Blocks in the COP network act similar to compile blocks in the SOP network, as they encapsulate a bunch of nodes that you can treat as a single object. They typically consist of a Block Begin node and a Block End, with other nodes in between. Turning on Simulate enables simulation mode, which ties the process to the frame bar, allowing for caching and checkpointing in memory for faster recooking and scrubbing.

Another important feature is Live Simulation, which allows Houdini to continuously animate in almost real-time, providing real-time feedback for changes. This is similar to a video game world, where things play whether or not you are actively pressing anything. This mode is not tied to the playbar, but it’s still recooking all the time. So if you make a change in the network, you will see its results being played back live in the viewport.

Reaction-Diffusion simulations are complex 2D simulations of chemicals reacting and diffusing. The simulation generates dynamic and unique patterns based on the interactions of two chemicals as they react with each other, diffuse through the medium, and are replenished at specific rates.

Create a Reaction-Diffusion simulation block by creating a Reaction-Diffusion Block Begin and Reaction-Diffusion Block End, and adding other COPs between them. You can also create a Reaction-Diffusion Block node, which automatically places down the Reaction-Diffusion block begin and end nodes. Use the passthrough input to bring additional inputs inside the block. Use the feedback input to add additional values that must be updated every frame.

This solver can be interacted with in a few ways: hard iteration counts, cached simulation, or sandbox-style live simulation. You can also wire a Reaction-Diffusion block into another Reaction-Diffusion block (for example, to input the raw chemical values).

For example, use Reaction-Diffusion to create organic patterns such as zebra print, coral, and fish scales.

1. Create a Reaction-Diffusion Block COP in your COP network. This makes Reaction-Diffusion Block Begin and Reaction-Diffusion Block End nodes.

   Note

   Instead of a block, you can individually add Reaction-Diffusion Block Begin and Reaction-Diffusion Block End nodes. In Reaction-Diffusion Block Begin, set Block Path to the path of the Reaction-Diffusion Block End and connect their chemical output and input.

2. Create an activation mask for the solver.

   Shapes or harsh noise create an active reaction. You can also add decorator recipes on the Block Begin node to set up initial conditions. Some presets and conditions may die out immediately because they require a large contrast between neighboring pixels in order to activate, so if a pattern isn’t working try making it smaller or adding noise.

3. Wire additional nodes between the chemical output and input of the block. There’s a selection of nodes that you can connect to this chemical wire.

   Note

   The reaction can be unstable (see Reaction Parameters).

   Tip

   Use a Distort by Slope recipe to advect the chemical reaction into different directions.