# Transforming quadratic parametric curve to implicit form

I have a algebraic parametric curve $$\mathbf{p}(t) = (x(t), y(t))$$ where $x$ and $y$ are both polynomials of degree $\leq p$. Now, I want to find the implicit form $f(x, y) = 0$.

A document I'm reading claims without argumentation that there is always an equivalent implicit formulation of the form $f(x, y) = 0$, where $f$ is a polynomial of degree $\deg(f) \leq p$.

This is not obvious to me. I found some documents stating similar results. For example, in [1]:

Algebraic geometry provides us with the following key facts about algebraic curves: e.g. [2]: Every plane parametric curve can be expressed as an implicit curve. Some, but not all, implicit curves can be expressed as parametric curves.

Unfortunately, I can't get my hands on a copy of [2].

This question considers the case of a quadratic parametric curve.

I was wondering if there is a simple proof for the statement every algebraic parametric curve on the plane can be expressed as an implicit curve. Moreover, I am curious if $x$ and $y$ being polynomials of degree $\leq p$ implies that $\deg(f) \leq p$ as well.

Since I don't know the nature of the answer, pointing me to a resource might be more appropriate.

[1]: Conversion methods between parametric and implicit curves and surfaces, Christoph M. Hoffmann, from http://graphics.stanford.edu/courses/cs348a-17-winter/Handouts/a228715.pdf [2]: R.J. Walker, Algebraic Curves, Springer Verlag, New York, 1978.

• I don't think I am qualified to give an algebraic geometric answer, but maybe you can get help from gradient of level set polynomial being orthogonal to tangent of the curve. (Or maybe I am having a brain fart?) – mathreadler Nov 26 '17 at 12:41
• Great idea! I am trying to work this out right now: it seems promising. – Ruben Nov 26 '17 at 12:46
• Cox, Little and O'Shea Ideals, varieties and algorithms 3$\S$3. – Jan-Magnus Økland Nov 26 '17 at 13:03

To begin with, you are dealing with the "great old theory" of elimination.

In this case, the answer is : yes, it is always possible to eliminate parameter $t$ between polynomial expressions and obtain an implicit polynomial expression.

Let us show it on an example, with a parametric curve defined by these equations :

$$\tag{1}\begin{cases}x=t^2+t+1\\y=t^3-1\end{cases}$$

If you use a Computer Algebra System such as Mathematica, elimination process is implemented under the following form: Eliminate[{x==t^2+t+1,y==t^3-1},t] giving the following implicit equation :

$$\tag{2}x^3-3x^2-3xy-y^2=0$$

as its result.

But there is more to say. You need for this to be familiar with resultants (see remark below) (https://en.wikipedia.org/wiki/Resultant). The theorem you are looking for is the fact that the implicit equation can be obtained as the resultant of the $\color{red}{2}$nd degree polynomial $t^2+t+(1-x)$ (I was tempted to write $=0$...) and $\color{red}{3}$rd degree polynomial $t^3-(1+y)$ which the $5 \times 5$ parametric determinant (5 = $\color{red}{2+3}$):

$$d=\left|\begin{array}{ccccc}1& 1& (1-x)& 0 &0 \\ 0& 1& 1& (1-x)& 0\\ 0& 0& 1& 1& (1-x)\\ 1& 0& 0& -(1+y)& 0\\ 0& 1& 0& 0& -(1+y)\end{array}\right|=0$$

(one writes $\color{red}{3}$ times the $\color{red}{2}$nd degree polynomial, then $\color{red}{2}$ times the $\color{red}{3}$rd degree one, with a right shift for every "carriage return").

The expansion of determinant $d$ gives back (up to a certain unimportant constant factor) implicit equation (2).

This example can evidently been extended to a general case.

Remark 1: The excellent reference (Cox, Little, O'Shea "Ideals, Varieties and Algorithms") given by @Jan-Magnus Økland can be found online. See pages 155-162 devoted to Sylvester's resultant.

Remark 2: the main idea behind the concept of resultant is that it expresses a necessary and sufficient condition between parameter(s) present in coefficients of 2 equations for these equations to have at least a common root. Let us take an example : consider a quadratic polynomial $at^2+bt+c$ (with $a\ne0$) and its derivative $2at+b$. They have a common root $t_0$ which is a double root iff the following resultant is zero:

$$\delta=\left|\begin{array}{ccc}a& b& c \\ 2a& b& 0\\ 0& 2a& b\end{array}\right|=-a(b^2-4ac)$$

This value is $0$ iff $b^2-4ac=0$, which is the classical criteria for a quadratic equation to have a double root. We have thus found back the concept of discriminant by using the resultant!

• I didn't understand your example the first time, but your second remark was very enlightening. Even though I don't understand why this result holds (I am still planning to read Cox, Little, and O'Shea), it is very beautiful! :) – Ruben Dec 2 '17 at 20:48
• Tom Sederberg has done work applying resultants to Bezier curves, primarily for curve-curve intersections. – CADJunkie Feb 8 '18 at 18:10
• @CADJunkie Thank you very much for this indication : I just uploaded his book (tom.cs.byu.edu/~557/text/cagd.pdf) where indeed the resultant is used in this context. – Jean Marie Feb 8 '18 at 18:19

Hint Another idea based on my comment which may help. The tangent vectors along the parametric curve (using same notation as Jean Marie) will be given by

$$\left[\begin{array}{c}\frac{\partial x}{\partial t}\\\frac{\partial y}{\partial t}\end{array}\right]$$

These should be orthogonal to the gradient of the polynomial level-set you are seeking. This should give you a set of quite specific equations as the only thing remaining should be constant (quite literally the particular level you want to set your curve on).

• I don't see the relationship with the problem. – Jean Marie Dec 2 '17 at 8:39
• Any f(x, y) that satisfies p'(t) · ∇f = 0 also satisfies f(x(t), y(t)) = c, so f(x, y) = c is an implicit formulation of the curve defined by p. (This reasoning is not completely correct since x, y not on the curve might also satisfy f(x, y) = c, for example when f = 0). This seems like a reasonable idea, but I was not able to work this out in practice (which does not mean the method in itself does not work). – Ruben Dec 2 '17 at 10:09
• @Ruben: Yes, I have not tried it myself yet, so there may be cases when it does not work. – mathreadler Dec 2 '17 at 17:04
• @mathreadler I'm not arguing that there are cases where it doesn't work. I'm saying I don't see how to solve this system of equations. It is nonlinear and I don't think there is an easy algorithm one can use. – Ruben Dec 2 '17 at 20:42