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First, I should say I am at best an advanced amateur at Mathematica, and that generally speaking my knowledge of programming (in any language) is more in computation than visualization.

There is a model I am working on for which I have recently shown there is a feasibility region: $$\mathcal{R} = \{(x,y) \in [0,\infty)\times[0, \infty) : x+y \leq L\}.$$ Of course this is a triangular subregion of the first quadrant. I wish to restrict the domain of the streamplots I am generating to $\mathcal{R}$.

From looking at the documentation I have constructed the following code:

StreamPlot[{dx/dt, dy/dt}, {x, 0, L}, {y, 0, L}]

which works fine, but of course gives me a large amount of unneeded data that clutters up my attempted visualization. So then, my question is:

How do I restrict the domain of StreamPlot to $\mathcal{R} $?

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    $\begingroup$ Welcome to Mathematica.se! Full minimalistic code to demonstrate the issue, please! Then it is much easier for us to help you out! $\endgroup$ – Johu Sep 22 '18 at 21:07
  • $\begingroup$ Closely related to DensityPlot with equal mesh and a certain boundary $\endgroup$ – Johu Sep 22 '18 at 21:14
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Try

StreamPlot[{dx/dt, dy/dt}, {x, 0, L}, {y, 0, L}, RegionFunction -> Function[{x, y}, x+y <= L]]
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StreamPlot[{-1 - x^2 + y, 1 + x - y^2}, {x, -3, 3}, {y, -3, 3}, 
 RegionFunction -> Function[{x, y, z}, 2 < x^2 + y^2 < 9]]

enter image description here

or

StreamPlot[{-1 - x^2 + y, 1 + x - y^2}, {x, y} \[Element] 
  StadiumShape[{{0, 0}, {2, 3}}, 2]]

enter image description here

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  • $\begingroup$ Thank you very much for your help. This code will be very helpful if I ever need to plot a circular region. $\endgroup$ – GeauxMath Sep 22 '18 at 21:19
  • $\begingroup$ Just to demonstrate that the solution is not limted to circlar region, I updated the answer with a more compelx shape. $\endgroup$ – Johu Sep 22 '18 at 21:28

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