Volume 1 is a medical image (3D head), volume 2 is computed, containing just zeros and ones, in order to create a "mask" volume. The data values of the data volumes are multiplied together, causing some portions of volume 1 to become completely transparent. Defining LIGHTING_AND_PREINTEGRATION in file GPUDataCompose.cxx will apply lighting and pre-integration.
SoVolumeData, SoDataCompositor, SoMultiDataSeparator, SoVolumeShader