @@ void ParticleSystem::initParticle(particle *p)
 		p->life = (float) rng.random(min, max);
 	p->lifetime = p->life;
 
-	p->position[0] = position.getX();
-	p->position[1] = position.getY();
+	p->position[0] = pos.x;
+	p->position[1] = pos.y;
 
 	switch (areaSpreadDistribution)
 	{
@@ -265,7 +269,7 @@ void ParticleSystem::initParticle(particle *p)
 	max = direction + spread/2.0f;
 	p->direction = (float) rng.random(min, max);
 
-	p->origin = position;
+	p->origin = pos;
 
 	min = speedMin;
 	max = speedMax;
@@ -748,7 +752,7 @@ void ParticleSystem::emit(uint32 num)
 	num = std::min(num, maxParticles - activeParticles);
 
 	while(num--)
-		addParticle();
+		addParticle(1.0f);
 }
 
 bool ParticleSystem::isActive() const
@@ -847,25 +851,6 @@ void ParticleSystem::update(float dt)
 	if (pMem == nullptr || dt == 0.0f)
 		return;
 
-	// Make some more particles.
-	if (active)
-	{
-		float rate = 1.0f / emissionRate; // the amount of time between each particle emit
-		emitCounter += dt;
-		while (emitCounter > rate)
-		{
-			addParticle();
-			emitCounter -= rate;
-		}
-		/*int particles = (int)(emissionRate * dt);
-		for (int i = 0; i != particles; i++)
-			add();*/
-
-		life -= dt;
-		if (lifetime != -1 && life < 0)
-			stop();
-	}
-
 	// Traverse all particles and update.
 	particle *p = pHead;
 
@@ -940,6 +925,28 @@ void ParticleSystem::update(float dt)
 			p = p->next;
 		}
 	}
+
+	// Make some more particles.
+	if (active)
+	{
+		float rate = 1.0f / emissionRate; // the amount of time between each particle emit
+		emitCounter += dt;
+		float total = emitCounter - rate;
+		while (emitCounter > rate)
+		{
+			addParticle(1.0f - (emitCounter - rate) / total);
+			emitCounter -= rate;
+		}
+		/*int particles = (int)(emissionRate * dt);
+		 for (int i = 0; i != particles; i++)
+		 add();*/
+
+		life -= dt;
+		if (lifetime != -1 && life < 0)
+			stop();
+	}
+
+	prevPosition = position;
 }
 
 bool ParticleSystem::getConstant(const char *in, AreaSpreadDistribution &out)