Houdini 21.0 Copernicus

Flow solver

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Blocks in Copernicus

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.

Flow Blocks

Flow blocks function like a 2D fluid solver, although no actual fluid is involved. It is basically a Flow Block Begin and a Flow Block End with simulation mode turned on by default, and inputs/outputs for color, velocity, and temperature, are already set up. However, just like other blocks, you can turn off simulations, run fixed iterations, or enable live simulation.

Due to its 2D nature where everything has to rotate on a plane, it is not a real fluid simulation. While you could use this to make fire or smoke or liquid, it is generally intended to be an artistic tool to create interesting advection effects for motion graphics.

Using flow blocks

Once you use the tab menu to put down a Flow Block, you can put down a File. For this example, we will use Mandril.pic for the File Name. Then you can wire the c output of the File COP to the color input of the Flow Block Begin node. The following image is what you should see in the viewport.

To...Do this

Add some velocity

  1. Put down a Fractal Noise COP.

  2. Put down a Slope Direction COP.

  3. Connect the Fractal Noise noise output to the Slope Direction height input.

  4. Connect the Slope Direction output to the default velocity v input of the Flow Block Begin node.

  5. Set the Angle on the Slope Direction node to 90 degrees.

  6. Press Play on the playbar to see the distortion.

Use temperature to drive simulation

  1. Put down an SDF Shape COP.

  2. Put down an SDF to Mono COP.

  3. Connect the output of the SDF Shape to the input of the SDF to Mono node.

  4. Decrease the radius on the SDF Shape node to 0.2 to create a smaller source.

  5. Connect the SDF to Mono to the temperature input of the Flow Block Begin node to set the initial temperature.

  6. Press Play on the playbar to see the temperature rising up.

Tip

You can use the Time Scale parameter on the Flow Block End to speed up or slow down the simulation.

Blend in another image

  1. Complete the steps in Use temperature to drive simulation.

  2. Put down another File COP. We will use the default.pic butterfly for this example.

  3. Put down a Blend COP.

  4. Connect the color output on the Flow Block Begin node to the bg input of the Blend node.

  5. Connect the c output of the File node to the fg input of the Blend node.

  6. Connect the output of the the Blend node to color input of the Flow Block End node.

  7. Reduce the Mask parameter on the Blend node to 0.01 so that it slowly blends into the original image.

This will put the second File COP inside the block, which isn’t a problem for small files like this. However, if you were using a larger 2K or 4K texture, it would run significantly slower because it’s rerunning the file every frame. To solve this issue, you can instead use the passthrough input so that it’s only cooked outside of the network once. Re-wire the output of the File node to the passthrough input of the Flow Block Begin node, and connect the passthrough output to the fg input of the Blend node. This will significantly improving performance when using large files.

Use collisions

  1. Complete the steps in Use temperature to drive simulation.

  2. Put down an SDF Shape COP.

  3. Change the Basic Shape to Squircle and reduce the Blend to 0.75 to create in interesting shape.

  4. Put down an SDF to Mono COP and turn on the Invert checkbox.

  5. Connect the SDF Shape node to the SDF to Mono node and wire them into the passthrough input of the Flow Block Begin node.

  6. Press Press Play on the playbar to see the temperature rising up, but only in the designated shape.

Note

The flow solver uses a soft collision technique with a mask field. Increasing collision iterations will improve quality. The cache duration can be set on the Flow Block End node or managed with the Cache COP for expensive file reads.

Use divergence

  1. Put down an Chorma Key COP and connect the c output of the File node to the source input.

  2. Adjust the Hue/Saturation to filter out the colour red.

  3. Put down an Invert COP to invert the mask, and connect the matte output of the Chroma Key node to the source input of the Invert node.

  4. Connect the Invert node to to the passthrough input of the Flow Block Begin node.

  5. Press Press Play on the playbar to see the red areas exploding outwards.

The other input/output of the Flow Block is feedback. Color, velocity, and temperature are all fed back by default. This input lets you have extra fields, if you need more than just these three.

Note

feedback and passthrough don’t have to be a single layer. These two inputs can accept cables as well.

Content library example

The Houdini 21 splash screen project demonstrates the Flow Solver working in synergy with both SOP and DOP contexts. It shows how you can generate organic, intricate velocity fields in just a few minutes and then apply that data across different areas of Houdini. Download the file here.

Copernicus

Basics

Next steps

  • Slap comp

    Slap composite (slap comp) is a fast image manipulation you can use to view approximate and live results of a final composite.

  • OpenFX

    Describes what OpenFX is and how to use it in your network.

  • Hatching

    Describes how to use hatching in your Copernicus network.

  • VDBs

  • How to use ONNX Inference

    Describes how to apply inference using a model in the ONNX Inference node.

  • Filter List

    The Filter List window lets you quickly apply effects in your scene.

  • Copernicus tips

    Useful tips and information while using COPs.

Dynamics

Advanced concepts

  • Normals

    Defines the normals that the Copernicus network uses.

  • Spaces

    Defines the spaces that the Copernicus network uses.