Writing New Behaviour Ops — NDK Developers Guide

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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 plug-ins 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 checks to see whether the given particle is 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 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 is retrieved 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 | The position the particle was created at
> Vector3 | position | The position the particle is at now
> Vector3 | velocity | The velocity of the particle, expressed in units/frame
> Vector3 | size | The size of the particle, expressed in units
> Quarternion | orientation |
> Vector3 | rotationAxis |
> float | rotationAngle |
> float | rotationVelocity |
> float | mass | The mass of the particle
> float | life | The maximum lifetime of the particle (in frames)
> float | expirationChance | The chance that this particle dies each frame (for halflife)
> float | t | The number of frames (possibly fractional) this particle has been alive
> float | startTime | The time this particle was emitted
> int | id | A unique ID for the particle
> ParticleChannelSet | channels | The channel sets the particle belongs to
> int | pathMask | The pathMasks the particle is active for (internal)
> bool | active | Whether the particle currently exists or not
> Op* | representation | The Iop or GeoOp to use to as the particle for rendering

All of these are 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. For example, if you’re writing a plug-in that reduces the particle’s mass slightly each frame, then you need to take ctx.dt() into account too. If, on the other hand, you are writing a plug-in that sets the particle’s color to a different value depending upon its value t, there is no need to take into account ctx.dt().

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