Set particle attributes with a ramp texture

Use color ramps

You can use a color ramp to change vector array (per particle) attributes as particles age. When you create a ramp for a vector array (per particle) attribute, the attribute is connected to a color ramp.

For information on the Ramp Attributes, see Ramp.

To use a color ramp

Select the particle object.
Open the Per Particle (Array) Attributes section of the Attribute Editor, then right-click the desired attribute and select Create Ramp.
NoteTo control the position, velocity, or acceleration, right-click the rampPosition, rampVelocity, or rampAcceleration attribute box and select Create Ramp. You cannot use a ramp directly on position, velocity, or acceleration.
If you use a ramp and an expression to set the position, velocity, or acceleration, the ramp sets the value before the expression controls its value in a frame.

The rampPosition, rampVelocity, or rampAcceleration can receive input only from ramps. Each frame, Maya calculates the ramp attributes first, then calculates dynamics based on any value in the ramp attributes.

This creates a default ramp that controls the attribute as the particles age.
In the Lifespan Attributes section of the Attribute Editor, set the Lifespan Mode to Constant and set Lifespan to the number of seconds you want the particles to exist in your animation.
In the Per Particle (Array) Attributes section of the Attribute Editor, right-click the attribute box for which you just created a ramp, then select arrayMappern.outColorPP > Edit Ramp.
The Attribute Editor displays the default color ramp that controls the attribute. The three component values of the attribute are set to the RGB values of the vertical component (V) of the ramp over the particle’s lifetime. Values at the bottom of the ramp are used at the beginning of the lifespan, values at the top are used at the end of the lifespan.

By default, at the beginning of each particle’s lifespan, the left, middle, and right vector components of the attribute equal 1, 0, 0 because the RGB value at the bottom of the ramp is red (1, 0, 0).

At the middle of the lifespan, the vector components equal 0, 1, 0 because RGB is green (0, 1, 0). At the end of the lifespan, the vector components equal 0, 0, 1 because RGB is blue (0, 0, 1). Maya interpolates vector components between these three points.
Edit the ramp’s RGB color values as desired (see Rendering for details).
You can use any numerical values for R, G, and B in the Color Editor when you edit ramp values. Values outside the range 0 to 1 are valid, but they have no meaningful color equivalent in the display of the Texture Sample swatch. Do not edit HSV values or you’ll likely get incorrect results.

The only attributes for which the color in the swatch necessarily represent the component values of the attribute are rgbPP and incandescencePP. For other attributes that don’t have color equivalents, you can edit color values in the Color Editor by entering values in the R, G, and B boxes rather than by clicking colors directly.

For example, you can set rampAcceleration to 100, 0, 0 at some point in the lifespan by setting R, G, and B values of the corresponding point in the ramp to 100, 0, 0. These values create a red color in the ramp that’s identical to 1, 0, 0. However, adjacent points in the swatch will be interpolated with 100, 0, 0 differently than with 1, 0, 0.

You can also change the color sequence by changing other ramp attributes in the Attribute Editor such as Noise and Noise Frequency. See Rendering for details on the ramp attributes.

Remember, the horizontal component (U) of the ramp has no effect on the attribute by default. More precisely, the controlling component of the ramp is the leftmost vertical edge of the Texture Sample swatch in the Attribute Editor. To use the horizontal component as well, see Customize per particle attribute control with ramps.

An example of a default ramp applied to emitted particles follows:

Use grayscale ramps

You can use a grayscale ramp to change float array attributes as particles age. When you create a ramp for a float array (per particle) attribute, the attribute is connected to a grayscale ramp. In grayscale ramps, black represents a value of 0 and white represents a value of 1.

For information on advanced ramp features, see the following topics:

Working with the Ramp Editor in a separate large window
Mapping the color of a ramp index to a texture
Mapping the position of a ramp index to a texture.
Note
If you create a ramp for emitterRatePP, you control the emission rate over time for a directional or omni emitting particle object. The attribute exists in the particle object that emits, not in the emitted particles. You cannot control lifespanPP with a ramp.

To use a grayscale ramp

Select the particle object.
Open the Per Particle (Array) Attributes section of the Attribute Editor, then right-click the desired attribute and select Create Ramp.
This creates a default ramp that controls the attribute as the particles age.
In the Lifespan Attributes section of the Attribute Editor, set the Lifespan Mode to Constant and the Lifespan to the number of seconds you want the particles to exist in your animation.
If you are working with emitterNameRatePP, set the Lifespan to the number of seconds you want the emitter to exist. The emitting particle object will disappear after the Lifespan value specified.
In the Per Particle (Array) Attributes section of the Attribute Editor, right-click the attribute box for which you just created a ramp, then select arrayMappern.outColorPP > Edit Ramp.
The Attribute Editor displays the default grayscale ramp that controls the attribute. The values of the attribute are set to the R color values in the vertical component (V) of the ramp over the particle’s lifetime. Values at the bottom of the ramp are used at the beginning of the lifespan, values at the top are used at the end of the lifespan.

By default, at the beginning of each particle’s lifespan, the value of the attribute equals 1 because the R component at the bottom of the ramp is 1 (The color is white, with an RGB value of 1, 1, 1.)

At the middle of the lifespan, the value of the attribute equals 0.5, because the R component at the bottom of the ramp is 0.5. (The color is gray, with an RGB value of 0.5, 0.5, 0.5.)

At the end of the lifespan, the value of the attribute equals 0, because the R component at the bottom of the ramp is 0. (The color is black, with an RGB value of 0, 0, 0.)

Maya interpolates the values between the three points in the ramp.

Tip
Age is the attribute used with ramps by default. You can also use other attributes.
Edit the ramp’s R color values as desired (see Rendering for details).
You can use any numerical values for R in the Color Editor when you edit ramp values. Values outside the range 0 to 1 are valid, but they have no meaningful color equivalent in the display of the Texture Sample swatch. Do not edit HSV values or you’ll likely have incorrect values applied to the attribute.

Typically, opacityPP is the only attribute for which the color in the swatch represents the values in the attribute. For other attributes that don’t have color equivalents, you can edit color values in the Color Editor by entering values in the R box rather than by clicking colors directly.

You can also change the color sequence by changing other ramp attributes in the Attribute Editor such as Noise and Noise Frequency. See Rendering for details on the ramp attributes.

Remember that the horizontal component of the ramp has no effect on the attribute. To use the horizontal component as well, see Customize per particle attribute control with ramps below.

Customize per particle attribute control with ramps

A ramp has two inputs—U (horizontal) and V (vertical). By default, the V input of the array mapper is connected to the particle’s age and the U input is not connected. You can change these input connections when you are creating the ramp.

Other ramp options such as circular, diagonal, or box change on both axes. You select these in the ramp Attribute Editor.

To tune ramp control of per particle attributes, you can:

Use an existing ramp or customize U and V control of the ramp.
Ensure the ramp colors portray attribute values meaningfully.
Disconnect a ramp’s control of an attribute.

To specify how particle’s attributes get values from the ramp

If necessary, add the attribute to the particle.
Open the Per Particle (Array) Attributes section of the Attribute Editor, then right-click the desired attribute and select Create Ramp > .
This displays the Create Ramp options window.
To use an existing ramp, select it from the Map To menu.
To customize which attributes to control the U (horizontal) and V (vertical) ramp components, pick one of the following for Input U and Input V.
Click the attribute from the Input U or Input V menu.
TipIf you use Particle’s Age for both Input U and Input V, the part of the ramp that controls the attribute over the particle lifespan is indicated by the diagonal line in the following figure:

For the default ramp above, using Particle’s Age for both Input U and Input V has no advantage, because the ramp color doesn’t change in a U (horizontal) direction.

If you edit ramp attributes to create a ramp such as the following example, selecting Particle’s Age for both Input U and Input V adds versatility to your control of the attribute:

You can even use a texture as input to the ramp’s color. For example, you can map a 2D checker texture onto the color ramp as follows:

Editing the ramp

You can also edit the ramp to change the colors.

For information on advanced ramp features, see the following topics:

To edit the ramp

Display the Attribute Editor for the particle shape. To the right of rgbPP, right-mouse-click <-arrayMapper1.outColorPP and select Edit Ramp from the popup menu.
The ramp for rgbPP is displayed in the Attribute Editor.
Edit the colors in the ramp. See “Ramp” in Rendering for information on editing the colors in the ramp.

Map to an existing ramp

In addition to mapping to a default ramp, you can map the destination attribute to a ramp that you have already used in the scene.

To map to an existing ramp

Place the pointer in the data box of the per particle attribute for which you want to create a ramp. Click the right mouse button to display the pop-up menu and select Create Ramp > to display the Create Ramp options window.
Click the Map to button to display the options menu.
The ramps you have used in the scene are listed in the pop-up menu.
Select one of the ramps from the list.

Ensure ramp colors portray attribute values meaningfully

When you edit ramp values for RGB (or R) in the Color Editor, values outside the range 0 to 1 are valid but have no meaningful color equivalent in the display of the Texture Sample swatch. You can create meaningful color equivalents as follows:

To ensure ramp colors portray attribute values meaningfully

Set the lowest RGB (or R) values in the ramp to 0, and set the highest RGB (or R) values in the ramp to 1.
Right-click the attribute box of the per particle attribute that’s connected to a ramp, slide the pointer to the arrow to the right, and select Edit Array Mapper.
The array mapper lets you scale the range of attribute values applied to the particles.
Click the array mapper tab of the Attribute Editor.
For the Min Value and Max Value, enter the lowest and highest values you want to use for the attribute.
By default, the Min Value sets the lowest value the attribute has at the beginning of its lifespan—the value at the bottom of the ramp. By default, the Max Value sets the highest value the attribute has at the end of its lifespan—the value at the top of the ramp.

Disconnect a ramp’s control of an attribute

To discontinue a ramp’s control of an attribute, right-click the attribute box of the per particle attribute that’s connected to a ramp, slide the pointer to the arrow to the right, and select one of these:

Break Connection: Breaks the array mapper’s input to the particle shape. This menu item doesn’t delete the ramp node or array mapper. Use this menu item if you don’t want to remove the ramp when you break the connection.
Delete Array Mapper: Breaks the array mapper’s input to the particle shape by deleting the array mapper. The ramp will also be deleted if it has no other connections. Use this menu item if you no longer need the ramp in the scene.

Ramp example

Coloring particles according to distance from origin

In this example, we’ll use the V coordinate of a ramp to color particles according to their distance from the origin. This example uses emitters and fields, which are covered in the following chapters. You may want to read about emitters and fields before doing this example. Or you can cut and paste the MEL code into the Script Editor to create the emitters and fields.

To use a ramp to color particles according to their distance from origin

Create a sphere volume emitter. Set the Volume Speed attributes as follows: Away From Center to 1; all the other speeds to 0. Set Scale (10, 10, 10).

To do this, you can copy the following MEL commands from the Maya Help and paste them into the Script Editor:

emitter -pos 0 0 0 -type volume -r 100 -sro 0 -nuv 0 -cye none -cyi 1 -spd 1 -srn 0 -nsp 1 -tsp 0 -mxd 0 -mnd 0 -dx 1 -dy 0 -dz 0 -sp 0 -vsh sphere -vof 0 0 0 -vsw 360 -tsr 0.5 -afc 1 -afx 1 -arx 0 -alx 0 -rnd 0 -drs 0 -ssz 0 ;
scale 10 10 10;
particle;
connectDynamic -em emitter1 particle1;

Apply a radial field with Volume Shape set to Sphere to the particles. Set the Magnitude to -2, set Attenuation to 0, and disable Use Max Distance. Turn on Volume Exclusion and set Scale to 10, 10, 10. This will keep the particles inside the volume.
To do this, you can copy the following MEL commands from the Maya Help and paste them into the Script Editor:
```
radial -pos 0 0 0 -name pushIn -m -2 -att 0 -typ 0 -mxd -1 -vsh sphere -vex 1-vof 0 0 0 -vsw 360 -tsr 0.5 ;
scale 10 10 10;
connectDynamic -f pushIn particle1;
```
Add per-particle color to the particles.
To do this, you can copy the following MEL command from the Maya Help and paste it into the Script Editor:
```
addAttr -ln "rgbPP" -dt vectorArray particleShape1;
```
Select the particle and display the Particle Attribute Editor. In the space to the right of rgbPP, click the right mouse button and select Create Ramp > to display the Create Ramp Options.
Set Input U to None (the default), Input V to rgbVPP, and Map to New Ramp (the default). Click OK.
Notice that a new attribute called rgbVPP appears in the Attribute Editor.
Right-mouse-click next to rgbVPP and select Runtime Expression (before or after dynamics calculation). In the Expression Editor, copy and paste the following line, and click Create:
```
 rgbVPP = mag( position ) / 10;
```
The quantity mag(position) is always equal to the particle’s distance from the origin. We’ve divided by 10, which is the maximum distance any particle can be from the origin in our setup.

In other words, this expression says that the V input to the ramp will have a value 0 if the particle is at the origin, and 10 if it’s all the way out at the boundary of the sphere.
You may also want to make the particles a little bigger.
To do this, you can cut the following MEL commands from the Maya Help and paste them into the Script Editor:
```
addAttr -is true -ln "pointSize" -at long -min 1 -max 60 -dv 2 particleShape1;
setAttr "particleShape1.pointSize" 4;
```
Now turn on smooth shading, rewind. and play the scene.