Why this time-frequency representation of a pure sine function with Stockwell Transform is different in the boundary? This time-frequency decomposition was made with Stockwell Transform to a pure sine function and graphed with matlab. But as you can see in the boundary of the image the pattern is different. I need to know why is this happening?Time-frequency representation

The S-transform is defined using an integral over an infinite time interval. Supposedly your pure sine function is considered zero outside some finite interval? Or it wraps around to the other end, but the phases at the ends do not match? Then it's not so pure at the ends of that interval. There will be some discontinuity in the cropped/wrapped-around signal or its derivative; such discontinuities spread the spectrum.
As with Fourier analysis, use of window functions helps in reducing such phenomena.
