How to get the envelope of this picture

[ziqian]

The black line is drawn by myself, not generated by code

Answer 1

[Sulaymon Eshkabilov]

You can employ data confidence bounds in your data plotting. Here is the MATLAB doc: https://www.mathworks.com/help/matlab/creating_plots/line-plot-with-confidence-bounds.html

[Charalambos Hadjipanayi]

That is not exactly what he wants.

[DGM]

I'd probably agree, as it looks like an imagesc() or pcolor() plot.

Considering OP never objected, clarified, or provided example data or threshold criteria, I don't see what's left to do.

If you yourself want to provide an example that you feel fits your understanding of the question, you're welcome to do so. If you yourself have a similar question, feel free to ask it.