How To Get Coordinates Of Transparent Area In Image

How to find coordinates of transparent area in the image. I working on .png image which has transparent background and transparent area in the middle of the image. The transparent area in the middle look a like ellipse, so i want to find coordinate of top, bottom, left, and right of that area. I am using opencv.

I have tried to find pixels and from result that i got, i understand that pixel with rgb that equal to 255.0 255.0 255.0 is transparent. what i have in my mind is, if rgb with value that equal to 255 255 255 detected, i will put 1 into arraylist named transparent, and if it not equal to 255 255 255 i will put 0 into the list. So when i look into the list, if there is 1 0 or 0 1 it means that border between transparent area and colored area or vice versa. But, how to know if that border is between transparent area in the middle of image and the image, and not between background and the image. Am i doing this correctly?

Here snippet of code.

Mat imgMask = Highgui.imread(imgfile);
double[] pixels = new double[3];
System.out.println("channel " + imgMask);
for(int x = 0; x < imgMask.cols(); x++) {
for(int y = 0; y < imgMask.rows(); y++) {
pixels = imgMask.get(y, x);

[Code] .....