Photon Shaders

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

Photon shader takes care of how the material interacts with photons - how the surface reflects, transmits and absorbs photons. If the surface has a diffuse component, the photon shader will store photons.
This section shows how to use photon shaders with global illumination and caustics. We will take the plain Cornell box scene with material shaders and a light shader, and add global illumination, caustics and finally volume caustics.

References:

mental ray User Manual. Chapter 3.15

mental ray Shaders Guide. Chapter 2.4

Rendering with mental ray. Chapter 7, Appendix D.

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

The following photon shaders are shipped with Maya:

• dgs_material_photon
• dielectric_material_photon
• mib_photon_basic
• parti_volume_photon
• transmat_photon
  
  

Default behavior

The proper connection for a photon shader is from photonShader.message to shadingEngine.miPhotonShader.

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

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 cornellBox_photon_start.ma.

cornellBox_photon_start.ma

The Cornell box scene was used in material shaders, light shaders, volume shaders.
Note that a physical_light is attached to the lightShape node to provide physically accurate lighting with inverse-square distance falloff.

It is important to use physically correct light and material shader, photon shader to prevent the scene from flooding with energy.

This scene uses material shaders from physics.so package for this reason.
Another small change to notice is that the transmatSG now has a shadow shader transmat2, which will pass through any shadow and light rays.

This connection was made to prevent the light cover from casting a shadow to the whole scene.

Step 2. Create photon shaders

• transmatSG : light cover, front wall, back wall.
  We want the light cover, front wall and the back wall be invisible, and not participate in photon tracing, transmat_photon is just perfect for this case. The purpose of the invisible transmatSG is to create a volume caustics later.
• Open the Attribute Editor of transmatSG .
• From the "mental ray" section, open the tab named "Custom Shaders". It shows that transmat1 is connected as its material shader and transmat2 as shadow shader.
• Click the map button next to "Photon Shader", and pick transmat_photon .
• Create a dgs_material_photon shader for the red wall
• From Hypershade > Create mental ray Nodes > Photonic Materials
• Select dgs_material_photon.
• Rename it red_phot. This photon shader will be used for the red wall.
• Most of the time, you want the photon shader and the material shader have the same parameter values.
  Because the red wall uses dgs_material as its material shader, we use dgs_material_photon as its photon shader.
• Connect red_phot to redSG .
• CTRL+MMB red_phot to redSG.
• Set the photon shader parameter values same as the material shader.
• Set diffuse to 0.8 0 0
• Set glossy to 0 0 0
• Set specular to 0 0 0
• Set shiny to 0
• Do the same for whiteSG which is shared by the floor and the ceiling.
• Assign dgs_material_phogon to whiteSG, and rename it to white_phot .
• Match the values of white_pot to white_dgs_mat
• Set diffuse to 0.8 0.8 0.8
• Set glossy to 0 0 0
• Set specular to 0 0 0
• Set shiny to 0

Step 3. Using Map Visualizer
Let's render and see what we have.
Before rendering, make sure that the global illumination is on.

• Turn on global illumination.
• Go to the mental ray render globals, mental ray tab. Open "Caustics and Global Illumination" section. Check "Global Illumination".
• The two parameters below the check box is used to fine tune the scene, decrease the noise, etc. We can use the default values for this example.
• Make the light emit photons
• From the attribute editor of the light shape, open "mental ray" section.
• Check "Emit Photons" under "Caustics and Global Illumination".
• Make sure that "exponent" is set to 2, so that the energy of each photon decreases correctly as it travels around the room.

Now we are ready to render, but let's do one more extra step:

• Go back to the mental ray render globals, mental ray tab, Caustics and Global Illumination.
• At the bottom of this section, turn on "Photon Map Rebuild" so that mental ray creates new photon map.
• Provide a file name to "Photon Map File" edit box to be written out to a disk.

Finally, render.

The rendering takes longer with global illumination.

It's usually a good idea to turn up the verbosity, so that the output window shows the progress.

The verbosity is controlled from mental ray Render Globals > mental ray tab > Translation.

It is set to Progress Messages in this example, so that you can check photon emission progress.

You should notice red tint on the ceiling and faint pink on the floor, caused by photons reflected by the red wall.

The ceiling rendered black without global illumination before, because the light did not hit the ceiling directly.

The color showing up on the ceiling is solely from the photons.

Surfaces without photon shader did not participate in this process at all. For example, the blue wall did not affect the color of the ceiling at all.
To see the global illumination effect more clearly, set the color of physical_light1 black .

This disables the direct illumination.

Render, and you will see the effect of global illumination more clearly.

Set the physical_light1.color back

There is another way of checking the photon map process.

Because we specified the photon map file name, the file is now stored in the disk. A Map Visualizer provides a convenient way of diagnosing the photon map.
The Map Visualizer is accessible from Menu > Window > Rendering Editors > mental ray > Map Visualizer.
For the Map File Name, select the photon map file just created. It should be in the photonMap directory.
The photon map is displayed in the model view if the shading mode is Wireframe or Smooth Shade All.

As you may see, photons are only stored on the ceiling, floor and the red wall - only on the surfaces with a photon shader.
Delete the Map Visualizer by the button in the middle and quit.

Step 4. Finishing up
Assign photon shaders for the rest of the materials.

• Blue wall
  Assign a new dgs_material_photon, rename it to blue_phot, and set the same values as its material shader.
• Mirror ball
  Assign a new dgs_material_photon, rename it to mirror_phot, and set the same values as its material shader.
• Glass ball
  Assign a new dielectric_material_photon, rename it to glass_phot, and set the same values as its material shader.

Render again, and this time, both the red wall and the blue wall affected the image.

Following settings can be adjusted to fine-tune the image and performance.

• Number of photons emitted (from a light)
• Global Illum Accuracy (render global / shape node)
• Global Illum Radius (render global / shape node)
• Globalillum flag (transform node. Mostly for performance)
  
  

Photons are stored only if a surface has a photon shader, and a diffuse component.

For example, if the surface is specular like those balls in the scene, photons reflect and transmit on the surface if they have a photon shader, but won't be stored.

You can double check this from the Map Visualizer.

Global illumination
cornellBox_photon_globillum.ma

The scene is saved as cornellBox_photon_globillum.ma

Step 5. Caustics
Caustics and global illumination are handled separately in mental ray, even though caustics are a subset of global illumination.
Global illumination is mostly for diffuse light interaction, and caustics is dedicated to localized effect from specular light interactions.

Caustics are created by specular reflection or transmission of photons onto diffuse surface or volumes.
In the Cornell box scene, we can expect caustics from the glass ball - photons from the light transmitted through the glass ball, and focused on the diffuse floor.
The mirror ball also creates caustics by reflecting the photons, but it is hard to see because the reflection is not focused on a localized area.

The scene is almost set up for caustics. Specular objects (two spheres) as well as diffuse walls have proper photon shaders.
The light emits photons, and the number of caustics photons is reasonable.
The only thing left is to turn on Caustics from the mental ray render globals > mental ray tab > Caustics and Global Illumination.

Render again, and this time, we see the caustics on the floor.

Check the photon map from the Map Visualizer. You can select to see global illumination photons only, caustics photons only, or both. Volume photons will be used later.

To adjust the quality of the caustics, use:

• Number of caustics photons emitted (from a light)
• Caustics Accuracy (render global / shape node)
• Caustics Radius (render global / shape node)
• Caustics Filter Type (render global)
• Caustics Filter Kernel (render global)
• Caustics flag (transform node)

The scene is saved as cornellBox_photon_finish.ma

Global illumination and Caustics.
cornellBox_photon_finish.ma

Step 5. Volume Caustics

Now we are going to add some atmospheric to the scene, and create a volume caustics effect.

Caustics photons transmitting through the floating glass ball will create a prum on its way out.
If you are not familiar with volume shader, check out volumeShaders first, to get familiar with how volume effect is created.
Volume caustics as well as other participating media effect can be simulated using photon volume shader. A photon volume shader is similar to a volume shader, except that it is applied to the volume, and stores photons in the volume.

We will use a photon volume shader parti_volume_photon which is a counterpart to parti_volume volume shader.
Let's define the bounding box for the volume first.
You might have guessed, the transparent surfaces - front wall, back wall and the light cover plays important role.
The front wall and the back wall define the bounding planes for the volume effect.

• Assign a volume shader to transmatSG .
• Create a parti_volume shader from Hypershade > Create mental ray Nodes > Volumetric Materials.
• Rename it transmat_volume.
• Connect transmat_volume to transmatSG. transmat_volume.message to transmatSG.miVolumeShader
• Adjust volume shader settings.
  Let's use the following settings. (mental ray Shaders Guide, Chapter 2.5 Participating Media, explains each parameter.)
• scatter 0.05 0.05 0.05
• extinction 0.05
• min_step_len 0.13
• max_step_len 1
• light_dist 2

• Render and you can see the volume effect, especially around the light.
• The scene is saved as cornellBox_photon_volume.ma

Global Illumination in Volumes.

cornellBox_photon_volume.ma

• Add photon volume shader.
  Let's add a photon volume shader to transmatSG.
  The light cover has been waiting for this moment.
  The photon volume shader is why the light cover exists.
  We could have assigned a photon volume shader to the whole scene, but it would have caused the loss of many photons.
  By adding a photon volume shader to a light cover - a small object that covers a light, we give a hint to mental ray where we want photons be emitted.
  Volume photons are stored in the light frustum that the light cover defines.
• Assign a parti_volume_photon to the light cover .
• Create a parti_volume_photon from Hypershade > Create mental ray Nodes > Photonic Materials.
• Rename it as transmat_volume_phot
• Connect transmat_volume_phot to transmatSG. From transmat_volume_phot.message to transmatSG.miPhotonVolumeShader.
• Adjust the settings
  As we use the same settings for material shader and photon shader, same rule applies to volume shader and photon volume shader.
• scatter 0.05 0.05 0.05
• extinction 0.05
• min_step_len 0.13
• max_step_len 1
• light_dist 2

Render .

Volume caustics effect shows up below the glass ball.

The final scene is cornellBox_photon_volume_caustics.ma

Volume Caustics.

cornellBox_photon_volume_caustics.ma