

This definition works with new image sampler component. To work correctly make sure you're choosing Value(brightness) in filter option in the image sampler component and X e Y domain of the image sampler ( Setting option) longer than the texture image( easy way is to create a texture image big than the projection of the surface on the x,y plane).
The radius of the circle that have center at U,V points surface is connected with the brightness value of the texture image at the u,v points.
