# Formula for ellipse formed from projecting a tilted circle onto the xy plane

A circle is drawn in the center of a piece of paper. The piece of paper is placed on a table and a camera is positioned directly overtop to look at the circle.

A piece of glass is placed between the camera and the piece of paper and is parallel to the piece of paper. A cartesian plane is drawn onto the piece of glass with point (x1, y1) marking the center of the circle on the paper below.

A point on the circumference of the circle is chosen and its (x,y) coordinate on the piece of glass is marked.

The paper is then tilted about the line y=x1 (the horizontal line through the center of the circle).

Note that as the paper is tilted the camera will observe an ellipse on the piece of glass: the projection of the circle onto the piece of glass, and the center point of the ellipse will still be (x1, y1).

However, this is magic paper: as the paper tilts the circle drawn on the piece of paper grows or shrinks such that the projection of the circle onto the piece of glass always intersects the point (x, y).

What is the formula that describes the projected ellipse on the piece of glass given the tilt angle theta, the point (x, y), the circle’s center coordinates (x1, y1)? Note that we do not know the radius of the original circle.

The motivation for this question is that I am working on an mobile application that renders a map onto the screen. The user's current location is marked with a blue dot. I want the blue dot to be placed near the bottom-left corner of the screen. But the dot itself cannot be moved: I must instead specify the latitude/longitude to be used as the center of the map and I need to calculate the map center to use in order to locate the blue dot at the desired location.

When the phone is rotated then the map needs to rotate about the blue dot (not about the center of the map). The math to figure this out is easy if the "camera" is looking at the map head-on (tilt=0) as I just need to trace the circumference of the circle centered at the center of the map that goes through the desired (x,y) position of the blue dot; however, if the map is tilted then I need to instead trace the circumference of the ellipse passing through the blue dot, the ellipse formed by projecting a circle lying on the plane of the ground onto the screen, and I can't seem to figure out the formula for this ellipse.

• The projected image will only stay centered on the camera’s axis (line of sight) for a parallel projection, i.e., for a camera at infinity. If the camera is at a finite distance from the image plane, the center of the projected image will drift away from this axis. In general, you’ll need to both shift the ellipse back to this visual center and adjust its size so that it will pass through $(x,y)$. – amd Aug 12 '17 at 1:47

To make things a little bit simpler, let us assume that the centre of the circle corresponds with the origin of the cartesian reference frame. Then the circle is characterised by $$\frac{x^2}{R^2} + \frac{y^2}{R^2} = 1$$ where $R$ is its radius of the circle and since the point $(x_p,y_p)$ lies on the circle we have $R = \sqrt{x_p^2 + y_p^2}$.

Rotating the circle about the line $y=0$ will result in a projected ellipse with a long axis $R$ that is along the $x$-direction and a shortened axis of length $R \cos\theta$ in the $y$-direction. The projected ellipse is therefore characterised by $$\frac{x^2}{R^2} + \frac{y^2}{R^2 \cos^2\theta} = 1$$ This projected ellipse will in general not pass through the point $(x_p,y_p)$ anymore and we therefore make an isotropic expansion of the ellipse by a factor $f\geq1$. This gives for the scaled ellipse the curve $$\frac{x^2}{f^2 R^2} + \frac{y^2}{f^2 R^2 \cos^2\theta} = 1$$ Since this has to pass again through the point $(x_p,y_p)$ this gives a simple equation $$\frac{x_p^2}{f^2 R^2} + \frac{y_p^2}{f^2 R^2 \cos^2\theta} = 1$$ which we can easily solve for the scaling factor $f$ and find $$f^2 = \frac{x_p^2 \cos^2\theta + y_p^2}{R^2 \cos^2 \theta}$$ which can be inserted back into the characterisation we obtained for the scaled ellipse and gives the final answer $$\frac{x^2 \cos^2\theta}{x_p^2 \cos^2\theta + y_p^2} + \frac{y^2}{x_p^2 \cos^2\theta + y_p^2} = 1$$ The correctness can easily be confirmed by checking that it is an ellipse that passes through the point $(x_p,y_p)$ and of which the ratio of the long and short axis is indeed $\frac{1}{\cos\theta}$. For an arbitrary centre $(x_o,y_o)$ of the circle this would result in $$\frac{(x-x_o)^2 \cos^2\theta}{(x_p-x_o)^2 \cos^2\theta + (y_p-y_o)^2} + \frac{(y-y_o)^2}{(x_p-x_o)^2 \cos^2\theta + (y_p-y_o)^2} = 1$$

• Thank you for the wonderful explanation! I've implemented this in code but still find a significant drift that gets worse as the tilt becomes more exaggerated. Just wondering if you think that perhaps this is due to "stretching" of the map below the x axis and compression above? For example, if a rectangle is drawn onto the map's surface and then the map is tilted this produces an apparent trapezoid (see cloud.githubusercontent.com/assets/1231218/10617325/…). Do you think this same effect may need to be applied to the ellipse? – denversc Aug 11 '17 at 5:40
• You are absolutely right there. The problem above is for a normal projection, but what you are trying to do is a projection in perspective. The problem with that is that things that are "further" away get projected closer together. This also means that a projected circle would become a distorted ellipse with a bit more complicated characterisation, and hence the solution above will get worse on increasing viewing angle. Also that problem can be solved, but additional information of the location of the viewpoint (now not infinitely far away) needs to be specified. – Ronald Blaak Aug 11 '17 at 9:41
• @RonaldBlaak I’m not sure that I’d characterize the ellipse that results from central projection of a circle as “distorted.” It’s a perfectly ordinary ellipse, albeit one that’s related to the circle by a more complex formula. – amd Aug 12 '17 at 0:55
• @amd Thanks for pointing out my mistake there. – Ronald Blaak Aug 12 '17 at 16:32
• @denversc For a finite camera, the center of the ellipse will, as you’ve discovered, be offset from $(x_0,y_0)$ due to the asymmetry of the projection: the ray to the edge of the circle that’s tilted away from the camer makes a smaller angle with the camer’s axis than does the ray to the near edge, so the foreshortening is greater for that side of the pivot line. – amd Aug 13 '17 at 22:10

This situation is somewhat more complicated than you’ve assumed in your question. The center of the ellipse that’s the projection of the circle will in general not be the projection of the circle’s center. It will “drift” away from this point. This is because for a finite camera, i.e., one positioned a finite distance from the image, the projection is asymmetric: rays from the camera to the edge of the circle that’s tilted away from the camera make a smaller angle with the camera’s axis than do the corresponding rays on the opposite edge, so the foreshortening parallel to the $y$-axis is greater on that side. This asymmetry also means that uniformly stretching the magic paper doesn’t correspond to a uniform scaling of the image (glass) plane, and vice-versa. We’ll come back to this later.

Aside from the obvious dependency on the tilt angle $\theta$, for a camera positioned a finite distance $f\gt0$ from the image plane, the size, shape and location of the circle’s image also depend on the distance $d\gt0$ of the circle’s center from the image plane and radius $r$ of the circle. Note that even though we’re not given this radius explicitly, we can compute it from the two given points: $r=\sqrt{(x-x_0)^2+(y-y_0)^2}$. It’s reasonable to restrict $\theta$ to the open interval $\left(-\frac\pi2,\frac\pi2\right)$: When $\theta=\pm\frac\pi2$ we’re looking at the paper edge on, and beyond that we’re looking at the back of the paper.

It seems easiest to me to proceed in two stages: compute the raw projection of the circle and then adjust it to fit the constraints. We’ll do this by computing a homography—a planar perspective projection—between the two planes. W.l.o.g. assume that $(x_0,y_0)=(0,0)$ since we can always translate to make it so. Place the glass on the $x$-$y$ plane. Working in homogeneous coordinates, the camera is at $\mathbf V=[0,0,f,1]^T$ and the circle’s center at $[0,0,-d,1]^T$. A normal to the paper’s plane is $[0,\cos\theta,\sin\theta,0]^T$. The image plane is then $\mathbf\Pi_{\text{im}}=[0,0,-1,0]^T$ and the paper’s plane is $\mathbf\Pi_{\text{src}}=[0,\sin\theta,-\cos\theta,-d\cos\theta]^T$, although we won’t make direct use of the latter. The matrix $\mathcal P$ of the projection onto the image plane is $$\mathcal P=\mathbf V\mathbf\Pi_{\text{im}}^T-\mathbf V^T\mathbf\Pi_{\text{im}}\mathcal I_4=\begin{bmatrix}f&0&0&0\\0&f&0&0\\0&0&0&0\\0&0&-1&f\end{bmatrix}.$$ To complete the unadjusted homography we choose convenient bases for the two planes. For the image plane, we’ll simply drop the $z$-coordinate. For the source plane, we place the origin at the circle’s center, take the “$x$”-axis parallel to the world $x$-axis and use the normal for the “$z$”-axis. Putting these together with $\mathcal P$ produces the homography $$\mathcal H=\begin{bmatrix}1&0&0&0\\0&1&0&0\\0&0&0&1\end{bmatrix}\begin{bmatrix}f&0&0&0\\0&f&0&0\\0&0&0&0\\0&0&-1&f\end{bmatrix}\begin{bmatrix}1&0&0\\0&\cos\theta&0\\0&\sin\theta&-d\\0&0&1\end{bmatrix} = \begin{bmatrix}f&0&0\\0&f\cos\theta&0\\0&-\sin\theta&f+d\end{bmatrix}.$$

The general equation of a plane conic can be written in matrix form as $[x,y,1]\mathcal C[x,y,1]^T=0$, where $\mathcal C$ is a symmetric $3\times3$ matrix. This equation has the same form when “homogenized” by replacing $x$ and $y$ by $x/w$ and $y/w$, respectively, and we can identify conics with these matrices. They are covariant: if points transform according to $\mathbf p'=\mathcal M\mathbf p$, then $\mathcal C$ transforms as $\mathcal M^{-T}\mathcal C\mathcal M^{-1}$. Our circle is represented by the matrix $\mathcal C=\operatorname{diag}(1,1,-r^2)$ and its image is: $$\mathcal C'=\mathcal H^{-T}\mathcal C\mathcal H^{-1}=\begin{bmatrix} (f+d)^2&0&0 \\ 0&(f+d)^2\sec^2\theta-r^2\tan^2\theta&-r^2f\tan\theta \\ 0&-r^2f\tan\theta&-r^2f^2 \end{bmatrix}.$$ (I’ve multiplied through by $f^2(f+d)^2$, which doesn’t change the conic.) $\det\mathcal C'\lt0$, so this is an ellipse when $(f+d)^2\gt r^2\sin^2\theta$, an hyperbola when $(f+d)^2\lt r^2\sin^2\theta$, while equality produces a parabola. These conditions correspond to zero, two and one intersections, respectively, of the tilted circle with the plane $z=f$. This ellipse is axis-aligned, but its center is not at the origin. $\mathcal H$ preserves colinearity, so the center of this ellipse is the midpoint of the points $\mathcal H[0,\pm r,1]^T$, $$\mathbf c'=\left[0,{rf\sin\theta\over(f+d)^2-r^2\sin^2\theta}r\cos\theta,1\right]^T.$$

By linearity, a translation in one plane corresponds to a translation in the other plane, so the projected image’s center can be adjusted by “sliding” the paper a bit in the direction of its “$y$”-axis. On the other hand, because of the asymmetry in the projection noted at the beginning, uniformly stretching the paper to hit the target point will change the shape of the ellipse. Not only that, because the center of the projection depends nonlinearly on the circle’s size, this stretch will also affect the shift needed to place the center where it’s supposed to go. I don’t think that’s what you really had in mind when you described the “magic paper,” so I’ll go through how to make the necessary adjustments after projection.

Since we already have the ellipse center $\mathbf c'$, it’s easiest to first translate and then scale. The effect of this translation is easy to compute: the translated ellipse is in standard position, so the resulting matrix is diagonal, with the first two elements the same as those along the main diagonal of $\mathcal C'$, while the lower-right element becomes $\mathbf c'^T\mathcal C'\mathbf c'$. We can then normalize this matrix so that the lower-right element is $-1$, after which the semi-axis lengths can be read off as the reciprocal square roots of the other two diagonal elements. That is, \begin{align}a^2 &= -{\mathbf c'^T\mathcal C'\mathbf c' \over \mathcal C'_{1,1}} \\ b^2 &= -{\mathbf c'^T\mathcal C'\mathbf c' \over \mathcal C'_{2,2}}.\end{align} However, before going to the trouble of calculating these values, remember that this ellipse will be scaled, so we really only care about its eccentricity, which depends on the ratio of $a^2$ to $b^2$. Thus, we can simply use the diagonal elements $\mathcal C'_{1,1}$ and $\mathcal C'_{2,2}$ directly without computing $\mathbf c'^T\mathcal C'\mathbf c'$ (or, for that matter, the center $\mathbf c'$ itself) and even discard other common factors to get $$a^{-2}=1 \\ b^{-2}=\sec^2\theta-{r^2\over(f+d)^2}\tan^2\theta.$$

(To satisfy my curiosity, I computed the actual values, anyway: $$a^2 = {f^2\over(f+d)^2-r^2\sin^2\theta}r^2 \\ b^2 = \left({f(f+d)\over(f+d)^2-r^2\sin^2\theta}\right)^2r^2\cos^2\theta.$$ As $f\to\infty$, i.e., as the projection becomes parallel, $a\to r$, $b\to r\cos\theta$ and $\mathbf c'\to[0,0,1]^T$.)

To find the necessary scale factor so that this translated ellipse passes through $\mathbf p'=[x_p,y_p,1]^T$, observe that scaling the ellipse ${x^2\over a^2}+{y^2\over b^2}=1$ by a factor of $s>0$ changes the equation to ${x^2\over a^2}+{y^2\over b^2}=s^2$, therefore the correct scale factor is $$s=\sqrt{{x_p^2\over a^2}+{y_p^2\over b^2}}$$ and the equation of the adjusted ellipse is $${x^2\over a^2}+{y^2\over b^2}={x_p^2\over a^2}+{y_p^2\over b^2}$$ with $a^2$ and $b^2$ as above. Substituting for $a^2$ and $b^2$ and rearranging finally gives the sought-after equation: $$(x^2-x_p^2)+(y^2-y_p^2)\left(\sec^2\theta-{r^2\over(f+d)^2}\tan^2\theta\right)=0.\tag{*}$$ In the limit as $f\to\infty$, this becomes $(x^2-x_p^2)+(y^2-y_p^2)\sec^2\theta$. Note that the final adjusted ellipse doesn’t so much depend on the actual values of $r$, $f$ and $d$ as on the ratio of the size of the circle to the distance of the camera from its center. If this distance is large relative to the radius of the circle, or the tilt angle very large or small, the limit ellipse is a good approximation to the true image.

In matrix terms, the final ellipse is obtained via the transformation matrix $$\begin{bmatrix}s&0&-sc'_x\\0&s&-sc'_y\\0&0&1\end{bmatrix}\mathcal H = \begin{bmatrix}sf&0&0 \\ 0&s(f\cos\theta+c'_y\sin\theta)&-sc'_y(f+d) \\ 0&-\sin\theta&f+d \end{bmatrix}.$$ Here, $s$ should be computed using the true values of $a$ and $b$. This matrix allows you to map the entire source plane, not just the circle, if so desired.

If you want to do the scaling on the paper side instead, that will entail a similar computation. Instead of starting out with $\mathcal H$, however, you’ll use $$\mathcal H'=\begin{bmatrix}sf&0&0\\0&sf\cos\theta&0\\0&-s\sin\theta&f+d\end{bmatrix}.$$ The center of the ellipse and its semi-axis lengths are found as above, but they will all depend on the unknown scale factor $s$. Plugging the coordinates of $\mathbf p'$ into the resulting ellipse equation will let you solve for this scale factor.

• The projective viewpoint certainly eases the visualization and the algebra. (+1 of course.) – J. M. is a poor mathematician Aug 14 '17 at 7:04
• Wow I've never worked with perspective geometry before! Thank you for the detailed explanation. Would it be possible to write the equation in parametric form? That is, given an angle phi from the circle's center coordinates what is the (x,y) coordinates that satisfy the relation? I was able to convert the first response from ronald-blaak into the x=acos(t) and y=bsin(t) parametric form but I don't have the skills to convert the final equation provided by amd into parametric form. – denversc Aug 15 '17 at 5:14
• @denversc Write it as $(x^2-x_p^2)+\mu(y^2-y_p^2)=0$ and rearrange it into standard form. – amd Aug 15 '17 at 5:53
• @denversc Actually, you can just go back one equation and divide that one through by the r.h.s. to get an equation in standard form. – amd Aug 15 '17 at 17:58