  Context. We present a model-free method for mapping surface brightness
variations. Aims. We aim to develop a method that is not dependent on either
stellar atmosphere models or limb-darkening equation. This method is optimized
for exoplanet transit surveys such that a large database of stellar spot
coverage can be created. Methods. The method uses light curves from several
transit events of the same system. These light curves are phase-folded and
median-combined to for a high-quality light curve without temporal local
brightness variations. Stellar specific intensities are extracted from this
light curve using a model-free method. We search individual light curves for
departures from the median-combined light curve. Such departures are
interpreted as brightness variations on the stellar surface. A map of
brightness variations on the stellar surface is produced by finding the
brightness distribution that can produce a synthetic light curve that fits
observations well. No assumptions about the size, shape, or contrast of
brightness variations are made. Results. We successfully reproduce maps of
stellar disks from both synthetic data and archive observations from FORS2, the
visual and near UV FOcal Reducer and low dispersion Spectrograph for the Very
Large Telescope (VLT).
