Figure 4.
Portrait of a Lady (c. 1460) by Rogier van der Weyden. Inequality signs indicate the luminance relations between adjacent surfaces (oriented in accordance with the corresponding boundary).
Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.