# points on implicit curves

Cfmattox shared this problem 1 year ago
Not a Problem

I typed "y^2 = x^3" into the algebra panel and attached an arbitrary point, A = (.90, .85) to the curve. I then zoomed in one mouse click. The point moved to A = (.66, .53). The location of the point seems to depend upon the window size. It happens for every implicit curve I tried. Attached are two ggb files (before and after I zoomed) which demonstrate the problem.

As I was filling out this report, I noticed that the point also moves when I change the geogebra window size on my macbook pro. Let me know if you need more information.

Fletcher

2

Sorry, that's expected. Try Curve(t^2,t^3,t,-5,5) for more control

1

Well, I gave that curve as a simple example. It happens to be easily parameterized. What should I do for a more complicated curve like, say, sin(x^2y) = sin(x + y)?

1

I just discovered Implicitcurve, but implicitcurve(sin(x^y) - sin(x+y)) results in the same bug I reported. Am I doing something wrong?

2

try point(implicitcurve, slider) being slider a slider from 0 to 1

2

That's an interesting feature and I am glad to learn of it, but it still suffers from the same behaviour: when I zoom, the coordinates of the point change.

Is this bug that difficult to fix? Calling it "expected" behaviour sounds like you are content with living with it. I'm not trying to be snarky, although it may sound that way. I would think fixing it would be in everyone's best interest. I do appreciate your efforts!

Fletcher

1

In my opinion this is either an undocumented restriction or a bug.

But definitely not a "Not a Problem". In both cases an action of the development team is necessary.

Please note the questions to the reproducible facts in the attachment.

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note: I can guess that a windows independing implementation of PathParameters for impliciteCurves could be very laborious to impossible. But that is no reason to let the questioner run into the void.

1

It's not just when zooming or changing window size, it also occurs when one tries to drag the whole plane.

Here's a partial workaround. My implicit curve is, e.g., y^2 = x^3 -x. Start by placing an arbitrary point, P, on the X-axis. If your curve is symmetrical about the X-axis (or Y-axis) and if You don't mind dealing with duplicate points on either side of the symmetry axis, then create a line perpendicular to the axis at P, and intersect that line with the implicit curve.

If You don't like the duplicate (or multiple) points, perhaps You can mimic what I did for my curve: First vertically translate the axis point P by an arbitrary amount, then construct the ray based at P, through P's translation, and intersect this ray with the implicit curve. If one has a more complicated curve, to where the ray intersects the curve in more than one place, You might need to get creative, perhaps: use a segment instead of the ray; use a circle or other curve instead of the ray.

Of course if You are wanting to animate the point around the implicit curve, or if You want to create a locus based on that point, the above kluge-workaround doesn't help much.

I agree with rami & cfmattox that the current behaviour is not "Not A Problem", and it deserves some kind of improvement or at least a published discussion.