Bias on simulation sets C and D. Left: bias on the half-light radius (top) and total magnitude (bottom) as a function of the spiral-arm-to-total flux ratio, obtained by fitting a Sérsic profile to images from simulation set C. Right: bias on half-light radius (top) and total magnitude (bottom) as a function of the ratio R_sky/R_e, smo for simulation set D. The quantity R_sky is defined as the radius at which the surface brightness of the smooth component equals the amplitude of the sky background fluctuation for a single pixel.