J = VL_IMWBACKWARD(I, X, Y) returns the values of image I at locations X,Y. X and Y are real matrices of arbitrary but identical dimensions. I is bilinearly interpolated between samples and extended with NaNs to the whole real plane.
[J,JX,JY] = VL_IMWBACKWARD(...) returns the warped derivatives JX and JY too.
By default, VL_IMWBACKWARD() assumes that the image I uses the standard coordinate system. VL_IMWBACKWARD(XR,YR,I,X,Y) assumes instead that I is defined on a rectangular grid specified by the vectors XR and YR.
VL_IMWBACKWARD() is less general than the MATLAB native function INTERP2(), but it is significantly faster.
See also: IMWFORWARD(), INTERP2(), VL_HELP().