All fields in
Maya are unique. A thorough understanding of each field's behavior is a
crucial first step towards an artist's ability to efficiently and
effectively design a dynamic effect. There are two fields, however,
which are commonly thought to yield identical results: Gravity and
Uniform. While similar, they have an important difference: how they deal
with mass.
While Gravity and Uniform have different default attribute settings, they can yield identical behavior with a couple attribute changes. Gravity, by default, accelerates particles along the negative Y-axis. The field also has the same effect on particles anywhere in worldSpace, regardless of the location of the field. Uniform, on the other hand, accelerates particles along the positive X-axis. Particles which are closer to the field accelerate faster than those which are further away.
While Gravity and Uniform have different default attribute settings, they can yield identical behavior with a couple attribute changes. Gravity, by default, accelerates particles along the negative Y-axis. The field also has the same effect on particles anywhere in worldSpace, regardless of the location of the field. Uniform, on the other hand, accelerates particles along the positive X-axis. Particles which are closer to the field accelerate faster than those which are further away.
gravity vs uniform default attribute settings
These
differences, however, are not intrinsic to the field types, but a result
of the settings on field attenuation and field axis. While gravity has
an attenuation value of '0', by default, Uniform has a value of '1'. As
with all fields, attenuation determines a decay/falloff effect. If a
gravity field's attenuation is set to '1', it will no longer behave like
real 'gravity', in the sense that particles closer to the field will
accelerate faster. The axis that the particles move is also just a
matter of attribute defaults. Both Uniform and Gravity have X/Y/Z axis
attributes. With this said, it should be clear that one can get Uniform
to behave like a default gravity field by simply setting the attenuation
to '0' and the Y-axis to '-1'.
So then, what is the real difference between Gravity and Uniform other than different default attribute values? As mentioned above, the difference is 'mass'.
It is important to know that all dynamic objects in Maya have a 'mass' attribute. For particles, this exists has a Per-Particle attribute found in the 'per-particle array attributes' folder. If particles are individually selected in component mode, and the Component Editor is opened, looking in the 'particles' tab will display the fact that all particles have a default mass value of '1'. What is imporant to note is that ALL fields in Maya use this mass attribute to determine their effect on particles... except for Gravity. The reason for this is that gravity, from a human perspective, causes all objects at human scale to fall at the same rate of 9.8 m/sec/sec. A marble and a boulder dropped from 50 feet will hit the ground at the same time. If you want to get scientific about it, based on Newton's Universal Law of Gravitation, where objects attract each other based on their mass, the boulder and marble do not actually fall at the same rate... but from our human perspective they might as well be. Gravity fields, therefore, are most effectively implemented when you need to simulate gravity... or when you need all particles to move in a specific direction at the same speed. To reiterate, gravity fields ignore mass
So then, what is the real difference between Gravity and Uniform other than different default attribute values? As mentioned above, the difference is 'mass'.
It is important to know that all dynamic objects in Maya have a 'mass' attribute. For particles, this exists has a Per-Particle attribute found in the 'per-particle array attributes' folder. If particles are individually selected in component mode, and the Component Editor is opened, looking in the 'particles' tab will display the fact that all particles have a default mass value of '1'. What is imporant to note is that ALL fields in Maya use this mass attribute to determine their effect on particles... except for Gravity. The reason for this is that gravity, from a human perspective, causes all objects at human scale to fall at the same rate of 9.8 m/sec/sec. A marble and a boulder dropped from 50 feet will hit the ground at the same time. If you want to get scientific about it, based on Newton's Universal Law of Gravitation, where objects attract each other based on their mass, the boulder and marble do not actually fall at the same rate... but from our human perspective they might as well be. Gravity fields, therefore, are most effectively implemented when you need to simulate gravity... or when you need all particles to move in a specific direction at the same speed. To reiterate, gravity fields ignore mass
Component Editor with individual particles selected, mass highlighted
So what about Uniform? Well, when you need particles to move in the same direction but you do not
want them to move at the same speed, Uniform is the answer. Example
applications include dust, debris, transporter effects, wind blown
objects and so on.
In regards to particle mass, it can be set in the Component Editor on a per-particle basis, although this is tedious for particle objects which have more than a handful of particles. The most effective way to set mass is with a creation/runtime expression.
In regards to particle mass, it can be set in the Component Editor on a per-particle basis, although this is tedious for particle objects which have more than a handful of particles. The most effective way to set mass is with a creation/runtime expression.
Expression Editor; creation expression to randomize mass values
Effect of Uniform field on particle grid with random mass values
Example:
Disintigrating Vase
Lets say you need to disintegrate an object into dust which gets blown by a gusting breeze. This can be accomplished by baking lighting into the textures on an object, then getting the surface to emit particles which inherit color from the baked texture. With a nice animated transparency map the surface can disappear while a similar animated map controls the emission of the dust particles. By randomizing the mass of the particles with a creation expression (i.e./particleShape.mass = rand(1,5)) and applying a Uniform field, the dust can dissipate in a more natural uneven manner. You could even have the size of the dust clumps be derived from particle mass, so that larger clumps move slower then smaller ones... you get the idea. Mass can also be controlled via a runtime expression so that mass can change based on definable conditions (i.e./mass could decrease as velocity increases so that as particles move faster they eventually accelerate at the same rate).
Disintigrating Vase
Lets say you need to disintegrate an object into dust which gets blown by a gusting breeze. This can be accomplished by baking lighting into the textures on an object, then getting the surface to emit particles which inherit color from the baked texture. With a nice animated transparency map the surface can disappear while a similar animated map controls the emission of the dust particles. By randomizing the mass of the particles with a creation expression (i.e./particleShape.mass = rand(1,5)) and applying a Uniform field, the dust can dissipate in a more natural uneven manner. You could even have the size of the dust clumps be derived from particle mass, so that larger clumps move slower then smaller ones... you get the idea. Mass can also be controlled via a runtime expression so that mass can change based on definable conditions (i.e./mass could decrease as velocity increases so that as particles move faster they eventually accelerate at the same rate).
In the above
image, particles are emitted from a NURBS surface emitter. The texture
rate is controlled by an animated ramp, while the particle color is
derived from a file texture created by baking the vase's shading group
lighting. The particles are pushed to the right by a uniform field and
dissipate due to turbulence and an opacityPP age ramp.
When all the particles have the default mass values, as in the above image, another by-product is the usually undesirable patterns which can emerge within the particle object. This is a result of how often Maya calculates particle emission. One way to solve this problem is by increasing the 'oversampling' value found in Solvers > Edit Oversampling. The reason to avoid this is that it has a substantial effect on playback speed. By randomizing mass values, however, this pattern will disappear, without a performance hit.
When all the particles have the default mass values, as in the above image, another by-product is the usually undesirable patterns which can emerge within the particle object. This is a result of how often Maya calculates particle emission. One way to solve this problem is by increasing the 'oversampling' value found in Solvers > Edit Oversampling. The reason to avoid this is that it has a substantial effect on playback speed. By randomizing mass values, however, this pattern will disappear, without a performance hit.
Staccato pattern removed by randomizing per-particle mass.
Final Composite of HW rendered, motion blurred particles with SW geometry.
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