  Isospin-1/2 $D\pi$ scattering amplitudes are computed using lattice QCD,
working in a single volume of approximately $(3.6\; \mathrm{fm})^3$ and with a
light quark mass corresponding to $m_\pi\approx239$ MeV. The spectrum of the
elastic $D\pi$ energy region is computed yielding 20 energy levels. Using the
L\"uscher finite-volume quantisation condition, these energies are translated
into constraints on the infinite-volume scattering amplitudes and hence enable
us to map out the energy dependence of elastic $D\pi$ scattering. By
analytically continuing a range of scattering amplitudes, a $D_0^\ast$
resonance pole is consistently found strongly coupled to the $S$-wave $D\pi$
channel, with a mass $m\approx 2200$ MeV and a width $\Gamma\approx400$ MeV.
Combined with earlier work investigating the $D_{s0}^\ast$, and $D_0^\ast$ with
heavier light quarks, similar couplings between each of these scalar states and
their relevant meson-meson scattering channels are determined. The mass of the
$D_0^\ast$ is consistently found well below that of the $D_{s0}^\ast$, in
contrast to the currently reported experimental result.
