NURBS Knot Vector Direction Done RIght

Last post I revealed how I'd gotten myself tangled up when trying to add a "do the right thing" mode to Nubs.evaluate. I'm going to repeat that code here, as I've added one of the Nubs.evaluate functions to the mix.

	multi method KnotVector.N0_index(Int $p, $u, KnotBasisDirection $direction = Left)
	{
	given $direction
	{
	when Left { UpperBound(@.knots, $u) - $p - 1; }
	when Right { LowerBound(@.knots, $u) - $p - 1; }
	when Reasonable { die "Must specify Left or Right for KnotBasisDirection"; }
	}
	}
	multi method Nubs.Direction($t, KnotBasisDirection $direction)
	{
	return $t < ([+] self.ParameterRange) / 2 ?? Left !! Right
	if ($direction == Reasonable);
	return $direction;
	}
	multi method Nubs.evaluate($t, KnotBasisDirection $direction = Reasonable)
	{
	my $n0 = $.knot_vector.N0_index($.degree, $t, Direction($t, $direction));
	return [+] ($.knot_vector.N_local($n0, $.degree, $t)
	>>*<< @.control_points[$n0 .. ($n0 + $.degree)]);
	}

view raw gistfile1.txt hosted with ❤ by GitHub

These functions became overly complex because I tried to overload the KnotBasisDirection enum with an extra layer of meaning. Pulling that extra layer out and rearranging things a tad gives you vastly better code:

	multi method KnotVector.N0_index(Int $p, $u, KnotBasisDirection $direction = Left)
	{
	given $direction
	{
	when Left { UpperBound(@.knots, $u) - $p - 1; }
	when Right { LowerBound(@.knots, $u) - $p - 1; }
	}
	}
	multi method Nubs.Direction($t)
	{
	$t < ([+] self.ParameterRange) / 2 ?? Left !! Right;
	}
	multi method Nubs.evaluate($t, KnotBasisDirection $direction = self.Direction($t))
	{
	my $n0 = $.knot_vector.N0_index($.degree, $t, $direction);
	return [+] ($.knot_vector.N_local($n0, $.degree, $t)
	>>*<< @.control_points[$n0 .. ($n0 + $.degree)]);
	}

view raw gistfile1.txt hosted with ❤ by GitHub

Notice that we've eliminated an enum value here, two cases that needed to be checked for, and simplified a third line of code. The "trick" (almost too obvious to be called a trick) is to make the default value of $direction be determined by a called to Direction, rather than calling Direction with a special value that indicates it needs to do work.

It's the best of both worlds. By default, if you don't specify $direction it will do something reasonable. And for those rare weird cases where it is important (discontinuities in the curve), you can specify the direction to use for the evaluation.

Tomorrow: On the road to SVG in Perl 6.