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# Writing new behaviour ops

NukeX 6.3 has a particle system. It is possible to hook into this and write your own plugins that operate on particles.

The base class of interest is DD::Image::ParticleBehaviour in <DDImage/ParticleOp.h>. Subclasses should implement the following function:

    virtual void applyBehaviour( const ParticleContext& context, ParticleSystem* ps)

This should apply the operation to the particles in ParticleSystem.

A simple example is as follows:

    virtual void applyBehaviour( const ParticleContext& context, ParticleSystem* ps)
    {
       for (unsigned i = 0; i < ps->numParticles(); i++) {
          if ( conditionsApply( ps, i ) ) {
            applyAcceleration(ps, i, context, Vector3(0, -0.1, 0));
          }
       }
    }

This applies a consistent downwards acceleration to all particles. The conditionsApply function is a checks to see whether the given particle should be operated upon. It is important to call this, as the ParticleMerge node relies on it. Apart from checking which branch the particle was emitted on for ParticleMerge, it also checks whether or not the particle matches various criteria that can be set on knobs. These are the conditionKnobs (which allow execution to be made conditional on min/max age, channels, etc) and the domainKnobs (which allow execution to be confined to a certain region). To add these knobs call the addDomainKnobs and addConditionKnobs functions from your knobs() call, like so:

     void knobs(Knob_Callback f)
     {
       addConditionsKnobs(f);
       addDomainKnobs(f);
    }

It is possible to access properties of particles from within the applyBehaviour function by functions on the ParticleBehaviour. For example, the mass of the particle can be got with

    const float& particleMass(unsigned idx) const;

and can be set by

    float& particleMass(unsigned idx);

The properties that exist are as follows:

> Type | Name | Description
> --- | --- | ---
> Vector3 | initialPosition | position that the particle was created at
> Vector3 | position | position that the particle is at now
> Vector3 | velocity | velocity of the particle, expressed in units/frame
> Vector3 | size | size of the particle, expressed in units
> Quarternion | orientation |
> Vector3 | rotationAxis |
> float | rotationAngle |
> float | rotationVelocity |
> float | mass | mass of the particle
> float | life | maximum lifetime of the particle (in frames)
> float | expirationChance | chance that this particle will die each frame (for halflife)
> float | t | number of frames (possibly fractional) this particle has been alive
> float | startTime | time this particle was emitted
> int | id | a unique ID for the particle
> ParticleChannelSet | channels | the sets the particle is in
> int | pathMask | the pathMasks the particle is active for (internal)
> bool | active | whether the particle currently exists
> Op* | representation | The Iop or GeoOp to use to as the particle for rendering

Any of these could be set by an applyBehaviour implementation, although it would not make sense to set initialPosition/t/dtOffset/startTime/id/pathMask/active/representation.

applyBehaviour is given a ParticleContext which tells it how long to run the effect for. For example, a force applied for 0.5 frames ought to only result in half the acceleration that it would if it were run for 1 frame. This is used to implement sub-frame stepping. A couple of helper functions are provided on ParticleBehaviour

    void applyAcceleration(ParticleSystem* ps, unsigned idx, const ParticleContext& ctx, Vector3 accel);
    void applyForce(ParticleSystem* ps, unsigned idx, const ParticleContext& ctx, Vector3 force);

These take into account the ctx.dt(), and also whether or not the particle was only just emitted in the last frame and if so whether only part of that force should be applied. applyForce() takes into account the mass, dividing the force by the mass to get the acceleration. If you writing a plugin that, for example, reduces the particle’s mass slightly each frame, then you will need to take ctx.dt() into account too. If, on the other hand, you are writing a plugin that sets the particle’s colour to a different value depending upon its value t, there is no need to take into account ctx.dt().

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