Heading west on Sunset (that winding bit just north of the Los Angeles Country Club), when a charcoal grey Lamborghini Murcielago pulls out just ahead of me. Nice. It made a lovely sound and went from zero to 40 in about 2 seconds. Really expensive cars tend to actually be driven well (unlike the run-of-the-mill Mercedes/Porsche roadsters around here, mostly driven by the kind of women who take their dogs for a “walk” in a Prada bag).

## Archive for March, 2008

### LA Moments

Monday, March 3rd, 2008### Splines and modulation

Sunday, March 2nd, 2008My efforts to equally subdivide a curve along its length have, in part, been leading to this. First, I extended the sampling to work with splines (made up of cubic Bézier curves with c1 continuity). This shot shows 4 curves put together to form a spline:

Next, I wrote some code to modulate a curve onto another curve. Here’s a curve, and the same curve with another curve modulated onto it.

### And it was done

Saturday, March 1st, 2008Since I already had the binary search and interpolation code, it was just a matter of writing different samplers for ellipses and Bézier curves.

;; make a sampler function for a bezier curve (defun make-bezier-sampler (p0 p1 p2 p3) (lambda (k) (decasteljau p0 p1 p2 p3 k))) ;; make a sampler function for an ellipse (defun make-ellipse-sampler (center xradius yradius) (lambda (k) (let ((x (* xradius (cos k))) (y (* yradius (sin k)))) (make-point (+ x (xcoord center)) (+ y (ycoord center))))))

Equal angle increments:

Equal circumferential distances:

### More on ellipses

Saturday, March 1st, 2008I think I will use the same method as I do for Bézier curves to step along the circumference.

Another generalisation I had to make from the circle code is with respect to the normal. For a circle of radius r, centred on the origin, and parameterised by angle θ, a point on the circle is (r cos θ, r sin θ). And the normal is exactly the same: the point vector is normal to the circle.

For an ellipse, this is not the case. For an ellipse with half-dimensions a and b, centred on the origin, and parameterised by angle θ, a point on the ellipse is (a cos θ, b sin θ). But the normal is (b cos θ, a sin θ).

(Of course, the ellipse form degenerates to the circle form when a = b.)

### On ellipses

Saturday, March 1st, 2008Having conquered equidistant spacing along a Bézier curve, my thoughts now turn to the same problem for an ellipse. I have solved the problem for a circle of course, which is a special case of an ellipse. One would think that going from a circle to an ellipse would be mathematically easy: it’s easy to compute a point on an ellipse given centre and radii, and an ellipse is just a 2-way stretched circle, right?

Well, as with the Bézier curve, it’s not as simple as simply incrementing the angle parameter around the ellipse as one can with a circle, because obviously in that case the distance between successive points will vary.

And then one comes up against the rather startling fact that **there is no simple exact equation for the circumference of an ellipse**. No variant of 2πr here. As one reference puts it, “there are simple formulas but they are not exact, and there are exact formulas but they are not simple”. The exact formulas are infinite sums. The simple formulas can be glaringly inexact, depending on the ellipse.

I’m thinking on it some more. If, as I suspect, there proves to be no easy closed form solution to how much to vary the angle (or other alternative parameters of the curve) to achieve uniform spacing of radial points, I can fall back on the same solution as for the Bézier curve, i.e. sampling and interpolation.

### Bore da

Saturday, March 1st, 2008Happy St David’s Day to all my Wales-connected friends. I’m not remotely Welsh. But I can say *Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch*.

I suppose it’s been St David’s Day for 8 hours already, actually…