Particle Systems

You can specify the length of particles with the Length stretch parameter:

• With the value of 0, the length particles are simply square billboard.
• Increasing the value stretches the particles in the direction of their movement. The result is calculated by multiplying the stretch value by the particle velocity value. So, the higher the velocity is, the more stretched in the direction of their movement the particles are, while the other side remains of the same width.

Length Stretch = 0	Length Stretch = 1

Also, you can specify the Length Flattening parameter. This parameter makes the length particles not camera-oriented as usual billboard particles, but more perpendicular to the Z axis.

• With the value of 0, the particles wouldn't be flattered.
• With the maximum value of 1, the particles will be flat (perpendicular to the Z axis) when they are emitted.

Length Flattering = 0	Length Flattering = 1

Intersection stands for calculating collision of the mesh only with the center of the particle, not the whole particle shape. Intersection may look a bit unnatural as physical reaction starts only when half of the particle has already penetrated the mesh, but it is perfectly suitable if there are a lot of small particles. The main advantage of this method is its fastness and cheapness performance-wise.

Collision option, unlike the intersection, detects whether the whole particle shape has collided with the mesh. If the particles are big enough, this is the preferable method, as it provides the higher degree of visual realism: the particles react according to the set settings, when their edge or vertex comes in contact with the objects. Collision calculations are more expensive, so big particles systems should use this option carefully.

Another way of interaction of the particle systems is the interaction with the environment through physical nodes (also known as effects): wind, force and water.

Particles Physical Mass

Physical Mask

Delay option defines the time to pass between the parent system initialization and initialization of the child node. So at first the parent system starts to emit the particles, and only after the delay pause the generation of the child particles is activated. The delay can be set with some extent of randomization.

• With the mean and spread value of 0, the parent and child systems are initialized synchronously.
• With higher mean value, the initialization of child particle system will be delayed relative to the parent one for the strictly defined time period.
• Providing spread value allows to define the range for randomization of the delay pause.

The particle system evolve with time, so after encountering it in the virtual world, the system only starts to be generated, particle by particle, until the whole system gains the intended look. When the character comes out on the glade, we will see the fire gradually burn up. Warm start for the particles enables rendering the full-grown particle system straight away.

The technical realization of warm start is the following: when the particle system is initialized on encountering, its life is computed starting from the generation of first particle to its disappearance. After that, the particle system is considered evolved and is rendered in this state. The calculations are taken at a fixed frame rate of 25  fps, which is the minimum required for correct simulation of particle systems (see also information about correlation between framerates).

Warm start can be enabled without any detrimental effect for any particles systems, except for huge ones.

Deptho sorting is required, if particles use alpha blending. If not enabled, then basing on the depth buffer data, further positioned opaque objects can be wrongly rendered over the top of transparent particles. With depth sorting, geometry will be rendered in the order from back to front, which rules out visual artifacts.

The emitter can generate particles not only constantly, but also in intervals. Duration parameter controls the time period, during which emission occurs.

• The higher the value, the longer are the periods when particles are generated.
• Setting the Duration to 0 means that particles are spawned over an infinitely small time interval, that is, only during one frame, simultaneously. This value can be used when you need to simulate a one-particle gun shot, for example. The exact number of particles to be emitted at once is controlled via the Limit parameter. (Set the Spawn rate very high in order to generate particles in this case).

After the generation cycle is over, the emitter can make a pause, duration of which is defined by the Period parameter:

• If the mean value is set to 0, the particles are spawned constantly, without any pauses.
• If infinity (inf) is specified, the particles emitter becomes inactive after one generation cycle.

The Life Time parameter shows, how long will particles exist after emission in seconds. This parameter also has mean and spread values, that allow the particles flow to form a convincingly disappearing tail with changing density.

• If the mean value is set to 0, the particles will not appear.

The Direction parameter specifies the direction in which all the emitted particles move forming a flow.

• Emitting direction is specified along the X, Y and Z axes. The values are relative to each other, which means the particles flow is deflected in the direction with greater value.
• Negative values force the particles to move in the inverse direction of the axes.

If a Random emitter is used to generate particles on the mesh surface, it is possible to make the emitted particles move in the direction of the surface normal. For that, you need to set the following:

• Direction values to 1, 1, 1.
• Spread values to 0, 0, 0.

Shaping the particles flow allows to create different spawning models of the particles. Spread option introduces additional angular modulation of the emission direction. It represents the same spread values, that randomly vary direction of the emitted particles within the set limits.

• Spread controls the scattering of the particles along the X, Y and Z axes. The values are also relative to each other and the direction values.
• The higher the value, the wider the particles are scattered. If no direction values are specified, the scattering occurs in both the positive and the negative axis directions.

The velocity determines the speed of particles movement in the set direction. To grant the natural-looking flow, the final velocity of each separate particle can differ, if the spread value is set.

• The higher the mean value is, the faster the particles movement becomes.
• Negative mean values set the particles movement in the opposite direction to the set one.
• If the spread value is higher than the mean velocity value, the particles will move in two directions: the main flow in the specified direction, and some particles in the opposite one. To avoid it, the spread value should be less than or equal to the mean velocity value.

Particle velocity can be altered by the additional external force of gravity. With this parameter the particle direction can be interfered with, diverting the flow along the X, Y and Z axes.

• Positive values equal gravity vector directed upwards.
• Negative values equal gravity vector directed downwards.

Linear damping specifies the decrease in particles linear velocity with the time and is used to simulate effect of the medium friction on particles. In other words, this parameter shows how quickly the particles stall their linear velocity.

• If the value is set to 0, the velocity of the particles remains the same throughout all the particle life.
• The higher the number is, the faster the particles velocity decreases with the time until it stops completely.

To add the angular velocity to particles, use the Rotation parameter. Starting from the initial orientation angle, particles can spin around their own axis. Rotation parameter determines the angular velocity of their spinning motion.

• Positive numbers rotate the particles clockwise.
• Negative numbers rotate the particles counterclockwise.

Angular damping is responsible for the gradual decrease of particle rotation. This parameter shows how quickly the particles stall their angular velocity, add by the Rotation parameter.

• If 0 value is set, the particles will consistently rotate throughout all of their life.
• The higher the number is, the faster the particles lose their angular velocity, until they become completely nonspinning.

By using the Growth parameter, you can change the size of the particles dynamically, in spite of the particles radius range. The growth parameter introduces particle that change their initial size with the time. The growth can be either defined strictly (only mean value) or approximated with the range (by specifying also a spread value).

• Positive numbers result in particles increasingly growing after they are spawned. For example, fireworks grow and blossom in the sky.
• Negative number define the particles gradually lessening. For example, to imitate the smoke dissipating in air.

To control the Growth parameter, use the Growth Damping parameter. It defines the gradual decrease in the particles enlargement in size with the time. For example, with this parameter you can simulate explosion effects: in the beginning the particles grow rapidly, while afterwards their growth slows down.

• The value of 0 means the particles will be constantly increasing in size throughout all of their life time.
• The higher the value is, the faster the growth of particles stops and they live out with the constant size.

If the Culling option of the particle system is enabled, the engine will not render the particle if the intersection or collision have occurred. It can be useful if you don't want to render particles after the interaction with the geometry.

If the option is disabled, the behavior of particles will depend on other parameters. For example, they can appear on the geometry surface as decal objects or reflect from the surface and continue the movement.

After hitting the obstacle (if the culling option is disabled), they can bounce off the surface rather than slipping along it. This option is very convenient if simulating some watter effects that include splashes. The strength of the effect of bouncing depends both on the restitution parameter of the particles and the restitution surface option for the colliding object (Properties ->Parameters tab -> Restitution).

• With the minimum value of 0, the particles does not bounce off.
• With the maximum value of 1, the angle of particles fall equals the angle of their bouncing off.

Another characteristic of the particles, showing up on their contact with the obstacle, is the roughness of their surface. It determines whether the particles scatter in different directions or react as a uniformly directed flow.

• The minimum value of 0 means the trajectory of all particles after the collision is the same.
• The maximum value of 1 means the particles will scatter in different directions and a bit angularly. This angle arising from particle roughness also affects bouncing behaviour.

Attractor property allows to correct the linear direction of particles movement.

• If positive value is provided, the particles are repelled and move away from the force center point.
• If negative value is provided, all the particles are attracted to move in the direction of the force center point.
• By 0 value, the property is inactive and does not effect particles movement.

With Rotator property, the particles are spinned around the force center point.

• Positive values spin the particles in the clockwise direction.
• Negative values spin the particles counterclockwise.
• By 0 value, the property is inactive and does not effect particles movement.

To get more plausible visual result, especially by applying a set of different forces, their influence on the particles can gradually decrease when moving to the boundary.

• 0 equals no attenuation and the force's influence ceases abruptly when crossing the sphere boundary.
• The higher the attenuation value is, the closer to the force center point starts to weaken.

The following parameters are used for pixel sampling from the generated noise texture:

• Offset - sampling offset along the axes.
• Step - size of the sampling step along the axes.

Force - is a force multiplier. The higher the value is, the higher the value of the resulting force that affects an object inside the noise volume will be.

The following parameters are used to generate a noise texture that will be used in run-time.

• Scale - scale of the noise texture. The minimum value is 0, the maximum value is 1.
• Frequency - number of octaves for the noise. The higher the value is, the more details the noise texture has. The minimum value is 0, the maximum value is 32.
• Size - size of the noise texture image in pixels.

Reflector represents an invisible surface aimed for providing additional bouncing off and scattering of the particles.

Deflector Restitution

Deflector Roughness

Clipper is a surface field that clips all the particles it comes in contacts with. If the particle system is accidentally dispersed into the unnecessary area, it can be clipped simply and trouble-free by adding a clipper, that does not render, without exerting additional effort of readjusting the whole particle system.