How to make it expand vertically?

Please, see the image I attach.

Imagine the middle rectangle to be an active contour. Is there a way programmatically to make it expand up or down but NOT left or right, until a specific row? The row shall be estimated by a gray pixel value. The first row Matlab meets this x gray value.

Is it possible?

Thanks

[Jan]

See your former question: https://www.mathworks.com/matlabcentral/answers/441297-scan-the-image-until-the-first-and-last-points-i-set . Another duplicate of this question was closed already.

This seems to be the contour of a lossy JPEG compressed image. This will impede the automatic detection of edges massively. Is there really no way to obtain clean and clear inputs?

[Stelios Fanourakis]

What do you mean by clear inputs? Those are the exported jpg images from the ultrasound unit. There is no point of downsampling anywhere.

As I explained previously. By time, new outcomes and new findings are gathering, and after many comments the original post loses its main focus and thus I start a new question.

For example, this question is different from this

https://uk.mathworks.com/matlabcentral/answers/441297-scan-the-image-until-the-first-and-last-points-i-set#comment_664807

Since, the question above started by trying to develop a scanning algorithm to trace specific pixel values as a start and end point.

This question is about developing a rectangle active contour model to expand vertically.

Thank you