  We present the first study of the spatial distribution of star formation in
z~0.5 cluster galaxies. The analysis is based on data taken with the Wide Field
Camera 3 as part of the Grism Lens-Amplified Survey from Space (GLASS). We
illustrate the methodology by focusing on two clusters (MACS0717.5+3745 and
MACS1423.8+2404) with different morphologies (one relaxed and one merging) and
use foreground and background galaxies as field control sample. The
cluster+field sample consists of 42 galaxies with stellar masses in the range
10^8-10^11 M_sun, and star formation rates in the range 1-20 M_sun/yr. Both in
clusters and in the field, H{\alpha} is more extended than the rest-frame UV
continuum in 60% of the cases, consistent with diffuse star formation and
inside out growth. In ~20% of the cases, the H{\alpha} emission appears more
extended in cluster galaxies than in the field, pointing perhaps to ionized gas
being stripped and/or star formation being enhanced at large radii. The peak of
the H{\alpha} emission and that of the continuum are offset by less than 1 kpc.
We investigate trends with the hot gas density as traced by the X-ray emission,
and with the surface mass density as inferred from gravitational lens models
and find no conclusive results. The diversity of morphologies and sizes
observed in H_alpha illustrates the complexity of the environmental process
that regulate star formation. Upcoming analysis of the full GLASS dataset will
increase our sample size by almost an order of magnitude, verifying and
strengthening the inference from this initial dataset.
