You can rotate individual particles to create many different effects, such as billowy smoke, flying debris, and swirling snowflakes. You can set a particle’s initial rotation (orientation) when it’s emitted and also set its rotation speed (velocity) per particle type to be used for the duration of the simulation.
Rotations don’t really consider collisions. That is, if a rotated particle collides with an object, it doesn’t change its rotation as might happen in real life (such as with faster rotation or changed rotational direction upon impact). You could animate the particle’s roll, pitch, or yaw; or emit another particle type without yaw upon collision.
3D or Billboard Rotations
You can rotate particles in either 3D (XYZ) or 2D (billboard style). 3D rotations can face any direction, allowing the particles to rotate in all three dimensions (roll, pitch, and yaw). Billboard-rotated particles always face the camera, allowing you to spin the particles.
To see the particle rotation, you must set the appropriate Face Direction of Particle Rotation value in the attached shader (see The Basic Particle Render Type Shaders).
Roll, Pitch, and Yaw
When you rotate particles in 3D style, you can choose which of its axes (any any combination of them) around which the particle rotate. Like the classic airplane example:
• Roll makes the particle rotate around its Z axis.
• Pitch makes the particle rotate around its X axis (the airplane’s tail and nose move up or down).
• Yaw makes the particle rotate around its Y axis (the airplane’s tail and nose move laterally).
Setting the Particle’s Initial Orientation
You can set the initial orientation of the particles at emission time with the Orientation parameters in the Emission property editor. These determine the angle at which the particle is emitted, with 0 degrees being the default position with the particle upright and facing the camera.
• For Billboard, the value is simply set in degrees.
• For 3D rotations, the value is expressed with three angles in degrees: Roll (Z), Pitch (X), and Yaw (Y).
Variance (the Var parameter) is also available for these parameters (see Adding Variation to Particles).
Setting the Particle’s Rotation Over Time
You can also set the rotation used by a particle type during the length of the simulation. The rotation is updated at every frame using the Rotation Velocity values defined in the Particle Type property editor’s General page.
Rotation is expressed in degrees per second. For example, setting the Roll to 180 would be rotate the particle 180 degrees in one second. The higher the angle, the faster the particle rotates. For example, set Billboard to 180 to make nice rolling particles.
The rotation takes into account the particle’s orientation angle at emission time, as set on the Emission page (see previous page).
The rotation is also interpreted from the cloud’s reference frame (the cloud’s XYZ coordinates), as opposed to the global coordinates.
However, with Align On Velocity active, the rotation is expressed in a reference frame as defined by the particle’s velocity vector (direction). This is very useful to control local rotations on particles.
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