Is there a visual difference b/w anterior view of knee on x-ray and posterior view of knee on x-ray? can the patella block view on the anterior x-ray

YES: There would be some difference in what is magnified more, so the accepted x-ray format is an ap. Everyone does it the same. As in a chest x-ray it is always a pa. This uniform practice means any one reviewing a knee x-ray it is always an ap. As far as the patella blocking any thing, yes it does to some degree but also in a pa. This standard is universal so thre is no deviation worldwide.