ac.commutative algebra - Any implemented algorithm to compute the closure of an affine variety in a product of projective spaces?

This can be done in a few steps in probably any computer algebra package. You take the generators of your original ideal $I$, and bi-homogenize them, as described in the question. Then saturate with respect to the two hyperplanes at infinity, which are defined by the equation $x_0 y_0$.

For example, the diagonal in $mathbb A^3 times mathbb A^3$ is defined by $x_1 - y_1$, $x_2 - y_2$, and $x_3 - y_3$. If I wanted to use this to compute the ideal of the diagonal in $mathbb P^3 times mathbb P^3$, I would use the following commands in Macaulay2:

 r = QQ[x0,x1,x2,x3,y0,y1,y2,y3]
 i = ideal(x1*y0-y1*x0, x2*y0-y2*x0, x3*y0-y3*x0)
 saturate(i, x0*y0)

The code in Singular would be:

 ring r = 0, (x0,x1,x2,x3,y0,y1,y2,y3), dp;
 ideal i = x1*y0-y1*x0, x2*y0-y2*x0, x3*y0-y3*x0;
 LIB "elim.lib";
 sat(i, x0*y0);