Re: Line intersection point is wrong

Emre Hasegeli <emre(at)hasegeli(dot)com> writes:
>> Hm. I don't think I believe the vertical-line cases there either.
>> They seem to be assuming A = -1 in a vertical line, which would be
>> true if the line was computed by line_construct_pts, but otherwise
>> not necessarily.

> I think we can just remove those cases.

That would be nice.

>> Also: your formulation of the general case assumes that
>> (l1->A * l2->B - l2->A * l1->B) is not zero, which I'm
>> not entirely convinced of. In principle the line_parallel test
>> would catch the case, but seeing that that is not exactly how
>> line_parallel computes its result, roundoff error could bite us
>> here. I wonder if line_interpt_internal should skip the
>> line_parallel call and instead do its own tests for zero divide
>> to detect parallel lines.

> If it would do its own test, the result would be inconsistent with ?#
> and ?||.

Hmm, well, what if we change line_parallel() so that what it tests
is whether (l1->A * l2->B - l2->A * l1->B) is zero? That seems
simpler and more symmetric than what it does now.

regards, tom lane