link to updated notebook

https://observablehq.com/@d3/pie-settings illustrates corner radius, pad angle, and centroids.

README.md

@@ -110,7 +110,7 @@ If the arc generator has a [context](#arc_context), then the arc is rendered to
 
 Computes the midpoint [*x*, *y*] of the center line of the arc that would be [generated](#_arc) by the given *arguments*. The *arguments* are arbitrary; they are simply propagated to the arc generator’s accessor functions along with the `this` object. To be consistent with the generated arc, the accessors must be deterministic, *i.e.*, return the same value given the same arguments. The midpoint is defined as ([startAngle](#arc_startAngle) + [endAngle](#arc_endAngle)) / 2 and ([innerRadius](#arc_innerRadius) + [outerRadius](#arc_outerRadius)) / 2. For example:
 
-[<img alt="Circular Sector Centroids" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/centroid-circular-sector.png" width="250" height="250">](http://bl.ocks.org/mbostock/9b5a2fd1ce1a146f27e4)[<img alt="Annular Sector Centroids" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/centroid-annular-sector.png" width="250" height="250">](http://bl.ocks.org/mbostock/c274877f647361f3df7d)
+[<img alt="Circular Sector Centroids" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/centroid-circular-sector.png" width="250" height="250">](https://observablehq.com/@d3/pie-settings)[<img alt="Annular Sector Centroids" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/centroid-annular-sector.png" width="250" height="250">](https://observablehq.com/@d3/pie-settings)
 
 Note that this is **not the geometric center** of the arc, which may be outside the arc; this method is merely a convenience for positioning labels.
 
@@ -150,7 +150,7 @@ function cornerRadius() {
 
 If the corner radius is greater than zero, the corners of the arc are rounded using circles of the given radius. For a circular sector, the two outer corners are rounded; for an annular sector, all four corners are rounded. The corner circles are shown in this diagram:
 
-[<img alt="Rounded Circular Sectors" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/rounded-circular-sector.png" width="250" height="250">](http://bl.ocks.org/mbostock/e5e3680f3079cf5c3437)[<img alt="Rounded Annular Sectors" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/rounded-annular-sector.png" width="250" height="250">](http://bl.ocks.org/mbostock/f41f50e06a6c04828b6e)
+[<img alt="Rounded Circular Sectors" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/rounded-circular-sector.png" width="250" height="250">](https://observablehq.com/@d3/pie-settings)[<img alt="Rounded Annular Sectors" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/rounded-annular-sector.png" width="250" height="250">](https://observablehq.com/@d3/pie-settings)
 
 The corner radius may not be larger than ([outerRadius](#arc_outerRadius) - [innerRadius](#arc_innerRadius)) / 2. In addition, for arcs whose angular span is less than π, the corner radius may be reduced as two adjacent rounded corners intersect. This is occurs more often with the inner corners. See the [arc corners animation](http://bl.ocks.org/mbostock/b7671cb38efdfa5da3af) for illustration.
 
@@ -192,7 +192,7 @@ The pad angle is converted to a fixed linear distance separating adjacent arcs,
 
 If the [inner radius](#arc_innerRadius) or angular span is small relative to the pad angle, it may not be possible to maintain parallel edges between adjacent arcs. In this case, the inner edge of the arc may collapse to a point, similar to a circular sector. For this reason, padding is typically only applied to annular sectors (*i.e.*, when innerRadius is positive), as shown in this diagram:
 
-[<img alt="Padded Circular Sectors" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/padded-circular-sector.png" width="250" height="250">](http://bl.ocks.org/mbostock/f37b07b92633781a46f7)[<img alt="Padded Annular Sectors" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/padded-annular-sector.png" width="250" height="250">](http://bl.ocks.org/mbostock/99f0a6533f7c949cf8b8)
+[<img alt="Padded Circular Sectors" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/padded-circular-sector.png" width="250" height="250">](https://observablehq.com/@d3/pie-settings)[<img alt="Padded Annular Sectors" src="https://raw.githubusercontent.com/d3/d3-shape/master/img/padded-annular-sector.png" width="250" height="250">](https://observablehq.com/@d3/pie-settings)
 
 The recommended minimum inner radius when using padding is outerRadius \* padAngle / sin(θ), where θ is the angular span of the smallest arc before padding. For example, if the outer radius is 200 pixels and the pad angle is 0.02 radians, a reasonable θ is 0.04 radians, and a reasonable inner radius is 100 pixels. See the [arc padding animation](http://bl.ocks.org/mbostock/053fcc2295a445afab07) for illustration.