Why the x value of the pFilm is changed to the negation of it in RealisticCamera?

[kanition]

In line 910, the x value is assigned as -pFilm2.x. However there is no explain to it in the context. Could you give more details about this operation? Thanks a lot.

pbrt-v4/src/pbrt/cameras.cpp

Lines 906 to 910 in 9afc099

	// Find point on film, _pFilm_, corresponding to _sample.pFilm_
	Point2f s(sample.pFilm.x / film.FullResolution().x,
	sample.pFilm.y / film.FullResolution().y);
	Point2f pFilm2 = physicalExtent.Lerp(s);
	Point3f pFilm(-pFilm2.x, pFilm2.y, 0);