This effect is not difficult, the key is to set the position and proportion,
Capture mouse position, determine mouse position area, and onmouseover event, onmousemove event, onmouseout event
Set the proportion of the large image displayed. The proportion of the cut image displayed on the small image must be accurate, preferably 2 times or 4 times.
Mainly pay attention to the width. The image m.jpg I have here is 1440X900....
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