Geometry tools

Lines and distances

You can find the midpoint between two points using midpoint.

The following code places a small pentagon (using ngon) at the midpoint of each side of a larger pentagon:

sethue("red")
ngon(O, 100, 5, 0, action = :stroke)

sethue("darkgreen")
p5 = ngon(O, 100, 5, 0, vertices=true)

for i in eachindex(p5)
    pt1 = p5[mod1(i, 5)]
    pt2 = p5[mod1(i + 1, 5)]
    ngon(midpoint(pt1, pt2), 20, 5, 0, action = :fill)
end

A more general function, between, finds for a value x between 0 and 1 the corresponding point on a line defined by two points. So midpoint(p1, p2) and between(p1, p2, 0.5) should return the same point.

sethue("red")
p1 = Point(-150, 0)
p2 = Point(150, 40)
line(p1, p2)
strokepath()
for i in -0.5:0.1:1.5
    randomhue()
    circle(between(p1, p2, i), 5, action = :fill)
end

Values less than 0.0 and greater than 1.0 appear to work well too, placing the point on the line if extended.

center3pts finds the radius and center point of a circle passing through three points which you can then use with functions such as circle or arc2r.

perpendicular finds the foot of a perpendicular line which lies on a line through two points perpendicular to a another point.

A, B = Point(-150, 0), Point(150, 50)

sethue("grey50")
fontsize(18)
setline(4)

line(A, B, action = :stroke)

# point perpendicular to line
C = Point(-50, -80)
D = perpendicular(A, B, C)

# line perpendicular to line
E, F = perpendicular(A, B)

# point perpendicular to extended line
G = Point(230, -200)
H = perpendicular(A, B, G)

sethue("grey50")
label.(string.(["A", "B", "C", "D", "E", "F", "G", "H"]),
    :ne, offset=10,
    (A, B, C, D, E, F, G, H))

sethue("red")
arrow(C, D)
sethue("green")
arrow(E, F)
sethue("orange")
arrow(G, H)
sethue("purple")
circle.([A, B, C, D, E, F, G, H], 4, action = :fill)

Points and arcs

Use isarcclockwise(c, p1, p2) to check whether an arc centered at c running from p1 to p2 is clockwise.

The pointinverse function finds the inverse of a point relative to a reference circle (centerpoint and radius). In the following example, each vertex on the shape inside the circle is linked by an arrow to its inverse outside the circle.

d = @drawsvg begin
radius = 60
circle(O, radius + 20, action = :stroke)

points = polycross(O, radius, 7, vertices=true)
poly(points, action = :stroke, close=true)

antipoints = last.(pointinverse.(points, O, radius+20))

for (n, pt) in enumerate(zip(points, antipoints))
    sethue(HSB(length(points) * n, 0.8, 0.8))
    @. circle(pt, distance(O, pt)/6, action = :fill)
    sethue("black")
    arrow(pt...)
end
end 800 500

Use anglethreepoints to find the angle formed by two lines connecting three points:

function showangle(pt1, pt2, pt3)
    θ = anglethreepoints(pt1, pt2, pt3)
    label(string(round(rad2deg(θ), digits=2), "°"), :w, pt2)
    newpath()
    carc(pt2, 50, 0, -θ)
    strokepath()
end

let
    background("grey20")
    sethue("white")
    fontsize(12)
    tiles = Tiler(800, 800, 4, 4)
    for (pos, n) in tiles
        @layer begin
            translate(pos)
            pg = [polar(50, 0), O, polar(50, n * -2π/16)]
            poly(pg, action = :stroke)
            for n in 1:3
                pt1 = pg[1]
                pt2 = pg[2]
                pt3 = pg[3]
                showangle(pt1, pt2, pt3)
            end
        end
    end
end

Other functions that help with geometry include:

• distance distance between two points

• getnearestpointonline drop perpendicular

• pointlinedistance distance of point from line between two points

• slope angle of line between two points

• perpendicular find perpendicular

• dotproduct scalar dot product of two points

• @polar convert radius and angle to point

• polar convert radius and angle to point

• ispointonline true if point lies on line

• ispointonpoly true if point lies on any edge of polygon

• isarcclockwise true if arc is clockwise

• pointinverse inverse of point with respect to circle

• anglethreepoints angle formed by two lines defined by three points

• determinant3 find determinant of matrix of 3 points

• rotatepoint rotate point around another by angle

Triangle centers

To find the center of a triangle, use one of:

• trianglecircumcenter center of circumcircle/intersection of the perpendicular bisectors.

• triangleincenter intersection of the interior angle bisectors

• trianglecenter centroid

• triangleorthocenter intersection of the altitudes

▲ = Point[Point(-100.0, 0.0), Point(110.0, 30.0), Point(65.0, 90.0)]

@layer begin
    sethue("red")
    setline(2)
    poly(▲,  :stroke, close=true)
end

# circumcenter
circle(▲..., action = :stroke)
cp = trianglecircumcenter(▲...)
circle(cp, 2, action = :fill)
label("circumcenter", :N, cp)

# incenter
cp = triangleincenter(▲...)
circle(cp, 2, action = :fill)
pt1 = getnearestpointonline(▲[1], ▲[2], cp)
@layer begin
    sethue("black")
    circle(cp, distance(cp, pt1), action = :stroke)
    label("incenter", :S, cp)
end

# center
cp = trianglecenter(▲...)
circle(cp, 2, action = :fill)
label("center", :w, cp)

# orthocenter
cp = triangleorthocenter(▲...)
circle(cp, 2, action = :fill)
label("orthocenter", :e, cp)

Intersections

intersectionlines finds the intersection of two lines.

sethue("black")
P1, P2, P3, P4 = ngon(O, 100, 5, vertices=true)
label.(["P1", "P2", "P3", "P4"], :N, [P1, P2, P3, P4])
line(P1, P2, action = :stroke)
line(P4, P3, action = :stroke)

flag, ip =  intersectionlines(P1, P2, P4, P3)
if flag
    circle(ip, 5, action = :fill)
end

intersectionlinecircle finds the intersection of a line and a circle. There can be 0, 1, or 2 intersection points.

l1 = Point(-100.0, -75.0)
l2 = Point(300.0, 100.0)
rad = 100
cpoint = Point(0, 0)
line(l1, l2, action = :stroke)
circle(cpoint, rad, action = :stroke)
nints, ip1, ip2 =  intersectionlinecircle(l1, l2, cpoint, rad)
sethue("black")
if nints == 2
    circle(ip1, 8, action = :stroke)
    circle(ip2, 8, action = :stroke)
end

intersection2circles finds the area of the intersection of two circles, and intersectioncirclecircle finds the points where they cross.

This example shows the areas of two circles, and the area of their intersection.

c1 = (O, 150)
c2 = (O + (100, 0), 150)

circle(c1... , action = :stroke)
circle(c2... , action = :stroke)

sethue("purple")
circle(c1... , action = :clip)
circle(c2... , action = :fill)
clipreset()

sethue("black")

text(string(150^2 * π |> round), c1[1] - (125, 0))
text(string(150^2 * π |> round), c2[1] + (100, 0))
sethue("white")
text(string(intersection2circles(c1..., c2...) |> round),
     midpoint(c1[1], c2[1]), halign=:center)

sethue("red")
flag, C, D = intersectioncirclecircle(c1..., c2...)
if flag
    circle.([C, D], 5, action = :fill)
end

Bounding boxes

The BoundingBox type allows you to use rectangular extents to organize and interact with the 2D drawing area. A BoundingBox holds two points, the opposite corners of a bounding box.

You can make a BoundingBox from:

• the current drawing
• two points
• a text string
• an existing polygon
• a stored path
• a table or table cell

and by modifying an existing bounding box, or using the results of functions such as circle() or box().

BoundingBox without arguments defines an extent that encloses the drawing (assuming that the origin is at the center of the drawing—see origin). Use centered=false if the drawing origin is still at the top left corner.

This example draws circles at three points: at two of the drawing's corners and at the midway point between them:

origin()

bb = BoundingBox()
setline(10)
sethue("orange")

circle(bb[1], 150, action = :stroke) # first corner

circle(bb[2], 150, action = :stroke) # second corner

circle(midpoint(bb...), 150, action = :stroke) # midpoint

sethue("blue")
circle.([bb[1], midpoint(bb[1:2]), bb[2]], 130, action = :fill)

sethue("red")
circle.([first(bb), midpoint(bb...), last(bb)], 100, action = :fill)

You can make a bounding box from a polygon:

p = star(O, 100, 5, 0.1, π/3.3, vertices=true)
sethue("antiquewhite")
box(BoundingBox(p), action = :fill)

sethue("black")
poly(p, action = :stroke, close=true)

The resulting bounding boxes can be passed to box or poly to be drawn.

To convert a bounding box b into a box, use box(b, vertices=true) or convert(Vector{Point}, BoundingBox()).

To obtain the coordinates of the corners or key points on the bounding box, use the functions with names combining box and top|middle|bottom and left|center|right. So boxtopleft(bbox) finds the top left corner of the bounding box bbox.

You can also do some arithmetic on bounding boxes. In the next example, the bounding box is created from the text "good afternoon". The bounding box is filled with purple, then increased by 40 units on all sides (blue), also scaled by 1.3 (green), and also shifted by (0, 100) (orange).

translate(-130,0)
fontsize(40)
str = "good afternoon"
sethue("purple")
box(BoundingBox(str), action = :fill)
sethue("white")
text(str)

sethue("blue")
modbox = BoundingBox(str) + 40 # add 40 units to all sides
poly(modbox, action = :stroke, close=true)

sethue("green")
modbox = BoundingBox(str) * 1.3
poly(modbox, action = :stroke, close=true)

sethue("orange")
modbox = BoundingBox(str) + (0, 100)
poly(modbox, action = :fill, close=true)

You can find the union and intersection of BoundingBoxes, and also find whether a point lies inside one. The following code creates, shrinks, and shifts two bounding boxes (colored yellow and pink), and then draws: their union (a bounding box that includes both), in black outline; and their intersection (a bounding box of their common areas), in red. Then some random points are created (you can pass a bounding box to rand() to get a random point inside the box) and drawn differently depending on whether they're inside the intersection or outside.

origin()
setopacity(0.75)
setline(8)

bbox1 = BoundingBox()/2 - (50, 30)
sethue("yellow")
box(bbox1, action = :fill)

bbox2 = BoundingBox()/2  + (50, 30)
sethue("pink")
box(bbox2, action = :fill)

sethue("black")
box(bbox1 + bbox2, action = :stroke)

sethue("red")
bothboxes = intersectboundingboxes(bbox1, bbox2)
box(bothboxes, action = :fill)

for i in 1:500
    pt = rand(bbox1 + bbox2)
    if isinside(pt, bothboxes)
        sethue("white")
        circle(pt, 3, action = :fill)
    else
        sethue("black")
        circle(pt, 2, action = :fill)
    end
end

To find out where a line starting at the center of a bounding box passing through a point crosses or would cross the edges of the box, use pointcrossesboundingbox.

bx = BoundingBox(box(O, 200, 200))
setline(1)
box(bx, action = :stroke)
for i in 1:10
    pt = randompoint((1.5bx)...)
    pt2 = pointcrossesboundingbox(pt, bx)

    sethue("grey50")
    arrow(O, pt)
    sethue("red")
    circle(pt2, 3, action = :stroke)
end

Random points

You can use randompointarray to create an array of randomly placed points.

The randompointarray(boundingbox, distance) method fills the boundingbox with random points up to distance units apart using a Poisson Disk sampling method.

background("black")
b = blend(
        boxtopleft(BoundingBox()),
        boxbottomright(BoundingBox()),
        "red",
        "green")

addstop(b, 0.3, "orange")
addstop(b, 0.4, "magenta")
addstop(b, 0.5, "cyan")
addstop(b, 0.7, "yellow")
setblend(b)

for pt in randompointarray(BoundingBox() * 0.9, 15)
    d = rescale(distance(pt, O), 0, sqrt(800 * 500), 1, 0)
    circle(pt, 1 + 7d, action = :fill)
end

The randompointarray(point1, point2, n) method generates n random points in the area bounded by two points, using Julia's random number generator.

pt1 = Point(-300, -150)
pt2 = Point(300, 150)

sethue("purple")
map(pt -> circle(pt, 6, action = :fill), (pt1, pt2))
box(pt1, pt2, action = :stroke)

sethue("blue")
map(pt -> circle(pt, 4, action = :fill), randompointarray(pt1, pt2, 200))

Use rand(BoundingBox()) to return a single point somewhere inside a bounding box.

Noise

For artistic graphics you might prefer noisy input values to purely random ones. Use the noise function to obtain smoothly changing random values corresponding to input coordinates. The returned values wander slowly rather than jump about everywhere.

In this example, the gray value varies gradually as the noise function returns values between 0 and 1 depending on the location of the two input values pos.x and pos.y.

The top two quadrants use a lower value for the detail keyword argument, an integer ()>= 1) specifying how many "octaves" of noise you want.

The left two quadrants use a lower value for the persistence keyword argument, a floating point number specifying how the amplitude diminishes for each successive level of detail. There is more fine detail when the persistence is higher, particularly when the detail setting is also high.

tiles = Tiler(800, 400, 200, 200)
sethue("black")
for (pos, n) in tiles
    freq = 0.05
    pos.y < 0 ? d = 1      : d = 4
    pos.x < 0 ? pers = 0.3 : pers = 1.0
    ns = noise(freq * pos.x, freq * pos.y,
        detail=d,
        persistence=pers)
    setgray(ns)
    box(pos, tiles.tilewidth, tiles.tileheight, action = :fillstroke)
end

Use initnoise to initialize the noise behaviour.