From 58539171a31287bbbc6a200fc7b28fa9399dfc91 Mon Sep 17 00:00:00 2001
From: Thijs Wenker <me@thoys.nl>
Date: Sat, 20 Feb 2016 22:46:19 +0100
Subject: [PATCH] give particle trails a nice smooth path between frames

---
 libraries/entities/src/ParticleEffectEntityItem.cpp | 9 ++++++---
 libraries/entities/src/ParticleEffectEntityItem.h   | 4 ++--
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/libraries/entities/src/ParticleEffectEntityItem.cpp b/libraries/entities/src/ParticleEffectEntityItem.cpp
index 16196aa129..4b798cbcd9 100644
--- a/libraries/entities/src/ParticleEffectEntityItem.cpp
+++ b/libraries/entities/src/ParticleEffectEntityItem.cpp
@@ -103,6 +103,7 @@ EntityItemPointer ParticleEffectEntityItem::factory(const EntityItemID& entityID
 // our non-pure virtual subclass for now...
 ParticleEffectEntityItem::ParticleEffectEntityItem(const EntityItemID& entityItemID) :
     EntityItem(entityItemID),
+    _previousPosition(getPosition()),
     _lastSimulated(usecTimestampNow())
 {
     _type = EntityTypes::ParticleEffect;
@@ -623,7 +624,8 @@ void ParticleEffectEntityItem::stepSimulation(float deltaTime) {
             }
             
             // emit a new particle at tail index.
-            _particles.push_back(createParticle());
+            _particles.push_back(createParticle(glm::mix(_previousPosition, getPosition(),
+                (deltaTime - timeLeftInFrame) / deltaTime)));
             auto particle = _particles.back();
             particle.lifetime += timeLeftInFrame;
             
@@ -637,15 +639,16 @@ void ParticleEffectEntityItem::stepSimulation(float deltaTime) {
 
         _timeUntilNextEmit -= timeLeftInFrame;
     }
+    _previousPosition = getPosition();
 }
 
-ParticleEffectEntityItem::Particle ParticleEffectEntityItem::createParticle() {
+ParticleEffectEntityItem::Particle ParticleEffectEntityItem::createParticle(const glm::vec3& position) {
     Particle particle;
 
  
     particle.seed = randFloatInRange(-1.0f, 1.0f);
     if (getEmitterShouldTrail()) {
-        particle.position = getPosition();
+        particle.position = position;
     }
     // Position, velocity, and acceleration
     if (_polarStart == 0.0f && _polarFinish == 0.0f && _emitDimensions.z == 0.0f) {
diff --git a/libraries/entities/src/ParticleEffectEntityItem.h b/libraries/entities/src/ParticleEffectEntityItem.h
index 5afcbe2ae1..ba6f32b51f 100644
--- a/libraries/entities/src/ParticleEffectEntityItem.h
+++ b/libraries/entities/src/ParticleEffectEntityItem.h
@@ -227,7 +227,7 @@ protected:
 
     bool isAnimatingSomething() const;
     
-    Particle createParticle();
+    Particle createParticle(const glm::vec3& position);
     void stepSimulation(float deltaTime);
     void integrateParticle(Particle& particle, float deltaTime);
     
@@ -275,7 +275,7 @@ protected:
     float _azimuthStart = DEFAULT_AZIMUTH_START;
     float _azimuthFinish = DEFAULT_AZIMUTH_FINISH;
     
-
+    glm::vec3 _previousPosition;
     quint64 _lastSimulated { 0 };
     bool _isEmitting { true };