plot 2D-function: x and y Range

Hello,

I have a function of 2 variables (X,Y). Of course, I can plot the function with surf(...).

But the function is only valid for X and Y Values, which describe a triangle. Outside the triangle, I do not want to see the Z-Values in my surface plot (or set the Z-Values to Zero there).

How can I solve that easily?

Thank you!