Ray tracing: Computing ray-circle intersections

01 Sep 2018
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Part 1: Ray tracing: Computing ray-circle intersections
Part 2: Ray tracing: Computing ray-sphere intersections

I'm currently making my way through Ray Tracing in One Weekend, now available for free, and its 500 lines of C++ code. Being a bit rusty on the math side, these posts are my notes and supplement the book.

In this part and the next, we focus on probably the most central aspect of ray tracing: computing 2D line-circle and 3D line-sphere intersections. Their points of intersection are what enables us trace a ray from the eye, or camera position, through a pixel on the screen, or image plane, and into the scene. As the ray travels through the scene, we want to know if it hits, or intersects with, a scene object such as a circle, and if so at which points in space. If, where, and with which scene object the ray intersects determines the image plane's color at that pixel.

We start out with 2D line-circle intersections because it's simpler to visualize and because once the scalar math is worked out, it generalizes nicely to 3D line-sphere intersection and vector math. What we're working toward is nicely illustrated in the Wikipedia article on ray tracing:

As a supplement to the illustration, Disney's Practical Guide to Path Tracing provides a humourous non-technical overview and Painting With Light adds both depth and historical context.

Circle centered around origin

A circle may be defined in terms of the position of its center and its radius. To start simple, assume the circle is centered around the origin at $$(0,0)$$. The circle equation, or circle on standard form, then becomes $$x^2 + y^2 = r^2$$ The way to read the circle equation is that for any point $$(x,y)$$, if $$x^2 + y^2 = r^2$$, then the point is on the circle, otherwise it's not. This circle equation follows from the Pythagorean theorem, or distance formula. We can visualize the relationship between the circle equation and the Pythagorean theorem by considering a point $$(x,y)$$ on the circle shown below:

For this right-sided triangle and the point on circle, $$\sqrt{x^2 + y^2} = r$$. By squaring both sides we get rid of the square root and end up with the circle equation.

Circle offset from origin

Instead of a circle centered at $$(0,0)$$, it may be centered at any point $$(c_x, c_y)$$, extending the circle equation: $$\begin{eqnarray*} (x - c_x)^2 + (y - c_y)^2 = r^2 \end{eqnarray*}$$ To make sense of this equation, imagine we want a circle with its center shifted along the $$x$$ axis to the right of origin. As we subtract $$c_x$$ from $$x$$, for the circle equation to hold, and points to be on the circle, its $$x$$ component must be greater to compensate for $$c_x$$, shifting its center. Say we take the circle above and shift its center to $$2,0$$, then a point on the circle would be $$(2 - 2)^2 + (3 - 0)^2 = r^2$$.

To keep the following examples short, we assume the circle is centered at $$(0,0)$$ such that $$c_x$$ and $$c_y$$ may be ignored.

Line and circle intersections

Assuming a circle $$x^2 + y^2 = r^2$$ and a line $$y = ax + b$$, both on standard form (if not, we first must bring those on standard form), three possible intersection scenarios exists:

As an example, let's go with the circle $$x^2 + y^2 = 3^2$$ and the line $$y = -x + 3$$. If the two intersect, it implies that for some value of $$x$$ and for some value of $$y$$, substituting those same values into both equations, each equation holds. It doesn't imply that the left side of one equation equals the right side of the other, but that the left and right side of each equation by itself is equal.

At a point of intersection, the value of $$x$$ and the value of $$y$$ in the two equations are the same. Thus, either $$y = -x + 3$$ or $$x = -y + 3$$ of the line equation may be substituted into the circle equation. Substituting in $$y$$ yields a new equation: $$x^2 + (-x + 3)^2 = 3^2$$ The goal of substituting one equation into the other is that instead of one circle equation with two variables ($$x$$ and $$y$$), out comes one quadratic equation with one variable ($$x$$): $$\begin{eqnarray*} x^2 + (-x + 3)(-x + 3) = 3^2 & & \textrm{\{expand $$y = -x + 3$$\}}\\ x^2 + x^2 - 3x - 3x + 9 = 9 & & \textrm{\{multiply out parenthesis\}}\\ x^2 + x^2 - 3x - 3x = 0 & & \textrm{\{subtract 9 from both sides\}}\\ 2x^2 - 6x = 0 & & \textrm{\{simplify\}} \end{eqnarray*}$$ Solving a quadratic equation of the standard form $$ax^2 + bx + c = 0$$ for $$x$$ means determining values of $$x$$ for which $$y = 0$$. In other words, determining where the quadratic equation intersects the $$x$$ axis.

The line, circle, and quadratic equations are depicted graphically below. From the graphs, it's easy to visually confirm that the solutions to the quadratic equation are $$x_1 = 0$$ and $$x_2 = 3$$:

The standard method for solving algebraically involves substituting the coefficients of $$ax^2 + bx + c$$, that is $$a$$, $$b$$, and $$c$$, into the equation below, with $$d$$ being the discriminant: $$\begin{eqnarray*} x = \dfrac{-b \pm \sqrt{d}}{2a} \textrm{, where $$d = b^2 - 4ac$$} \end{eqnarray*}$$ The discriminant indicates which of the three intersection scenarios to expect. If $$d = 0$$, then one solution, or intersection, exist as the square root term disappears. If $$d > 0$$, two solutions, or intersections, exist as the square root term must be both added and subtracted. If $$d < 0$$, no solution, or intersection, exists as calculating the square root of a negative number is undefined:

$$d = (-6)^2 - 4(2)(0) = 36$$ Computing the discriminant without substituting it into the larger equation is useful only as a means to determine the number of solutions in advance. For the actual solutions, the points of intersection, the larger equation is required: $$\begin{eqnarray*} x_1 = \dfrac{-(-6) + \sqrt{36}}{2(2)} = \dfrac{6 + 6}{4} = \dfrac{12}{4} = 3\\ x_2 = \dfrac{-(-6) - \sqrt{36}}{2(2)} = \dfrac{6 - 6}{4} = \dfrac{0}{4} = 0 \end{eqnarray*}$$ Finally, inserting the values of $$x$$ into either equation gets us the corresponding values of $$y$$. The line equation is the simplest of the two: $$\begin{eqnarray*} y_1 = -x_1 + 3 = -0 + 3 = 3\\ y_2 = -x_2 + 3 = -3 + 3 = 0 \end{eqnarray*}$$ and so the points of intersection of the line and the circle are at $$(0,3)$$ and $$(3,0)$$.

Summary

At first sight, the math in this post may seem like unimportant high school math. Its true power, however, becomes evident next, when the techniques are generalized from lines and circles in 2D to lines and spheres in 3D and from scalars to vectors.