Houdini 20.0 Nodes VOP nodes

MtlX Surface Unlit VOP node

An unlit surface shader node.

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Since 19.5

An unlit surface shader, constructed from emission and transmission values. It represents a surface that can emit and transmit light, but does not receive illumination from light sources or other surfaces.

Create emissive particles

You can use this shader to emit light from particles, e.g. from a POP Network OBJ.


The following guide only explains how to create emissive particles, not a complete scene setup with Camera LOP, Karma Render Settings LOP, etc.

  1. On the stage level, press ⇥ Tab to open the Tab menu. From there, create SOP Create LOP.

  2. Click the node’s name and rename it to particles.

  3. Double-click the node to dive into it.

  4. Inside the SOP Create LOP, add a particle source object. This can be any geometry, e.g. a Sphere SOP.

  5. Add a downstream POP Network and connect its first input with the output of the sphere.

  6. Double-click popnet to dive into it.

Particle network

Inside the POP Network you can see a complete and fully working network. When you go to the Playbar and click to start the simulation, you will see that new particles are added, but they remain static. You need a force to move the particles.

  1. Press ⇥ Tab again, and add a POP Force DOP.

  2. Place the node between source_first_input and wire_pops_into_here to connect the force.

  3. On the Noise tab, set Amplitude to 1.

  4. Select source_first_input.

  5. On the Birth tab, change Const. Birth Rate to 200 to decrease the number of particles.

  6. Return to the stage level.

Emissive material

The material lets you define emission strength and color.

  1. Add a Material Library LOP and connect its input with the output of the particles SOP Create LOP.

  2. Double-click the new node to dive into it. There, lay down an USD MaterialX Builder VOP. This node is basically a modified Sub Network VOP.

  3. Dive into the mtlxmaterial node, create the MtlX Surface Unlit VOP node and connect its output with the input of surface_output.

  4. Increase Emission to make the lights brighter, while Emission Color tints the particles.

  5. Return to the stage level.

Assign the material

You must assign the material to the particles through paths to see the lights.

  1. Add an Assign Material LOP and connect its input with the output of materiallibrary1.

  2. For Primitives, enter /particles. This is the path to the SOP Create LOP.

  3. For Material Path, enter /materials/mtlxmaterial. This path points to the mtlxmaterial subnet inside the library.

You can see a graphic representation of the network’s hierarchy in the Scene Graph Tree pane. You can also drag elements from the Scene Graph Tree to the parameters of the Assign Material LOP.



Surface emission amount. Multiplier of the Emission Color.

Emission Color

Color emitted by the surface.


Surface transmission amount. Multiplier of the Emission Color.

Transmission Color

Color transmitted through the surface.


Surface presence, useful for cutting out shapes with images or other signals.

VOP nodes