Find large distinctions in Video
Implement a low pass 2D filter that will reduce greatly the noise. 
Following an example that you could complete yourself. 
You could subtract the filter bias to keep an average of 0 "addition" by the filter, controlling the dynamic range.

Example of filter

         1/16  1/8   1/4  1/8   1/16

    1/16  1/8   1/4   1/2  1/4   1/8     1/16

1/16  1/8   1/4   1/2   1    1/2   1/4     1/8     1/16

1/8   1/4   1/2   1     2    1     1/2     1/4     1/8

1/16  1/8   1/4   1/2   1    1/2   1/4     1/8     1/16

   1/16  1/8   1/4   1/2  1/4   1/8     1/16

        1/16  1/8   1/4  1/8   1/16


You might be able to clearly see the desired image.

Than run an Edge a 2D edge detector which will enhance the location of the step.
You could try running two 1D filters: -1 1 in horizontal and vertical direction. If you know the relative "black"/"white" areas you could select which to use of the optional -1 1 or 1 -1.

You might be able to use frequency domain filtering for the low pass 2D filter. 
If you are not familiar with this type of challenges and this is not exercise you need to get a consultant to help you or study the subject in more depth.

Depending of the contrast of the "step" you might be able to detect automatically the "line" created by the edge detector.