8. Area light settings — photons and shadow rays

As mentioned in the previous section, we still need to set up two more shader connections for the complete volume caustics effect to render. But we can finally start emitting photons and rendering without photons getting lost in infinite calculation loops.

1. Select “caustics_areaLight1” and open the Attribute Editor (CTRL + A)

2. Scroll down to the “mental ray” rollout

   1. Enable “Emit Photons”  under “Caustics and Global Illumination”

   2. Set “Intensity: 2000”, “Exponent = 1.500”, and “Caustic Photons = 500,000” (if this option is greyed out, caustics have not been enabled in the mental ray render settings yet—see step 3.1)

3. “Raytrace Shadow Attributes” rollout: set both “Shadow Rays” and “Ray Depth Limit” to “4”

Test render 5

FINALLY. Something that is starting to look like caustics and doesn’t hang the renderer. Definitely heading in the right direction. As you can see though, the volume caustics are not showing up. Those require a bit more love. 

9. Water photon shader 

This is the second to last shader connection needed for the full volume caustics effect. The photon shader is the one that determines whether or not caustic photons get scattered by the water surface to create beams of light in the volume on the other side. Without shooting photons into a surface with this shader assigned to it, volume caustic photons will never be created.

1. Select “waterSurface_pPlane1” in the viewport again, and open the “Attribute Editor” (CTRL + A)

2. Select the “waterSurface_blinn1” tab, then hit the “go to output connection” button to get to “waterSurface_blinn1SG”
   
   

   

3. Scroll down to “mental ray” section > “Custom Shaders” > “Phot Volume Shader” channel, press the “assign shader” button , and
   

4. select “parti_volume_photon” from the list
   
   

5. Rename “parti_volume_photon1” to “waterSurface_parti_volume_photon1”
   

6. Since settings for both “parti” materials in this tutorial will be the same, let’s wire all parameters to each other so we only have to modify one of them to update them both when polishing settings later on. (NOTE: you may not want to do this so you can have separate control over the volume caustics, and the reflective / refractive ones.)

   1. Open Hypershade (Window > Rendering Editors > Hypershade)

   2. Middle mouse button drag “photonBoundary_parti_volume1” from the “Materials” tab (should already be selected when you open Hypershade) into the “Work Area” below

   3. SHIFT + Middle mouse button drag + Drop “waterSurface_parti_volume_photon1” from the “Utilities” tab onto “photonBoundary_parti_volume1” to open the connection editor with both shaders loaded

   4. Select each of the channels from “caching” to “no_glbl_where_direct” one matched pair at a time in sequence from left to right and top to bottom in the connection editor to connect them
      
            

If we were to render at this point (image NOT included), you’ll notice that nothing appears to have changed since the previous test render. Even when you turn up the Scatter settings on our “parti” shaders per the instructions below, there are still no volume caustic effects in the scene. This is because the camera still needs a reference to the volume shader.

1. Select “photonBoundary_parti_volume1” in the work area and open the Attribute Editor (CRTL + A)

2. Set “Scatter = 0.150“ and then test render again to see… the continued lack of volume cuastics! (image NOT included)
   
   (NOTE: "Scatter" determines the dispersion of photons as they pass through the water surface. It SHOULD make the volume caustics appear brighter the higher this number gets… once the camera is set up to "see" the photons on the next page, that is)