  An accurate method for warping images is presented. Differently from most
commonly used techniques, this method guarantees the conservation of the
intensity of the transformed image, evaluated as the sum of its pixel values
over the whole image or over corresponding transformed subregions of it. Such
property is mandatory for quantitative analysis, as, for instance, when
deformed images are used to assess radiances, to measure optical fluxes from
light sources, or to characterize material optical densities. The proposed
method enforces area resampling by decomposing each rectangular pixel in two
triangles, and projecting the pixel intensity onto half pixels of the
transformed image, with weights proportional to the area of overlap of the
triangular half-pixels. The result is quantitatively exact, as long as the
original pixel value is assumed to represent a constant image density within
the pixel area, and as long as the coordinate transformation is diffeomorphic.
Implementation details and possible variations of the method are discussed.
