What is a Gauss sign? I am reading the paper 
"A Method for Extraction of Bronchus Regions from 3D Chest X-ray
CT Images by Analyzing Structural Features of the Bronchus" by Takayuki KITASAKA, Kensaku MORI, Jun-ichi HASEGAWA and Jun-ichiro TORIWAKI
and I run into a term I do not understand:



In equation (2), when we say "[] expresses the Gauss sign", what does it mean?

From the context (a change of scale using discrete units), this should certainly mean floor as on page 5 of Gauss's Werke 2

per signum $[x]$ exprimemus integrum ipsa $x$ proxime minorem, ita ut $x-[x]$ semper fiat quantitas positiva intra limites $0$ et $1$ sita

i.e. the next lower integer.
