Volume Shaders

This section shows the basic workflow for mental ray volume shaders in Maya.

References:

mental ray User Manual, Chapter 3.10.

mental ray Shaders Guide, Chapter 2.5.

Rendering with mental ray, Chapter 6, Appendix D.

Where to find volume shaders
Material shaders appear in the Hypershade > Create mental ray Nodes > Volumetric Materials.

The following material shaders are shipped with Maya:

• mib_ray_marcher
• mib_volume
• parti_volume
  
  

Default behavior

The proper connection for a volume shader is:

• from volumeShader.message to shadingEngine.miVolumeShader
• from volumeShader.message to cameraShape.miVolumeShader

Ctrl+drag from a material shader to a shading engine to make the connections.

This section shows a workflow for using a volume shader to create smoke or fog effect.

Volume shaders can be connected to a shading engine or a camera shape.

If connected to a shading engine, the shader affects the inside of the object the shading engine is assigned to.

If connected to a camera shape, the shader affects the entire scene.

Volume rendering is an expensive procedure, using a bounding box for the participating medium is recommended, then assign a volume shader to the shape. In this example, we will use parti_volume to make a smoke in a shape of vase through which three balls are visible.

Step 1. Getting Ready

Make sure the mental ray for Maya plug-in is loaded.

• Windows > Settings/Preferences > Plug-in Manager...
• Find Mayatomr and check loaded

Open the template scene we will use for this section: volume_start.ma.

volume_start.ma

Step 2. Assign a material
We will create a fog effect around the balls, but also give some shape to the fog. For example, the vase can be used to shape up the fog.

• Position the vase so that the balls are inside the vase, then assign a material to the vase.
• Use a refractive or transparent material to see the effect of the volume shader because the volume shader assigned to a material will be called when the ray is inside the object.
• We’ll use transmat shader for this scene. transmat shader simply lets a ray pass through the object, making the object invisible.
  This is suitable for most atmospheric effects, but if you would like to create an object filled with smoke, a refractive shader can be used as a material.
• The ray tracing settings should be set high enough in the mental ray Render Globals.
• Create a transmat shader
• From Hypershade > Create mental ray Nodes > Materials
• Select transmat.
• Assign transmat to the vase
• Select the vase.
• RMB transmat1 from the hypershade and select Assign Material To Selection
• Check the connection between the shading engine and the vase.
  
  

Then Render

transmat

The vase is completely invisible, and the rest of the image should be the same as before.

transmat is useful when it comes to simulating atmospheric effects.

Step 3. Assign a volume shader
We’ll use the parti_volume shader.

• Create parti_volume.
• From Hypershade > Create mental ray Nodes > Volume Materials
• Select parti_volume.
• Connect parti_volume1 to transmat1SG.
• The connection is from message of parti_volume1 to miVolumeShader of transmat1SG.
• Ctrl MMB+drag parti_volume1 to transmat1SG to make the connection.

Now all the connections are made. Render again.

transmat and parti_volume

The vase is still invisible.

We need to adjust the parti_volume parameters to correct the look.

Step 4. Adjust parti_volume

• Open the Attribute Editor of parti_volume1.
• scatter is the scattering color. It's default to dark gray, which is too low for this scene.
• Set the scatter value to bright gray.
• Render, and finally the vase shows up in the image.

parti_volume.scatter

• Let's increase extinction so that light is absorbed.
• Render, and you can see the dark vase shape.

parti_volume.extinction

• Because the smoke absorbs light energy, we'd better increase the light intensity to see the effect more clearly.
• Render.

parti_volume.intensity

• The smoke in the vase has a sampling pattern, which we can reduce by increasing the number of volume samples.
• The default min_step_len and max_step_len is slightly high for this scene, so Decrease the min, max step size .
• Render again to see how the smoke looks quite smooth.
• (However, you may have noticed the increase in render time.)

parti_volume.min_step_len, max_step_len

• If the smoke looks too uniform, increase nonuniform to add variation to the density of the smoke.
• Render.
• (This will also increase the rendering time.)

parti_volume.nonuniform

volume_finish.ma

volume_finish.ma contains the scene at this point.

You can adjust more parameters to get the desired look.
mental ray Shaders Guide, Chapter 2.5 Participating Media, explains each parameter of the shader.

NOTE: lights is an array of lights the volume shader samples. Because mental ray for Maya 6.0 resolves the light linking in render time, and Illuminates by Default of a light shape is turned on by default, it is no longer necessary to connect light to this array. However, you can still manually connect the light, in which case mental ray for Maya would not perform the automatic light link for the shader.

Step 4. Cornell box
Another example.
cornellBox_volume_start.ma has the Cornell box scene with materials and a light shader.

cornellBox_volume_start.ma

The front wall, back wall, and a light cover share a shading engine transmatSG, which has transmat as its material shader.
As discussed in Material shaders, transmat is a totally transparent material, making the object invisible.
As a matter of fact, the above three surfaces are intended for volume effect.
Let's use parti_volume one more time.

• Assign parti_volume shader to transmatSG
• Use the following settings:
• scatter 0.05 0.05 0.05
• extinction 0.05
• min_step_len 0.13
• max_step_len 1
• light_dist 2

Then render.

cornellBox_volume_finish.ma

The glare around the light is very noticeable.

This scene will further enhanced in Photon Shaders using photon shaders and a photon volume shader with global illumination and caustics.