How do I define this without using piecewise function?
I think it has something to do with Bilinear Surface but not sure how to get started.
$x_1=-1, x_2=1, x_3=0, x_4=1$
$y_1=0, y_2=1, y_3=1, y_4=0$
$ x(u,v)=x_1+u·(x_2-x_1)+v·(x_3-x_1+u·(x_4-x_3-x_2+x_1))$
$y(u,v)=y_1+u·(y_2-y_1)+v·(y_3-y_1+u·(y_4-y_3-y_2+y_1))$
I don't understand what is this $u & v$ we have here.
I don't know exactly what you mean by "without using piecewise function", but the description
$$\{(x,y)| y\in[0,1] \land x\in[-1, 1-y]\}$$
has no piecewise functions...