TouchDesigner Tutorial: Instancing with CHOPs

This tutorial explores how Channel Operators (CHOPs) provide an intuitive and powerful way to work with instancing data in TouchDesigner. CHOPs offer one of the most accessible methods for understanding how instances work and controlling their behavior.

Part 1: Creating Instances with Pattern CHOPs

Setting Up Sine and Cosine Patterns

The first example demonstrates creating instances arranged in a circular pattern using trigonometric functions.

Step 1: Create the Pattern CHOPs

1. Add a Pattern CHOP to your network
2. Set the length parameter to 50 samples
3. Change the channel name to “sin” (this creates a sine wave pattern)

Step 2: Add a Second Pattern

1. Connect the first Pattern CHOP to another Pattern CHOP
2. Set the second pattern’s type to “cosine”
3. Rename the channel to “cosine” to match the pattern type

Step 3: Combine the Patterns

1. Use a Merge CHOP to combine both sine and cosine patterns into a single operator
2. Connect the Merge to a Null CHOP
3. Make the viewer active and right-click inside to select “dots per sample” - this shows a dot for each sample in your multi-sample CHOP

Connecting to Geometry

1. Add a Circle SOP with a radius of 0.1
2. Right-click on the Circle’s output and add a Geometry component
3. On the Geometry component’s Instance page:
   • Turn on instancing
   • Set the Translate OP to your Null CHOP
   • Set Translate X to the “sin” channel
   • Set Translate Y to the “cosine” channel

The result is a new instance for each sample, arranged in a circular pattern where the sine channel controls X positions and the cosine channel controls Y positions.

Scaling the Pattern

To adjust the size of your circular arrangement:

1. Right-click on the wire between Merge and Null
2. Select “Insert Operator” and add a Math CHOP
3. On the Multi-Add page, use the Multiply parameter to scale the diameter

Part 2: Creating a Trailing Instance Effect

This second example creates a more dynamic visualization using noise and trail data.

Setting Up the Noise Trail

Step 1: Create the Noise Source

1. Add a Noise CHOP to your network
2. On the Channel page, use pattern expansion t[xyz] to create three channels: TX, TY, and TZ
3. On the Common page, turn off “Time Slice”

Step 2: Collect Position History

1. Connect the Noise CHOP to a Trail CHOP
2. Set the Trail to measure in samples
3. Set the sample count to 200
4. Connect to a Null CHOP named “null_inst” (null instances)

Step 3: Connect to Geometry

1. Add a Box SOP and connect it to a Geometry component
2. On the Instance page:
   • Turn on instancing
   • Set the Null instance CHOP as your default instance OP
   • Map TX, TY, and TZ channels to their respective translate parameters
3. Reduce the box size to 0.02 to see all instances clearly

Step 4: Scale the Movement

Insert a Math CHOP after the Noise CHOP and set the Multiply parameter to 3 on the Multi-Add page to increase the spatial coverage.

Part 3: Calculating Rotation Vectors

To orient instances along their trajectory of movement, you need to compute a rotation vector.

Computing the Rotation Vector

Step 1: Capture Past Positions

1. Add a Delay CHOP after your Math CHOP
2. Set it to measure in frames with a delay of 1 frame
3. This represents positions from one frame ago

Step 2: Rename Past Channels

Use a Rename CHOP to rename the delayed channels to past_* (resulting in past_tx, past_ty, past_tz)

Step 3: Collect Past Positions in Trail

Connect the renamed past positions to your Trail CHOP as well.

Step 4: Separate Current and Past Positions

1. Add a Select CHOP to select current positions (t*)
2. Add another Select CHOP to select past positions (past_*)

Step 5: Calculate the Vector

1. Use a Math CHOP to subtract the past positions from current positions
2. On the Combine CHOPs page, select “Subtract”
3. Use a Rename CHOP to name the result rotv[xyz] (rotation vector X, Y, Z)

Step 6: Merge and Apply

1. Use a Merge CHOP to combine current positions and rotation vectors
2. Connect this to your null_inst
3. On the Geometry component’s Instance 2 page, set the Rotate to Vector parameters to rotvx, rotvy, and rotvz

Visualizing the Rotation

To better see the rotation effect, modify your Box SOP by increasing the TZ scale to 0.2. This elongates the boxes so you can clearly see them rotating along their movement vector.

Understanding the Math

The key concept is that the math operation happens for every sample. TX, TY, TZ and past_tx, past_ty, past_tz are subtracted from one another based on their sample index. This per-sample calculation gives each instance its own rotation based on its individual trajectory.

Part 4: Adding Color Based on Position

To add color that changes based on position in the trail:

1. Add a Pattern CHOP with a “ramp” type
2. Reference the Trail’s window length for the Pattern’s length parameter
3. Name the channel “r” (red)
4. Merge this with your other instance data
5. On the Geometry component’s Instance 2 page, set the Color R parameter to the “r” channel

The result shows instances transitioning from white (where G, B, A default to 1) to blue as the R channel goes from 0 to 1 along the trail.

Key Concepts Summary

1. Pattern CHOPs - Generate mathematical patterns (sine, cosine, ramp) that can drive instance positions
2. Trail CHOPs - Collect a history of values over time, perfect for creating trailing effects
3. Delay CHOPs - Capture past values for computing velocity and direction vectors
4. Math CHOPs - Perform arithmetic operations across channels, including subtraction for vector calculation
5. Sample-Based Operations - All math operations happen per-sample, giving each instance individual control
6. Rotate to Vector - The Instance 2 page feature that orients instances along a direction vector

Practical Applications

This technique is excellent for:

• Particle trail visualizations
• Motion graphics with directional elements
• Data visualization showing movement over time
• Creating organic, flowing animations

CHOPs provide an intuitive way to understand and control instancing because you can visualize the data at every step of your network.