What caused the different VolumeRender outputs between pure VTK and 3DSlicer

Support
1

This is a heart image rendered by pure VTK code:

vr-by-vtk
vr-by-vtk1644×1384 381 KB

This is the same heart rendered by 3D Slicer:

vr-by-3dslicer
vr-by-3dslicer1546×1176 330 KB

I used the same transfer functions and tried to tweak some parameters on the mapper (vtkOpenGLGPUVolumeRayCastMapper), but still couldn’t achieve the similar effects like 3D Slicer.
Can anyone give me some hints about why Slicer so smoothed?