Particles

 

MtoA supports Maya particles and nparticles, however there are some current limitations:

  • Tube, Numeric and Streak are not supported.
  • There are some limitations with UV coordinates in particles and different textures (the Maya Ramp texture for example).

  • The particle cloud shader is not supported.
  • IPR refresh has to be forced from time to time.
  • SSS is currently only supported for polymeshes.

Render Points As

Choose to render the particles as points, spheres or quads (when 'Particle Render Type' is set to 'Points').

Min Particle Radius

Sets the minimum radius for the size of the particles.

Radius Multiplier

Applies a global multiplier to the radius size of the particles. 

Max Particle Radius

Sets the maximum radius for the size of the particles.

Min Pixel Width

If this value is non-zero, particles in points mode with a small on-screen width will be automatically enlarged so that they are at least the specified size in pixels. The enlargement fraction is then used in the particle shader to adjust the opacity so that the visible thickness of the particle remains the same. For a given number of AA samples, this makes it a lot easier to anti-alias very small particles, at the expense of render time (because of the additional transparency/depth complexity). Good values are in the range 0.2 to 0.7. Values closer to 0 are faster to render but need more AA samples. So if your scene already uses very high AA settings, you should use a low value like 0.1.

Min Pixel Width doesn't work on particles with varying radius over time (which is the case when motion blur is enabled). Also, for the automatic transparency to work, it needs a special material that supports the internal geo_opacity shader such as the Standard Hair shader.

Interpolated Motion Blur

When the motion blur keys from the scene don't match the time where the nParticle cache was sampled, artifacts will appear. The Interpolated Motion Blur check will avoid this situation by using the nearest samples from the cache. This option requires that the nCache Evaluation Interval attribute be the same as the nCache Evaluate Every # frame(s) option that was used to create the nCache.

 

Delete Dead Particles

This option disables the rendering of particles that exceed the maximum particle Lifespan range when using motion blur.

 

Volume Step Size

Particles switch to rendering in volume mode when using this attribute. This controls the rate at which the volume shader gets called. The value indicates the average distance between adjacent volume shader samples along a given ray, in world space units. Generally you will want this setting to be as large as possible before visibly noticeable artifacts occur, and in the case of voxelized volumes no smaller than about half the size of your voxels (in world space). Low Step Sizes will increase render times. For example, when the step size is 0.1 and the volume is 10 units big in world space, there are approximately 100 primary samples and therefore the volume shader is called 100 times.

Cloud texture connected to scattering & attenuation of volume collector

The volume step size has a fixed size, therefore, the larger the container the more steps will be taken and the longer it will take to render. Also beware that if the step size is too small your render times will increase. If you cannot see anything rendered, you may need to reduce the Step Size value.

A Maya scene that demonstrates nparticles rendered as volumes can be found here.

Workflow Issues

Particle Sampler Info

MtoA does not export the particleSamplerInfo node. It exports some "userDataXXX" nodes and they require that the info is exported within the particles object. 

The example below has a particleSamplerInfo connected to the Uv Co-ordinates of a ramp texture and the Out Uv Type is set to Normalized Age:

The particleSamplerInfo is ignored and the particles remain the same color over time

 

By default this will not work. However, it can be achieved by selecting the particleShape node, and in the Arnold section, in the Export Attributes, type 'age':

Adding 'Age' to the Arnold Export Attributes for the ParticleShape node fixes the connection to the ParticleSamplerInfo node

 

 

A Maya scene file that demonstrates the above particle sampler info workflow can be found here.

A video that demonstrates the above particle sampler info workflow can be found here.

OpacityPP with particles will only be exported if the Opacity in the Arnold section of the particleShape is disabled.

Using Maya's Particle Instancer in combination with Stand-ins is a powerful way to produce scenes with a lot of complexity.

Stand-ins connected to particles via Particle Instancer (rollover image)

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