In 3D software, caustics rendering is set based on
luminance values (luminance value must be higher than
0) of the shader applied to a dummy CG object, which
receives caustics from transparent CG objects. The issue
is that if the caustics are received by the dummy CG
surfaces, the luminance value of the CG surfaces must be
0. Because in digital compositing, 0 means completely
transparent, any value above 0 means semi-transparent
and 1 means 100% opaque, thus if the value is 0, the
rendered caustics on dummy CG surfaces can be
composited with live-action footage seamlessly and
without showing any colors from the CG dummy surface
on the footage. However, there is a known issue with
Maya the mental ray engine, which is not fully
compatible with Maya nodes.