How can I convert a "tent" into a "cap"?

Question

I wanted to draw a "cap"-like paraboloid structure.

like this one ...

.

But, the function $$f(x, y) = -x^2-y^2+4$$
is giving me a "tent"-like structure.

See the link.

How should I modify my equation/function to achieve this?

Answer 1

generally $z=ax^2+by^2+c$ plots that paraboloid shape.the higher the magnitudes of a and b are the higher stiff the graph gets.change in c changes the vertical distance.if a and b are negative then the graph flips its face like your case.

to get your desired shape,it is just needed to change the sign of your coefficient "a","b".make them positive.you can make them higher in magnitude to get a stiffer graph...or simply you can change the domain(make it larger) of "x" and "y" in your plotting function to have a stiffer look .