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I understand how one can calculate a plane equation (ax+by+cz=d) from three points but how can you go in reverse?

How can you calculate arbitrary points from a plane equation?

math_Artist88
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  • Do you mean tell whether or not a point lies in the plane? Just plug in its coordinates into the equation; if ax+by+cz does not equal d, then it does not lie in the plane, if it does equal d, then it does lie in the plane. – Chill2Macht Apr 23 '16 at 20:59
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    You can convert the plane to parameter-form $\vec{x} = \vec{a} + \lambda \vec{u} + \mu \vec{v} \quad \text{with } \lambda, \mu \in \mathbb{R}$ and pick arbitrary values for $\lambda$ and $\mu$, then compute the resulting $\vec{x}$ which is in the plane. – Maximilian Gerhardt Apr 23 '16 at 21:01
  • no, i mean how can I calculate a set of unknown points from ax+by+cz=d. the x, y, z intercepts are one way to approach it but I wanted to know if there was another way (a=b=0, solve for z, etc.) – math_Artist88 Apr 23 '16 at 21:02
  • how could I convert the plane eqtn(ax+by+cz=d) to parametric form. I am not familiar with this concept – math_Artist88 Apr 23 '16 at 21:05
  • Think of $\vec{a}$ as the base-point, then you have two vectors $\vec{u}, \vec{v}$ which are two "direction vectors" from this point which describe the whole plane. You can compute these from the three original points $A,B,C$ quite easily. Pick one as the base-point (e.g. $A$), then $\vec{u} = \vec{AB}$ and $\vec{v} = \vec{AC}$. Else, there are tons of questions here and material on the internet about this. – Maximilian Gerhardt Apr 23 '16 at 21:12
  • ok, the only values I have to start with are a, b,c, d coefficient from the plane equation. I don't have a base point – math_Artist88 Apr 23 '16 at 21:15
  • Possible related question/answer on SO (http://stackoverflow.com/a/23474396/380384) – John Alexiou Apr 23 '16 at 21:35
  • Possible duplicate of How do I get three non-colinear points on a plane? – John Alexiou Apr 23 '16 at 21:36

2 Answers2

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From your comment I finally understood what you are looking for:

If you have a plane defined by $a x + b y + c z = d$ then you also have the following properties:

  • Plane normal direction: $$\hat{n} = \begin{pmatrix} \frac{a}{\sqrt{a^2+b^2+c^2}} \\ \frac{b}{\sqrt{a^2+b^2+c^2}} \\ \frac{c}{\sqrt{a^2+b^2+c^2}} \end{pmatrix}$$

  • Point on plane closest to the origin (position of plane) $$ \vec{r} = \begin{pmatrix} \frac{a d}{a^2+b^2+c^2} \\ \frac{b d}{a^2+b^2+c^2} \\ \frac{c d}{a^2+b^2+c^2} \end{pmatrix} $$

  • Distance of plane from the origin $$ r = \frac{d}{\sqrt{a^2+b^2+c^2}} $$

  • Directions along the plane (not unit vectors), and perpendicular to $\hat{n}$. $$ \begin{align} \hat{e}_1 & = \begin{pmatrix} c-b \\ a-c \\ b-a \end{pmatrix} & \hat{e}_2 & = \begin{pmatrix} a (b+c)-b^2-c^2 \\ b (a+c) -a^2-c^2 \\ c (a+b)-a^2 - b^2 \end{pmatrix} \end{align} $$

You can verify that $\hat{e}_1 \cdot \hat{n} =0$, $\hat{e}_2 \cdot \hat{n}=0$ and $\hat{e}_1 \cdot \hat{e}_2 =0$, where $\cdot$ is the dot (inner) product.

Confirmation via GeoGebra

pic

NOTES: Please edit the equation to make it clear you are looking for the plane properties when given a plane in equation form.

John Alexiou
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  • This post would be a good approach except I do not know how to calculate the center point of the rectangle http://stackoverflow.com/questions/22769430/find-corners-of-rectangle-given-plane-equation-height-and-width – math_Artist88 Apr 24 '16 at 02:29
  • a normal vector and a point will give you a plane equation. im trying to go backwards from the plane equation to find a point at the center of the plane – math_Artist88 Apr 24 '16 at 02:37
  • perhaps, i should rephrase the question. how could I generate a rectangle (corner1, corner2, corner3, corner4) from the plane equation – math_Artist88 Apr 24 '16 at 19:40
  • From the information I in this answer use the two direction vectors to form a rectangle. – John Alexiou Apr 24 '16 at 22:22
  • There were typos in the equations that I finally fixed. – John Alexiou May 22 '19 at 22:48
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You pick arbitrarely two values $a,b$ for $x$ and $y$, you plug them in the equation, you solve it for $z$ and obtain a value $c$. The point $(a,b,c)$ is a point of your plane.

Bérénice
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