How warpAffine works?

edit

when i use

warpAffine(InputArray src, OutputArray dst, InputArray M, Size dsize);

imshow("dst",dst);

there is visible area and invisible area in OutputArray dst according to Size dsize.

Then I think, if dsize is small, only small area in src need to be warped and this will be more efficient.

warpAffine work like this way?

In other words,

cropping area and warping or warping whole image (src) and cropping ROI?

(sorry for my poor english.)