let's translate these words literally and you can see the difference... 視 is easy to translate, as a noun = sight, as a verb = to see, espy 轻 = light hence 轻視 = to look at lightly and so the meaning can be extended to "look down upon", "to look at disparagingly" 怱 is more difficult as it has three meanings. The meaning as a quantifier for heaviness, length can be ruled out from this combination. It can either mean, "suddenly" or "ignore/neglect". Without context, it can mean "suddenly espied" or "look of neglect" or "ignore". 不要怱視我对你的心 = please don't neglect/ignore my feelings for you. 怱視飞霜几片落 = suddenly I espied a few flakes of snow came flying down.

Their meanings are same! ※忽视=漠視=渺視=忽略=輕視=疏忽=無視

忽视 means ignore 轻视 means despise