Contribution function of the ice absorption features. Top left: total dust density distribution in the model with internal shielding (radial A_V; dashed black line) at 0.03, 0.1, 0.3, 1, and 10 mag and external shielding (vertical A_V; white dashed line) at 0.01, 0.03, 0.1, and 1 mag from top to bottom. The other panels show the contribution function (CBF), i.e., the contribution of a specific region to the total absorption of the ice feature in percentages. The pure CO ice at high abundance is added for reference, but we argue in the main text that this component is not observed. The absorbing area is the region in the disk that contribute significantly to the total absorption. The red and yellow lines indicate the fiducial and best fit photodesorption snow surface, respectively, for reference. The thick, dashed white line shows the τ = 1 surface at the ice feature wavelength. We would like to note that viewing angle is really important and is 83° in this case