The alpha test attribute governs whether or not a part of a node will be rendered based on the alpha value of its texture. This is particularly useful for rendering complex geometry into the depth or stencil buffer with a textured card rather than explicitly creating the shapes. This test is different from rendering with respect to alpha transparency value. If you set an alpha test attribute on a node which is rendering into the color buffer, you may be surprised by the result. All pixels that pass the alpha test will be rendered just as if no test had been performed, including their appropriate transparency and pixels that fail the test will not be rendered at all.
Remember to set your attribute’s priority to override any other alpha test attributes inherited from higher in the scene graph. In the following example, we create an attribute that would cause objects to render only if their alpha value is below one quarter intensity.
lowPassFilter = AlphaTestAttrib.make(RenderAttrib.MLess,0.25)
And now, this attribute can be added to a node to enable the action.