Apply the tangential filter `tsmooth` to the image and estimate the reduction in image noise.
This filter smooths along the tangential direction to local edge structure in each pixel.
Again, repeat this for various noise levels.

- You should be able to determine an approximate constant factor of reduction. What is it?
- Compare the result with Gaussian smoothing; where are the largest differences between the
resulting images and why?

root 2018-07-17