Intersection of line and cylinder

[pauljurczak]

Is intersection of line and cylinder implemented? I tried this:

c = Cylinder()
l = Line(Point(2, 2, 2), Point(3, 3, 3))
print(l.meet(c))

without success.

[jan-mue]

It is implemented but you have to use the intersect method of the Quadric/Cylinder class which returns a list of points instead of the single point/subspace that meet returns. This code should work:

c = Cylinder()
l = Line(Point(2, 2, 2), Point(3, 3, 3))
print(c.intersect(l))

I'll close this issue but please let me know if you have any more questions.

[pauljurczak]

Thank you. One more question: what is the recommended test for a point in infinity, which is returned when there is no intersection?

[jan-mue]

Thank you. One more question: what is the recommended test for a point in infinity, which is returned when there is no intersection?

Points at infinity are exactly those that have a zero in their last coordinate. For convenience I added the property isinf to the Point class that checks if the last coordinate is close to zero.

[pauljurczak]

Which is the last coordinate? This snippet:

c = Cylinder()
l = Line(Point(3, 3, 2), Point(3, 3, 3))
print(c.intersect(l))

produces:

[Point(0j, 0j, (-0.7276068751089989+0.6859943405700353j)) at Infinity]

with non-zero last coordinate.

[jan-mue]

Which is the last coordinate? This snippet:

c = Cylinder()
l = Line(Point(3, 3, 2), Point(3, 3, 3))
print(c.intersect(l))

produces:

[Point(0j, 0j, (-0.7276068751089989+0.6859943405700353j)) at Infinity]

with non-zero last coordinate.

The last coordinate is not shown in the standard string representation of a point. Instead "at Infinity" is added when it is close to zero. You can see all the homogeneous coordinates like this: print(c.intersect(l)[0].array). You can also directly access the last coordinate like this: print(c.intersect(l)[0][-1]).