Working with nParticle attribute ramps
Some nParticle attributes have ramps that allow you to control the way the attributes are applied to your nParticle objects. Attribute ramps that set per particle attributes are known as internal ramps, and they are connected to the object’s nParticleShape node when it is created. Other attributes have ramps which provide additional control over how the attribute value is applied to the nParticle object. For example, the Point Field Scale ramp allows you to vary Point Field Magnitude values along a defined Field Distance. nParticle attribute ramps work similar to other Maya ramp editors.
The ramp's horizontal component is represented by a Selected Position, while the ramp's vertical component is represented by a Selected Value. Attribute ramps with color settings, such as the Color and Incandescence ramps, use Selected Color instead of Selected Value. Depending on the ramp, Selected Position is defined by the ramp’s Input Value, or property to which the ramp is mapped. For example, if a ramp’s Scale Input is set to Age, then the far left position on the ramp represents the beginning of the nParticle's lifespan (Age =0), while a Position of 1 represents the final Age value of the nParticle.
Selected Values specify the scaling value of the input attribute at the Selected Position. For example, for the Radius Scale ramp, each Selected Value specifies a scaling value for the nParticle object's Radius attribute. By default ramps define a scaling value of 1, meaning that only one point exists on the ramp and that point represents an attribute scaling factor of 1.
Values output from an internal per particle ramp are also affected by the ramp's Scale Input and Input Max values. Scale Input defines the property used to map the ramp's input value, while Input Max defines the value range used by the ramp. For example, if Radius Scale Input is set to Age and Input Max is set to 50, the ramp would map radius values for particles that have an age between 0 and 50 seconds. The radius of nParticles that have an age of 50 seconds or older would be set to the radius defined by the Selected Value of the ramp's right-most position marker.