  {We study biasing as a physical phenomenon by analysing geometrical and
clustering properties of density fields of matter and galaxies.} {Our goal is
to determine the bias function using a combination of geometrical and power
spectrum analysis of simulated and real data.} {We apply an algorithm based on
local densities of particles, $\delta$, to form simulated biased models using
particles with $\delta \ge \delta_0$. We calculate the bias function of model
samples as functions of the particle density limit $\delta_0$. We compare the
biased models with Sloan Digital Sky Survey (SDSS) luminosity limited samples
of galaxies using the extended percolation method. We find density limits
$\delta_0$ of biased models, which correspond to luminosity limited SDSS
samples.} {Power spectra of biased model samples allow to estimate the bias
function $b(>L)$ of galaxies of luminosity $L$. We find the estimated bias
parameter of $L_\ast$ galaxies, $b_\ast =1.85 \pm 0.15$. } {The absence of
galaxy formation in low-density regions of the Universe is the dominant factor
of the biasing phenomenon. Second largest effect is the dependence of the bias
function on the luminosity of galaxies. Variations in gravitational and
physical processes during the formation and evolution of galaxies have the
smallest influence to the bias function. }
