  Context. Multi-wavelength observations are indispensable in studying disk
geometry and dust evolution processes in protoplanetary disks. Aims. We aimed
to construct a 3-dimensional model of HD 163296 capable of reproducing
simultaneously new observations of the disk surface in scattered light with the
SPHERE instrument and thermal emission continuum observations of the disk
midplane with ALMA. We want to determine why the SED of HD 163296 is
intermediary between the otherwise well-separated group I and group II Herbig
stars. Methods. The disk was modelled using the Monte Carlo radiative transfer
code MCMax3D. The radial dust surface density profile was modelled after the
ALMA observations, while the polarized scattered light observations were used
to constrain the inclination of the inner disk component and turbulence and
grain growth in the outer disk. Results. While three rings are observed in the
disk midplane in millimeter thermal emission at $\sim$80, 124 and 200 AU, only
the innermost of these is observed in polarized scattered light, indicating a
lack of small dust grains on the surface of the outer disk. We provide two
models capable of explaining this difference. The first model uses increased
settling in the outer disk as a mechanism to bring the small dust grains on the
surface of the disk closer to the midplane, and into the shadow cast by the
first ring. The second model uses depletion of the smallest dust grains in the
outer disk as a mechanism for decreasing the optical depth at optical and NIR
wavelengths. In the region outside the fragmentation-dominated regime, such
depletion is expected from state-of-the-art dust evolution models. We studied
the effect of creating an artificial inner cavity in our models, and conclude
that HD 163296 might be a precursor to typical group I sources.
