Creating the Volumetric Caustic

This section covers the creation of the basic scene and the volumetric caustic effect.

If you have not already downloaded and installed the xphysics shader you can get it here.

To install it choose File->Addon->Install in XSI then browse to find the file you downloaded,
or you can simple drag the file on to the XSI workspace. For more detailed info on installing
addons consult your documentation.

You can get a scene which is set up ready to go straight to Creating the Materials here.

Theory

When creating a standard volumetric light the area of the volume is automatically defined
for you. However when using the xphysics shaders you're required to create geometry to
define the area the volumetric effects will appear in.

Setting up the Scene

The first step for us is to create the objects with which our volumetric caustics will be
interacting (our reflective and refractive objects). For this tutorial I've create a simple
nurbs sphere and a poly cylinder with the subdivisions increased in the geometry
approximation settings. I've then positioned them as below.

Next create a spot light which will be used to emit the caustic photons and shine
volumetic light. Shift it, aim it at the objects and enter the following settings. These
settings may not achieve the effect you desire - they are just the ones which have
been used in this example. Tweak them to achieve the look you want.

Now create some geometry to use as your caustic volume's area. I have just used a cube
and positioned to so it surrounds the area of the sphere and cylinder fairly well. The size
and shape of the object you require will depend on the strength of your caustic effect and
the geometry with which it interacts.

Creating the Materials

In this example I will be giving the sphere a glass material and cylinder a mirror material.

Any material with either refractive tranparency or reflectivity will do though. If you like you
can download these presets for the sphere and cylinder.

Now for the material required to make your caustics volume work. First select the cube
then choose Get->Material->More. Then browse to
C:\Softimage\XSI_2.03\Addons\xphysics\Data\DSPresets\Shaders\Material
or if XSI is installed in a different directory then the same location within that installation
directory. Once there choose xphysics_transmat and click ok. It will look like below so
don't be alarmed when there's no options.

Now with your geometry still selected hit 7 to bring up the render tree. Choose
Nodes->Volume->More. Then browse to
C:\Softimage\XSI_2.03\Addons\xphysics\Data\DSPresets\Shaders\Volume
and select xphysics_volume and click ok.

Now drag the output of the xphysics_volume node to your material node and connect it
to the Volume input. Then repeat connecting it the Photon Volume input. Your render
tree should look like below. You can get a preset for this material here.

Note: if it is not connected to both the Volume and Photon Volume nodes you won't
see any volumetric caustics.

Configuring the Visibility

You will need to alter the visibility for each of your pieces of geometry so that they
will interact with the caustics correctly. For the objects interacting with the caustics
other than the volume geometry enable both transmitting and receiving caustics. For
the volume geometry you need only enable the transmitting of caustics.

Preparing the Pass

For the volumetric caustics pass we are just going to modify the default pass. Go to
the explorer and change it to view passes (pressing "a" or selecting it from the top
left drop-down). You may wish to rename your default pass to volumetric_caustics.

Now select the Background_Lights_Partition. Choose Get->Property->Override. Then
browse to and select the specular and diffuse nodes as below.

Now click back on the override property editor and uncheck the boxes for both specular
and diffuse in the override. This is done so that we have only caustic light rendering in this
pass. This means that regardless of your scene setup all light should be left in the
background lights partition with this override applied.

Render Options

Choose render->options and configure your render options as you wish them to be. The
only things you'll need to be extra mindful of for this are to make sure you've switched on
caustics and you may wish to modify your photon depth settings to something more
appropriate to your scene then the default 20, 20, 40.

If you wish to run a test render you should end up with something like this.

Light Volume

Now that you've set up your volumetric caustics it's time to move on to the next step -
creating the light volume.