Sample Compositing

This section shows the workflow to use sample compositing shaders.

References:

mental ray User Manual, Chapter 3.8.

mental ray Shaders Guide, Chapter 1.5.

Rendering with mental ray, Appendix C.

Where to find sample composting shaders
Material shaders appear in the Hypershade > Create mental ray Nodes > Sample Compositing.

The following sample compositing shaders are shipped with Maya:

mib_dielectric
mib_opacity
mib_reflect
mib_refract
mib_refraction_index
mib_transparency
mib_twosided

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 that we will use for this section: sampleCompositing_start.ma.

Step 2. Assign a material shader to the sphere

This example uses mib_illum_phong for the material, but you can use any of mental ray material shaders. See Materials if you are not familiar with material shaders.

Step 3. Adjust Phong shader parameters

Open the Attribute Editor of mib_illum_phong1.

Adjust shader parameters.

Render with mental ray.

A highlight from the point light on the Phong material appears.

mib_illum_phong

Step 4. Create a sample compositing shader

We will use mib_refract to add refraction to the previous image.

Create a mib_refract shader

Hypershade > Create mental ray Nodes > Sample Compositing
Click mib_refract

Step 5. Setup mib_refract

refract is the color of refraction. Setting this to 0 won't show any refraction. Setting this to 1 will show refraction only, and no input in the final image. We will set this value to 0.5 for now.
ior is index of refraction. Setting this to 0 or 1 won't bend the refraction ray and makes the sphere transparent instead of refractive. You can achieve the same effect by using mib_transparent. Set this to 1.3 .
mib_refract composites refraction image to input. In order to achieve Phong material with refraction, the outValue of mib_illum_phong1 should be connected to input, and mib_refract1 should connected to the shading engine.
Select mib_illum_phong1 and check its outgoing connection. mib_illum_phong1.message is connected to mib_illum_phong1SG.miMaterialShader. The final connection between this 3 nodes should be:

mib_illum_phong1.outValue -> mib_refract1.input
mib_refract1.message -> mib_illum_phong1SG.miMaterialShader To make this connection,

MMB drag and drop mib_illum_phong1 to mib_refract1. Choose input. This will connect mib_illum_phong1.outValue to mib_refract1.input.
MMB drag and drop mib_refract1 to mib_illum_phong1SG. Choose default. The Connection Editor shows up.
Select message of mib_refract1 from the left panel, and select miMaterialShader of mib_illum_phong1SG from the right panel. If you cannot find message attribute, make sure that Connection Editor > Left Display > Show Hidden is turned on. Close the Connection Editor.
Check input-output connection of mib_refract1

Now we are ready to render.

mib_illum_phong and mib_refract.

Step 6. Adjust ray tracing settings
After all this effort, we still cannot see the refraction. This is because the raytracing setting is too low.

Open mental ray render globals
In the mental ray tab > Ray Tracing section, Refractions is set to 1, which is too low for this case.
We need at least 2. Set this value to 2 or higher Max Trace Depth should be also adjusted.
Set Max Trace Depth to same value as Refractions

Render again with mental ray. This time, refraction should appear.

The scene was saved out as sampleCompositing_refract.ma at this point.

Refraction with mib_illum_phong and mib_refract.
sampleCompositing_refract.ma

Step 7. Adjusting mib_refract parameters

Let's try changing parameter values of the shader.

Change ior to 0 or 1. and render .
Refraction ray passed through the sphere without bending, resulting transparency added.

mib_refract.ior = 0 or 1

Revert the change.
Set refract to 0. and render . mib_refract passes input to its output. It's same as using mib_illum_phong only.

Mib_refract.refract = 0

Set refract to 1. and render . mib_refract ignores input, and shows refraction layer only.

mib_refract.refract = 1

Set refract to red. and render .
Refraction of the checker shows up red and black, and the rest of the sphere is rendered cyan.
This is result of refract color being multiplied to the refraction, and (1 - refract) to input.

mib_refract.refract = 1 0 0

Other examples

sampleCompositing_ex1.ma : mib_opacity, mib_transparency

This scene shows how mib_opacity and mib_transparency can achieve the same result. Note that input values of two shaders are same. transp and opacity are set so that the addition of two values is exactly 1.
input is not connected to any material shader to keep the example simple.

mib_opacity, mib_transparency

sampleCompositing_ex2.ma : mib_refraction_index
This scene shows the use of mib_refraction_index.
The left sphere uses mib_refract shaders as shown in above step by step example. The right sphere uses mib_refraction_index connected to ior attribute of mib_refract. mib_refraction_index returns mtl_ior when entering an object, and 1/mtl_ior when exiting an object. In this example, refraction ray entering the sphere uses 1.3 for the index of refraction, and 1/1.3 for refraction ray that exits the sphere.
input is not connected to any material shader to keep the example simple.

mib_refract, mib_refraction_index

sampleCompositing_ex3.ma : mib_reflect

This is an example of mib_reflect. Both spheres have mib_reflect assgined to material shader of its own shading engine.
input is not connected to any material shader to keep the example simple. The difference between the two is that the mib_reflect2 assigned to the right sphere has notrace on.
If this parameter is on, the shader will sample the environment. In this example, there is no environment shader assigned, causing the right sphere to be rendered black.

mib_reflect

sampleCompositing_ex4.ma : mib_reflect

sampleCompositing_ex4.ma took the previous scene, and assigned an environment shader to persp camera. You can see that the left sphere shows reflection of the floor, as well as reflection from the environment, whereas the right sphere only shows reflection from the environment.
Please refer to Environment Shaders for more information.

mib_reflect

sampleCompositing_ex5.ma : mib_twoside

This scene shows a poly cube with mib_twoside. As the name of the shader suggests, this shader applies front to the front faces, and back to the back faces. Two different colors of mib_illum_lambert are assigned to front and back in this example.

mib_twoside