How can exactly detect the edge line of the image?

To extract ROI exactly, I need to detect the edge line of image. Firstly, I used sobel edge detection, but this cannot detect exactly the edge line. So, I used the thresholding for my images. Although this can detect most images, there are some remaining images cannot correctly detect the line. I also use morphological (dilate,erode,etc). But still there is disconnected edge lines. So, please could you help me some suggestions or codes for my problem? I want to get the edge line (e.g 3.bmp).

Answer 1

[Image Analyst]

I don't think you need edge detection. Why not just invert the images and use bwboundaries(), or scan column by column using find() to find top and bottom edges as a function of column number?