[Geotiff] how to keep coordinates after modifying a geotiff

[acid]

Hi,
I use geotiff example GTIFReportACorner() to get the coordinates of 4 corners of geotiff images,
and I wonder after modifying images,for example,cropping images, how can the coordinates of 4 new corners
 be obtained?
Really appreciate your help,

sincerely,Alison  ^_^