How to extract ROI by gray level of the image?

Hi, I am working on a project about stair detection and measurment. I first find and highlight the edges of the stairs by hough transform. Then I acquire the disparity map and here's the result.

Then I convert the disparity map into gray level image of [0 64], and the image shows a gradual change in gray level.

I tried to perform watershed segmentation for roi extract but the result is not good. Is there any other method for roi extraction by gray level? Thank you.