Light Shaders

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

References:

mental ray User Manual, Chapter 1.8, 3.13.

mental ray Shaders Guide, Chapter 1.10, 2.2.

Rendering with mental ray, Chapter 7.8, Appendix D.

Where to find light shaders
Material shaders appear in the Hypershade > Create mental ray Nodes > Lights.

The following material shaders are shipped with Maya:

· mib_light_infinite

· mib_light_photometric

· mib_light_point

· mib_light_spot

· physical_light

Default behavior

The proper connection for a light shader is from lightShader.message to lightShape.miLightShader.

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

Light shaders compute the contribution of a light to a point. Light color, direction, distance, cone angle (for a spot light), decay rate are the common example of parameters that affect the light contribution.
In mental ray for Maya, light shader is attached to a light shape node (under mental ray tab).

This section show to use the new light shader mib_light_photometric with a light profile file, and the popular light shader physical_light.

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

Step 2. physical_light
Open cornellBox_light_start.ma.

cornellBox_light_start.ma

This scene uses dgs_material, dieletric_material and transmat to make the traditional Cornell box scene. See material shaders for to see how to use material shaders.

The challenge with this scene is that the light intensity does not change along the distance from the light source. As a result, two points in the same direction from the light receive the same amount of light regardless of the distance to the light.
physical_light implements the physically correct light source; that is, the inverse-square falloff of the intensity. It is especially important to use a physically correct light when the material shaders are physically correct as well. dgs_material and dielectric_material are examples of physically correct material shaders that do not amplify the incoming light energy.
For more details about physically correctness of shaders, see Rendering with mental ray, Chapter 7.8.

Let's assign physical_shader to the light shape, so that the light intensity decreases properly.

• Open the attribute editor of the light shape lightShape.
• Open mental ray section.
• From the Custom Shaders sub section, click the map button next to Light Shader.
• The Create Render Node window appears.
• Go to Lights section of the mental ray tab.
• Click physical_light
• A new shader physical_light1 is created, and its message attribute is connected to lightShape.miLightShader.
  
  

Now render with mental ray.

Unfortunately, the image is black.

The first problem is that the light cover casts shadow to the whole scene. You can assign a transmat shader as the shadow shader of the light cover, but let's disable the shadow for now.

• From the mental ray render globals, go to the mental ray tab, Shadows section.
• Set Shadow Method to Off.
  
  

Now render with mental ray again.

cornellBox_light_finish.ma

Compared to the previous image, the light falloff is obvious on the wall and the floor.
The color of physical_light is very high (1000, 1000, 1000). This is necessary because the light intensity falls off inverse-square to the distance, the light energy reaching to the point in the scene is much smaller than before.

The scene is saved as cornellBox_light_finish.ma.

Step 3. mib_light_photometric
mib_light_photometric is a new shader introduced in mental ray 3.3. This shader reads the light distribution from IES or Eulumdat light profiles provided by physical lamp vendors. It can lead to more realistic images with real light patterns, compared to the uniform distribution the existing light shaders provide.
Light profiles are available on the web.

The workflow is similar to physical_light example above:

• Open lightprofile.ma.
  This simple scene has a poly plane with lambert shader, and a point light. Lambert is chosen so that the light pattern shows up clearly on the plane, less affected by other factors like viewing angle.
• Assign mib_light_photonmetric to pointLightShape1.
• Create mib_light_photometric shader from the Hypershade > Create mental ray Nodes > Lights.
• Ctrl MMB+drag mib_light_photometric1 to pointLightShape1.
• A connection is made from mib_light_photometric1.message to pointLightShape1.miLightShader.
• Setting the parameters
• Open the Attribute Editor of mib_light_photometric1.
• color should be set very high, because mib_light_photometric applies the inverse-square distance fall off to the light intensity. Set it to 3000 3000 3000 for example, to see the light profile more clearly.
• The rest of the parameters except profile are same as mib_light_point. See mental ray Shaders Guide - 1.10 Light, for more details.
• profile is the light profile to use. Click the button next to it. A new node mentalrayLightProfile will be created.
• See the Attribute Editor of mentalrayLightProfile1.
• Choose a light profile file for File Name. If you do not have one, download it from the web.
• Select the appropriate format. Either IES or Eulumdat.
• resolution is the resolution to sample the profile. Higher the value, closer the pattern sampled to the profile.
  
  

render with mental ray.

mib_light_photometric with pale blue light color