How does one look inside of the proton?

Presently Generalized Parton Distributions allow us novel three-dimensional description of the proton structure when both transverse and longitudinal degrees of freedom are taken into account. A tool to experimentally constraint GPDs is Deeply Virtual Compton Scattering. Main steps in extracting DVCS cross-section in the framework of COMPASS experiment are discussed in detail hinting path towards the proton tomography.