Controlling Particle Density and Color By Age Using MagmaFlow

 Applicable to Krakatoa 1.5.0 and higher

1 Controlling Particle Density and Color By Age Using MagmaFlow
- 1.1 Overview
- 1.2 Workflow

Overview

In previous versions of Krakatoa, fading off particles by Age required pre-processing of the Age, LifeSpan and Density data using either Script Operators or Box #3 DataOps inside of Particle Flow - see the following Tutorial for details.
In Krakatoa 1.5.0 and higher, the same can be done as a post-processing step as long as the PRT files contain the Age and LifeSpan channels generated by Particle Flow.
In addition, Krakatoa v1.5.1 introduces a Curve Operator which can be used to completely customize the falloff values.

Workflow

Creating a Sample Particle Flow

In the following example, we will use a simple Particle Flow - quite similar to the one found in the previous Tutorial mentioned above.
- We create a Standard PFlow, set the Emitter shape to Circle with Radius of 20.0.
- We set the Birth Operator to emit 100K particles from frame 0 to 100.
- We set the Speed Operator to 100.0 units per second without any variation and Flip the direction.
- We delete the Rotation and Shape Operators
- We add a Delete operator and set it to By Particle Age, Life Span 60 Variation 0.
The result is a simple column of particles rising up and dying after 60 frames.

Saving the Particles to PRT Sequence

We open the Krakatoa GUI, switch to "Save Particles To File Sequence" and specify a PRT file name to save to in the Save Particles rollout.
We expand the Channels rollout and specify the following channels for saving:
- Position
- Velocity (optional)
- ID (optional)
- LifeSpan
- Age
We can then save frames 0 to 100 of the Particle Flow to the PRT file sequence.
Once the saving is done, we can disable the Particle Flow

Creating a PRT Loader with KCMs

Now we can create a PRT Loader - if you have the Krakatoa MacroScripts installed on a toolbar, hold down the SHIFT key and click the PRT icon.
- In the PRT Loader created at the origin, select the PRT sequence saved in the previous step.
- Set the Viewport % to 100.0 to show all particles.
Add a KCM to the PRT Loader (if you have the Krakatoa MacroScripts installed, just press the KCM icon, otherwise add a Krakatoa Channels modifier from the Modifiers list).
Open the MagmaFlow editor and set the Output node to Density.
- Change the Input node to Age channel.
- With the Age Channel Input node selected, drag a ToFloat Operator node from the Converst category of the Depot, or use the right-click menu to add one. We have to do this because the Age and LifeSpan are generated and stored as Integer Ticks and not as Floats.
- Press SHIFT+L to create another Input node with the LifeSpan channel.
- With the LifeSpan Channel Input node selected, drag a ToFloat Operator node from the Convert category of the Depot, or use the right-click menu to add one.
- Select the two ToFloat Operators and hit the Divide / key on the Numeric Keypad to create a Divide Operator node. It will be connected to both ToFloat nodes.
- Drag a connection from the output socket of the Divide to the input socket of the Output operator.

Add a second KCM to the PRT Loader
- Switch its input node to Density.
- Since the Output is a Vector (Color) and the Input is a Float (Density), hit the T key with the Input node still selected to create a ToVector operator.
- Select the Input node and the ToVector Operator node and press the Spacebar key twice to connect the Input to the second and third sockets, thus turning the Density into a grayscale color.
- To avoid rendering the black&white gradient, you can optionally right-click the Modifier and set it to Off In Renderer. This way, the Density will be shown as Color only in the viewports!

In the viewport, you will notice that the Color of the particles (which is now representing their Density) is changing gradually from black (where Age is 0 and thus Age/LifeSpan=0) to white (where Age/LifeSpan=1.0)

Since we want our density to start at 1.0 and end at 0.0, we have to invert the values.
- Select the KCM_Density modifier and in the MagmaFlow, select the Divide Operator node.
- Hit the - (Minus) key on the Numeric Keypad to insert a Subtract Operator node between the Divide and the Output nodes.
- Press Ctrl+W to swap the first and second input sockets of the Subtract Operator.
- Press F to create a Float Input node with a value of 1.0.

As result, our particles are now colored white in the beginning and black at the end of their life.

Exponent Curve

Currently, our Density falloff is linear - particles with an Age of 30 frames have half the Density of particles with Age of 0.
We could change the falloff curve to an exponential one by adding a Power Operator between the Subtract and the Output Operators.
- Select the Subtract Operator and press the 6 key (it also has the ^ symbol on it!) to add a Power Operator.
- With the Power Operator still selected, hit Ctrl+6 to create a Float Input with a value of 6.0.

The result in the viewport will look like this:

By changing the Power Float Input to values below 1.0, we can change the curvature of the falloff in the opposite direction, producing more dense particles throughout most of their life.
- For example a value of 0.5 produces the following falloff:

Krakatoa 1.5.1 Curve Operator Node

 Available in Krakatoa v1.5.1 and higher

Instead of using mathematical operators to control the inversion and the falloff of the Age curve, we can use the Curve Operator in Krakatoa v1.5.1 and higher.
The Curve Operator takes an input in the range from 0.0 to 1.0 and outputs a value based on a user-defined curve.
We will have to remove some of the operators first:
- Select the Power Operator added in the previous step and press the Backspace key to remove it while preserving the flow.
- Select the now disconnected Float Input of the Power Operator and press the Backspace or Delete key to delete it.
- Select the Subtract Operator, switch it to Function and select Curve from the type list.
- The default Curve control is set to a line mapping input value of 0.0 to output value of 0.0 and 1.0 to 1.0.
- From the Presets [>>] list, we can select some preset curves, for example Accelerate:

The result in the viewport is flipped again, since the 0.0 value corresponds to black and not to white.
We can fix this by flipping the Curve horizontally using the Horizontal Flip button, thus mapping 0.0 to 1.0 and 1.0 to 0.0:

Similarly, we can select the Decelerate preset and flip it to get the slower falloff of Density:

The Curve Operator lets us define ANY curve using Corner, Bezier Corner and Bezier points. Thus, we can create rather fancy particle Density falloffs:

The results of the Accelerate, Accelerate Flipped, Decelerate Flipped and Custom Curves shown above can be seen on the four copies of the PRT Loader in the following screenshot:

Color Gradient By Age

Based on the above setups, we could easily produce gradients between two colors by just feeding the Density value in the range from 0.0 to 1.0 into a Blend Operator with two vector inputs.
If we want to use a large number of intermediate colors though, going the procedural way by introducing even more vector nodes and trying to blend between them inside the KCM would be rather convoluted.
A simpler approach would be to convert the Density into the U coordinate of the UVW Mapping Channel and use it to map a Gradient or Gradient Ramp map to the particles. This is a lot closer to the Particle Age Map approach of 3ds Max, but with a lot more flexibility!
- We change the second KCM to convert the Density into the U component of the Texture Coordinate and we set Y and W to 0 using a Float Input Node.
- Then we switch the Output to TextureCoord channel which corresponds to Mapping Channel 1 - we could use ANY of the 99 channels though.

We assign a Standard Material to all PRT Loaders and add a Gradient Ramp map to the Diffuse Channel.
- The Gradient Ramp is set to Explicit Map Channel 1 by default and we can define any colors we want in it.

The result in the viewport looks like this:

As you can see, the gradient is going in the wrong direction because the left of the map corresponds to U value of 0.0 and the right to 1.0, but we have flipped the Density to start with 1.0 and end with 0.0.
- Instead of reworking the whole Gradient Ramp flags, we can simply enter a U Tiling of -1 - this will mirror the map colors and give us the result we want:

Obviously, you could apply the same approach to Bitmap textures and paint the colors into a bitmap in Photoshop, pointing at the correct texel using the TextureCoord channel as needed.
Also keep in mind that every flag of the Gradient Ramp can be right-clicked and set to any Texture Map tree you want.
- For example, in the following example the left red flag was replaced with a Cellular Map, the middle cyan flag was replaced with a Checker Map (turquoise and orange) and the right red flag was replaced with a Noise Map: