This function simulates the texture being placed under a transparent body. The upper profile of this body, which causes the Refraction, is determined by the heightmap.

The heightmap is defined by the Source Texture, if you check the Use Texture/Source option. You can cause the Mask to be used as the heightmap by checking the Use Mask option.

The Pre-Smoothing parameter lets you control the smoothness of the refractive surface.

Regulate the refractive index of the surface with the Amplify value. Increasing this value results in stronger distortions.