Shading

The particles can be shaded like any other object in Guerilla, but you can also create complex particle effects using the particle attributes. We'll show some simple exemples here.

Using the default shaders

The particles can be shaded with the standard shaders, Library/Materials/Surface, Library/Materials/Curves, Library/Materials/Particle, or Library/Materials/Volume regarding their mode.

The same particles rendered with different shaders : Point with the Particle shader, spheres with the Surface shader, a blobby with the Surface shader (using sss), multi-points with a Volume shader

Here is a summary of the particle mode and shader compatibility :

Particle Mode	Particle Shader	Surface Shader	Volume Shader	Curve Shader	SSS Support
Sphere	Yes	Yes	Yes	No	No
Streaks	Yes	No	No	Yes	No
Sphere	Yes	Yes	Yes	No	No
Blobby	Yes	Yes	Yes	No	Yes
Sprites	Yes	Yes	No	No	Yes

Use the particle attributes to control color

You can read directly the attributes stored in each particle by adding a PrimAttr shader.

In this case, we use the common color attribute "Cd" (Color diffuse) to control the color of particle colors.

Use the particle Age to control the opacity

First, in the RenderGraph, add a Particle shader node and an Attributes node to affect the particle system. In the attribute node, activate the Shading -> Opacity Mode to Vertex to get a fast opacity rendering. See User Guide/Shading/Opacity for details.

Then, select the Particle node and drag'n drop the Opacity shader input in a viewport. This action creates a sub-shader network that overrides the Opacity shader input.

In this sub-shader, create a Maya Particle -> Age Normalized node. Todo do that, open the node picker Ctrl+Space and type "Age" or use to the library.

Add an Invert node and build this network.

In the Particle node, activate the sub-shader for the Shadow Opacity input, and select the sub-shader we just built to re-use it in the Shadow Opacity shader input. Here it is RenderGraph|Particle|Opacity.

The result without and with the effect.

Using Maya Particles, you can also compute the opacity in the per particle alpha channel and use Opacity -> Use Particle Opacity. The result is the same. Use the particle Id to select a color in a gradient

Use the same technique as above but override now the Color -> Color shader input with this network. We use here a Randomized Ids node and a Texture -> Gradient node.

Here is the gradient we use, three constant colors.

The result without and with the effect.

Use the particle Speed to modulate the particle color

Use the same technique as above but override now the Color -> Intensity shader input with this network. We use here a Maya Particle -> Velocity node, a Length node and a Multiply node.

The result without and with the effect.

Write SL shaders for the particles

The particle attributes can also be accessed by the SL Box, to write more complex effects.

Here is a summary of the SL parameters available to the shader for each particle mode :

Particle Mode	st (texture coordinates)	v (curve parameter)	N (normal)	float12 (particle id)
Sphere	No	No	Yes	Yes
Streaks	No	Yes	No	Yes
Sphere	No	No	Yes	Yes
Blobby	No	No	Yes	No
Sprites	Yes	No	Yes	Yes

Here is a summary of the SL parameter to use to access the Maya Particle attributes exported with the Guerilla plug-in :

Attributes	SL parameter	Comments
Acceleration	color3
Age	float12
Age Normalized	float12/float13	Computed with Age and Lifespan
CollisionNormal	color6
CollisionTime	float18
CollisionUV	st4
Emission	color4
Force	color5
Ids	float12
Randomized Ids	float12*0.61803398874989-floor(float12*0.61803398874989)	Computed with Ids
Lifespan	float13
Mass	float14
Opacity	alpha
ParentUV	st3
Radius	float11
RGB	Cs	Color in Guerilla
SpriteScale	st2
SpriteTwist	float16
Velocity	color2