rob

class CDustParticle : public CSimpleEmitter
{
public:
	
	void SetGravity(float gravity) {m_flGravity = gravity;}

	CDustParticle( const char *pDebugName ) : CSimpleEmitter( pDebugName ) {}
	
	//Create
	static CDustParticle *Create( const char *pDebugName="dust" )
	{
		return new CDustParticle( pDebugName );
	}

	//Roll
	virtual	float UpdateRoll( SimpleParticle *pParticle, float timeDelta )
	{
		pParticle->m_flRoll += pParticle->m_flRollDelta * timeDelta;
		
		pParticle->m_flRollDelta += pParticle->m_flRollDelta * ( timeDelta * -8.0f );

		if ( fabs( pParticle->m_flRollDelta ) < 0.5f )
		{
			pParticle->m_flRollDelta = ( pParticle->m_flRollDelta > 0.0f ) ? 0.5f : -0.5f;
		}

		return pParticle->m_flRoll;
	}

	//Velocity
	virtual void UpdateVelocity( SimpleParticle *pParticle, float timeDelta )
	{
		Vector	saveVelocity = pParticle->m_vecVelocity;

		//Decellerate
		static float dtime;
		static float decay;

		if ( dtime != timeDelta )
		{
			dtime = timeDelta;
			float expected = 0.5;
			decay = exp( log( 0.0001f ) * dtime / expected );
		}

		pParticle->m_vecVelocity = pParticle->m_vecVelocity * decay;
		
		//Cap the minimum speed
		if ( pParticle->m_vecVelocity.LengthSqr() < (32.0f*32.0f) )
		{
			VectorNormalize( saveVelocity );
			pParticle->m_vecVelocity = saveVelocity * 32.0f;
		}

		//factor in gravity!!!
		pParticle->m_vecVelocity[2] -= m_flGravity * timeDelta;

		Vector vecWind;
	
		GetWindspeedAtTime(gpGlobals->curtime, vecWind);

		VectorNormalize( vecWind );

		//and factor in wind!
		pParticle->m_vecVelocity += vecWind;

	}

	

	//Alpha
	virtual float UpdateAlpha( const SimpleParticle *pParticle )
	{
		float	tLifetime = pParticle->m_flLifetime / pParticle->m_flDieTime;
		float	ramp = 1.0f - tLifetime;

		//Non-linear fade
		if ( ramp < 0.75f )
			ramp *= ramp;

		return ramp;
	}

private:
	CDustParticle( const CDustParticle & ); // not defined, not accessible
	float m_flGravity;
};