about fisheye camera model

[hiro-oda]

Hi.
in cameras.py def _generate_rays_from_coords

elif CameraType.FISHEYE.value == cam_type:
                mask = (self.camera_type[true_indices] == CameraType.FISHEYE.value).squeeze(-1)  # (num_rays)
                mask = torch.stack([mask, mask, mask], dim=0)

                theta = torch.sqrt(torch.sum(coord_stack**2, dim=-1))
                theta = torch.clip(theta, 0.0, math.pi)

                sin_theta = torch.sin(theta)

                directions_stack[..., 0][mask] = torch.masked_select(
                    coord_stack[..., 0] * sin_theta / theta, mask
                ).float()
                directions_stack[..., 1][mask] = torch.masked_select(
                    coord_stack[..., 1] * sin_theta / theta, mask
                ).float()
                directions_stack[..., 2][mask] = -torch.masked_select(torch.cos(theta), mask).float()

For the above implementation of finding the ray direction from fisheye camera image coordinates, which paper or book did you refer to for the fisheye camera model? I would like to cite them. Thank you.