"crop image" randomly creates a white border

[Max Mugen]

sometimes, when i use the crop image function on a png image embedded,  it works and the image is cropped but I get randomly on either one side of the image a 1px border that is created. 

It is random and unpredictable, and if I go back to original size and crop it again, it sometimes not do the white border
it can appear on one of the four sides of the image 

Is there a way to fix that ? 
is the Adobe team aware of the problem and will fix it one day ? 

Normal png file size, win 10, latest AI version 

Thx 

[Monika Gause]

It probably wasn't snapped exactly to the image pixels. Or maybe you are also resampling the imageß

[Ton Frederiks · Accepted solution]

This often happens with images that have a resolution that is not a multiple of 72 ppi (like 144 or 288 ppi), but 150 or 300 ppi. When cropping, fractional pixels are calculated resulting in white line(s) at the edges.

Masking will give a better result.

[Max Mugen]

thank you for your answer ! nice to know
Just out of curiosity, should I understand that 72PPI is the base for math operations and computational calculation inside AI ?
 my images are indeed 300PPI 

[Ton Frederiks]

Yes 72 points per inch is the base. When Illustrator was created, the first Macintoshes had a screen resolution of 72 pixels per inch. So the 100% size on screen matched the printed output. That does not matter anymore, but the 72 was kept to decide the size of a pixel (a pixel does not have a fixed size untill you tell how many of them fit in an inch or centimeter). 

When working with images/pixels in Illustrator 72 or multiples thereoff are the best to work with.

There should be no visible difference between 288 ppi and 300 ppi images.

[Max Mugen]

thank you very much for the technical info, very appreciated ! 

[Ton Frederiks]

Good to hear you found it useful, Max.