arc

Builtin index · Plotting

arc(cx, cy, r, from, to)

Input: cx, cy, r, from, to · Output: no output

What it is

Draws part of a circle, as an open stroke. cx and cy (numbers) are the centre, in data coordinates; r (number, positive — a zero or negative radius is an error) is the radius, applied as the same figure to both axes, so a panel whose x and y scales differ draws an ellipse rather than a round arc — a square canvas is what makes it round, not axis equal, which only equalises the ranges; from (number, degrees) is the angle the sweep starts at, measured counterclockwise from the positive x axis, so 0 is the three-o'clock position; to (number, degrees) is the angle it ends at, and a to smaller than from sweeps clockwise instead. Optional segments= (number, default 128) is how many straight pieces approximate the curve, and rotate= (number, degrees) turns the whole arc about its centre. Unlike circle, an arc is open — it renders as a stroke (color=/width=/dash=), not a fillable shape.

Overloads

One call shape — this function is not overloaded.

Examples

From the documentation, and run by the build -- the output and the figure below are what the engine actually produced. Simplest first.

# A square canvas, so that equal ranges really are equal scales and the

# circle below comes out round. Without it the figure would render an

# ellipse while the paragraph above it explained how to avoid one.

figure_size(560, 560)
xlim(-2, 2)
ylim(-2, 2)
axis equal
circle(0, 0, 1, color = "royalblue", fill = "royalblue")
ellipse(0, 0, 1.6, 0.7, color = "crimson", rotate = 30)
arc(0, 0, 1.3, 0, 120, color = "seagreen", width = 2)
polygon([0.8, 1.6, 1.2], [-1.6, -1.6, -0.9], color = "purple", fill = "plum")
rectangle(-1.8, -1.8, -1.2, -1.2, color = "orange")
rect(1.0, -1.8, 1.8, -1.2, color = "teal")
point(0, 1.9)

from plotting

How it works

── shapes ────────────────────────────────────────────── circle(cx, cy, r), ellipse(cx, cy, rx, ry), arc(cx, cy, r, from_deg, to_deg), polygon(xs, ys). All are sampled in DATA space and mapped through the axis transform, so a circle stays a circle when the axes are equal and becomes the correct ellipse when they are not. Emitting an SVG <circle> at a pixel radius would be simpler and would draw the wrong shape on any panel whose scales differ -- which is most of them. segments= controls the sampling; 128 is smooth at any figure size a paper uses and cheap enough not to think about.

From the note above this builtin in the interpreter, where the reasoning was written down as it was built.