  We propose a new approach to 3D reconstruction from sequences of images
acquired by monocular endoscopes. It is based on two key insights. First,
endoluminal cavities are watertight, a property naturally enforced by modeling
them in terms of a signed distance function. Second, the scene illumination is
variable. It comes from the endoscope's light sources and decays with the
inverse of the squared distance to the surface. To exploit these insights, we
build on NeuS, a neural implicit surface reconstruction technique with an
outstanding capability to learn appearance and a SDF surface model from
multiple views, but currently limited to scenes with static illumination. To
remove this limitation and exploit the relation between pixel brightness and
depth, we modify the NeuS architecture to explicitly account for it and
introduce a calibrated photometric model of the endoscope's camera and light
source. Our method is the first one to produce watertight reconstructions of
whole colon sections. We demonstrate excellent accuracy on phantom imagery.
Remarkably, the watertight prior combined with illumination decline, allows to
complete the reconstruction of unseen portions of the surface with acceptable
accuracy, paving the way to automatic quality assessment of cancer screening
explorations, measuring the global percentage of observed mucosa.
