Calculate the facebook cover offset y

问题

I'm getting the user cover photo using Facebook SDK. I have the path to the img and the offset y.

  "source": "http://a5.sphotos.ak.fbcdn.net/hphotos-ak-ash3/526114_459046674110601_992101492_n.jpg",
  "offset_y": 19

The div where i put the image have 300px height; and right background position is center -146px;. How to calculate the top value when I have 300 and 19 ?

回答1:

offset_y is a proportion of space between upper area and lower area of cropped image.

So, actual pixel offset should be like this:

y = (offset_y / 100) * (upperArea_of_cropImage.height + lowerArea_of_cropImage.height)

Edited:

// where y is the actual pixels from the upper left corner of the real image to
// upper left corner of the cropped image

        -- ================================  --
    y {   ||                              ||    } a
        --||..............................|| --
          ||                              ||
          ||                              ||
          ||            Cover             ||
          ||                              ||
          ||..............................|| --
          ||                              ||    } b
          ||                              ||   |
           ================================  --

     offset_y that Facebook send to you actually was a/b not y

来源：https://stackoverflow.com/questions/11812364/calculate-the-facebook-cover-offset-y