particles: add parameter to delay particle birth in soft_start()

Also adds a get_tics_since_birth() getter.

This feature can be used to synchronize different particle effects.

Closes #769

direct/src/particles/ParticleEffect.py

@@ -221,10 +221,13 @@ class ParticleEffect(NodePath):
         for particles in self.getParticlesList():
             particles.softStop()
 
-    def softStart(self):
+    def softStart(self, firstBirthDelay=None):
         if self.__isValid():
             for particles in self.getParticlesList():
-                particles.softStart()
+                if firstBirthDelay is not None:
+                    particles.softStart(br=-1, first_birth_delay=firstBirthDelay)
+                else:
+                    particles.softStart()
         else:
             # Not asserting here since we want to crash live clients for more expedient bugfix
             # (Sorry, live clients)

panda/src/particlesystem/particleSystem.I

@@ -58,6 +58,18 @@ soft_start(PN_stdfloat br) {
   _tics_since_birth = 0.0f;
 }
 
+/**
+ * Causes system to use birth rate set by set_birth_rate(), with the system's
+ * first birth being delayed by the value of first_birth_delay. Note that a
+ * negative delay is perfectly valid, causing the first birth to happen
+ * sooner rather than later.
+ */
+INLINE void ParticleSystem::
+soft_start(PN_stdfloat br, PN_stdfloat first_birth_delay) {
+  soft_start(br);
+  _tics_since_birth = -first_birth_delay;
+}
+
 /**
  * Causes system to use birth rate set by set_soft_birth_rate()
  */
@@ -342,6 +354,14 @@ get_floor_z() const {
   return _floor_z;
 }
 
+/**
+
+*/
+INLINE PN_stdfloat ParticleSystem::
+get_tics_since_birth() const {
+  return _tics_since_birth;
+}
+
 /**
 
  */

panda/src/particlesystem/particleSystem.h

@@ -83,6 +83,7 @@ PUBLISHED:
   INLINE BaseParticleEmitter *get_emitter() const;
   INLINE BaseParticleFactory *get_factory() const;
   INLINE PN_stdfloat get_floor_z() const;
+  INLINE PN_stdfloat get_tics_since_birth() const;
 
   // particle template vector
 
@@ -96,6 +97,7 @@ PUBLISHED:
   INLINE void clear_to_initial();
   INLINE void soft_stop(PN_stdfloat br = 0.0);
   INLINE void soft_start(PN_stdfloat br = 0.0);
+  INLINE void soft_start(PN_stdfloat br, PN_stdfloat first_birth_delay);
   void update(PN_stdfloat dt);
 
   virtual void output(std::ostream &out) const;