Houdini 16.5 Particles

Instancing and Rendering

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Overview

When you use the shelf tools to set up your particle system, it creates a POP network node inside a geometry object. This lets you render the particle system directly (since Mantra always renders objects).

By default, Mantra replaces particles with tiny spheres. You can also render particles as points, or replace particles at render time with images (called sprites) or geometry.

Render particles as points

  1. Select the object containing the particles.

  2. In the parameter editor, click the Render tab, then click the Geometry sub-tab.

  3. Turn on Render As Points.

You can still use the particles scale (pscale) attribute to control the size of the points in the render.

Instance geometry on particles (particle replacement)

To...Do this

Instance geometry on particles

  1. Click the Instance button on the Drive Particles tab of the shelf.

  2. Click the particle system you want to be affected and press Enter to confirm your selection.

  3. Click the object you want the particles to be replaced with and press Enter to confirm your selection.

    You can see a preview by clicking the template flag in the network editor, which will replace each particle with a wire frame of the shape.

For specific parameter help see the Instance node help.

Rotate the instanced geometry to an angular value

  1. On the Instance node (see above).

  2. Set the Orientation to Fixed axis.

  3. Set the Angle parameter to the angle to rotate to, and the Axis parameter to the axis around which to rotate.

  4. Create a Rotation node and connect the particle flow to it.

  5. Set the Angle parameter to the angle to rotate to, and the Axis parameter to the axis around which to rotate.

Rotate the instanced geometry using torque

  1. Click the Torque button on the Drive Particles tab of the shelf.

  2. Click the particle system you want to be affected and press Enter to confirm your selection.

    Torque is like sticking a handle on something and rotating it by pushing on the handle.

  3. Set the Position of the offset point which you will apply the force to, and the Force to the force to apply to the offset point.

For specific parameter help see the Torque node help.

Make the instanced geometry look at an object

  1. Create a Look At node and connect the particle flow to it.

  2. Set Lookat Mode to Lookat point.

  3. Set the Reference parameter to Referenced object.

  4. Set the Object field to the path to the object you want the particles to point towards.

Make the instanced geometry look at a plane

  1. Create a Look At node and connect the particle flow to it.

  2. Set Lookat Mode to Lookat plane.

  3. Set the Reference parameter to World Space or This Object to choose in which space to interpret the direction.

  4. Set the Direction parameter to a vector describing the direction in which the particles should look.

Set the rendered appearance of particles

To...Do this

Control the appearance of non-sprite particles

  1. Use the tab menu to create a Render node.

  2. Use the Particle type parameter to control what raw particles look like in the rendered image.

  3. You can use the Render node to control the global size of all particles, or you can use the Property POP to set each particle’s scale (pscale) attribute.

Display sprites in the viewport

  1. In the viewport options menu, choose Display ▸ Display options, or press D.

  2. Click the Particles tab.

  3. Set Display particles as to Sprites.

Control the appearance of sprites

  1. Create a Sprite node and connect the particle flow to it.

  2. At the SHOP level, create a material with a VEX Decal and a Mantra: Sprite Procedural, and connect them to the Output Shaders node.

  3. Assign a Texture Map to the VEX Decal in the parameter editor.

  4. Go back to the SHOP level and drag your Material onto your particle system.

Note

You can rotate and scale the sprites by turning on the Rotate and Scale checkboxes and editing the values in the Sprite node parameter editor.

Note

Sprites always face the camera.

For specific parameter help see the Sprite node help.

If Resolve SHOP Path is on, the Sprite SHOP path is resolved relative to this node, and the spriteshop attribute is set to the full path of the shop. If it is off, no attempt is made to find the shop, and the exact string entered in the Sprite SHOP parameter is put into the spriteshop attribute. This will prevent the sprites from rendering properly in the POP Viewer. The Geometry and Object Viewers will still render the sprites properly if the SHOP paths are specified relative to the object that contains the POP Network.

Transparent sprites

To see particle alpha in the 3D viewport, the particles must be displayed as sprites. By default particles are displayed as points.

  1. Press D in the scene view to open the Display Options window.

  2. Click the Particles tab and set Display particles as Sprites.

  3. Close the window.

  4. Append a Color POP to your particle system.

  5. On the Ramp sub-tab of the Color tab, set the Lookup parameter to $LIFE.

Note

You will need to use a shader that will read the Alpha attribute in order to render. For example, VEX Layered Surface.

Motion blur

When calculating motion blur, Mantra by default assumes the geometry of an object is unchanging, and blurs it as a whole. For a particle system obviously the points are moving, so you need to turn on velocity blur to use the velocity (v) attribute of the points to blur them individually.

So, to enable motion blur on particles you must enable it on the object as well the render node.

To...Do this

Enable velocity blur on an object

  1. Select the object containing the particles.

  2. In the parameter editor, click the Render tab, then the Sampling sub-tab.

Turn on Geometry velocity blur.

Enable motion blur on a render node

  1. In the main menu open Render ▸ Edit Render Node and choose the render node.

    If you don’t already have a render node, you can create one by choosing Render ▸ Create Render Node ▸ Mantra.

  2. In the parameter editor, click the Properties tab and then the Sampling sub-tab.

  3. Turn on Allow motion blur.

Rendering Attributes for Points and Particles

To change the rendering style of particle systems, use a Render POP.

Note

You must re-simulate the POP network for any changes in the render POP to take effect.

You can add attributes to points and particles by creating an Attribute SOP or Attribute POP and connecting it to the particle system.

The following attributes can be added to points, vertices, entire particle systems, and primitives:

Points

vertices

Particle systems

Primitives

Particles

Getting started

Behavior

Next steps

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