Fits of a Sérsic profile to the smooth component of the surface brightness distribution of three example galaxies. Column 1: colour-composite image of the galaxy in the HSC g, r, and i bands. Column 2: grey-scale image of the galaxy in the HSC r band. Column 3: area over which the spiral arms dominate the surface brightness (green regions) and contaminants (red regions). Both are masked out from the fit. Column 4: best-fit model of the smooth component in the r band. Column 5: residual in the r band. The value of the r-band spiral-to-total flux ratio is shown in the bottom-right corner.