Maya 2012 Tutorial
How to Control Texture Alpha Transparency in Maya
The transparency channel specifies the object's transparency. Objects can have a uniform transparency throughout the geometry or in limited areas. By assigning a grayscale map to the transparency channel you can set the opaque sections (Black) and the transparent portions (White). The transparency option is available with Anisotropic, Blinn, Lambert and Phong surface materials.
Open a scene with multiple objects and add an upright polygon plane. Right click the plane and choose Assign New Material > Lambert.
The new Lambert texture panel opens in the Attribute Editor. To change the transparency, move the slider.
Click the checker-box next to the transparency slider to assign a grayscale map.
In the Create Render Node - 2D section, right-click the Bulge icon and choose Create Texture. This creates a Bulge texture that is UV mapped to the object.
Render your image an note the effect of the transparency map. You can also adjust the Bulge settings. In addition to procedural textures such as Bulge, you can also use grayscale jpg or tif files.
Shadows and Transparency
To render a transparent object with shadows, be sure to use Raytrace shadows. Depth Map shadows don't see the transparency map and the shadow is rendered as if the object is solid.
What's Wrong with the Shadow in Step 2?
Watch step 2 of the this topic and notice that the object is transparent, yet the shadow is still being cast on the floor. The shadow is a Ray Trace shadow.
To fix the problem, open the object's shader tab in the Attribute Editor. In the Ray Trace Options section, decrease the Shadow Attenuation value and the problem goes away.