how to avoid changes of image edges?

hi . i have changed the pixels of an image in edges and then i have saved it.and when i read the image and want to extract that pixels the palace of edges have changed.why this happen and how can i prevent them? and i use canny edge detection

[Adam]

You need to post some code. It is too difficult to know what exactly you did otherwise. If you save an image then reload it you will generally get exactly what you started with, provided you use the correct parameters for reading and writing.

[Jack Moriss]

my image is an rgb image and i select red channel and find the edges b=edge(red,'canny') and change pixels in edges and again rejoin channels but after that my image is double and i save it as png format im = im - min(im(:)); im = im / max(im(:)); imwrite(im,'rgg.png','Bitdepth',16); i =imread('rgg.png'); and in this step both the location of edges has changed and the pixel value is not exatly like befor saving the image