How to draw overley on volume rendering surface

Hello, every one!
I want to implement the features similar to the rendering widget of MRICron by VTK.
But I have no idea how the MRICron draw a overlay at volume rendering surface.

It looks like bellow:

The base volume is a T1 image. And the overlay is a PET image.
The two images is blended in MRP widgets looks like this:

It seems that the colored voxel of PET is attached to the T1 voxel at the time of volume rendering.
Has some class that can implement the similar effection in VTK?
Or,has any suggestion?

I tried to use vtkOpenGLGPUVolumeRayCastMapper, set the T1 to input data ,and set the PET to mask input.
But I have no ideal how covert the PET data to a label map, has some suggestion?