[FEATURE] display or filter the phase image such that background is black

[mattersoflight]

Problem
Phase measurements made with propagation-based quantitative phase imaging do not transfer the DC term of optical path length. As a result, the background looks gray, which is non-intuitive and can effect visual interpretation.

Proposed solutions

1. Use a custom colormap for the phase that sets the background to black, phase lower than the background to one color (e.g., blue), and phase higher than the background to another color (e.g., red, white).
   The approach is to automatically estimate the gray level of the background and set the colormap. If the phase -> RGB mapping is done carefully, the visualization improves significantly. See this rendering for example.

2. Develop high-pass filters that suppress the smooth background, but retain the structure of the phase information at mid-high spatial frequencies.

Structures we are typically interested in (organelles) have significantly high spatial frequencies relative to the passband of a microscope. We can use or adapt filters described in this paper by Nguyen,...,Popescu to suppress background without losing relevant information about the phase distribution of the specimen.