# Angle marker

Martin McBride, 2020-08-23
Tags geometry angle
Categories generativepy generative art The `angle_marker` function adds an angle marker to the path, that can then be stroked. Angle markers look like this: An angle marker can have 1, 2 or 3 arcs. The 2 and 3 arc forms are often used to indicate that two angles are equal. You can also draw a right angle marker, as shown.

## angle_marker function

```angle_marker(ctx, a, b, c, count=1, radius=8, gap=2, right_angle=False)
```
Parameter Type Description
ctx Context The Pycairo Context to draw to
a (number, number) Tuple (x, y) for point a
b (number, number) Tuple (x, y) for point b
c (number, number) Tuple (x, y) for point c
count int Number of arcs on angle, 1, 2 or 3.
gap number Gap between arcs in user units.
right_angle boolean True for a right angle.

Draws an angle arc for the angle abc. The arc is centred on point `b`, and shows the angle between lines ab and cb in the clockwise direction.

If you are using a flipped coordinate system (see the `setup` function in the drawing module), the angle is drawn in the counterclockwise direction.

`count` allows you to draw double or triple arcs to indicate different pairs of equal angles. If `count` is not 1, `gap` sets the distance between the arcs - typically a value that is about twice the line thickness usually looks best.

`right_angle` can be set to true to draw a right angle marker rather than an arc. In that case, `count` is ignored.

Note that it is up to your code do ensure that the angle actually is a right angle before setting this. If you set this flag when the angle isn't a right angle (or very close), you will get strange looking result. If you don't set this flag when the angle is a right angle, a normal angle arc will be drawn. This is to avoid spurious effects in animations if a changing angle happens to be exactly 90 degrees in one frame/

## Example

Here is an example that shows the `angle_marker`, `tick` and `paratick` functions in use:

```from generativepy.drawing import make_image, setup
from generativepy.color import Color
from generativepy.geometry import line, polygon, angle_marker, tick, paratick

'''
Create rectangles using the geometry module.
'''

def draw(ctx, width, height, frame_no, frame_count):
setup(ctx, width, height, background=Color(0.8))

ctx.set_source_rgba(*Color(0, 0, 0.5))
ctx.set_line_width(3)

## Draw lines with ticks and paraticks
a = (50, 50)
b = (50, 150)
line(ctx, a, b)
ctx.stroke()
tick(ctx, a, b, length=12, gap=6)
ctx.stroke()

a = (100, 50)
b = (150, 150)
line(ctx, a, b)
ctx.stroke()
tick(ctx, a, b, 2, length=12, gap=6)
ctx.stroke()

a = (250, 50)
b = (200, 150)
line(ctx, a, b)
ctx.stroke()
tick(ctx, a, b, 3, length=12, gap=6)
ctx.stroke()

a = (350, 50)
b = (350, 150)
line(ctx, a, b)
ctx.stroke()
paratick(ctx, a, b, length=12, gap=6)
ctx.stroke()

a = (400, 50)
b = (450, 150)
line(ctx, a, b)
ctx.stroke()
paratick(ctx, a, b, 2, length=12, gap=6)
ctx.stroke()

a = (550, 150)
b = (500, 50)
line(ctx, a, b)
ctx.stroke()
paratick(ctx, a, b, 3, length=12, gap=6)
ctx.stroke()

## Draw lines with angles
a = (50, 250)
b = (50, 450)
c = (150, 450)
polygon(ctx, (a, b, c), closed=False)
ctx.stroke()
angle_marker(ctx, a, b, c, radius=24, gap=6, right_angle=True)
ctx.stroke()

a = (250, 250)
b = (200, 450)
c = (300, 450)
polygon(ctx, (a, b, c), closed=False)
ctx.stroke()
angle_marker(ctx, a, b, c, 3, radius=24, gap=6)
ctx.stroke()

a = (300, 250)
b = (400, 300)
c = (500, 300)
polygon(ctx, (a, b, c), closed=False)
ctx.stroke()
angle_marker(ctx, c, b, a, radius=24, gap=6)
ctx.stroke()

a = (300, 350)
b = (400, 400)
c = (500, 400)
polygon(ctx, (a, b, c), closed=False)
ctx.stroke()
angle_marker(ctx, a, b, c, 2, radius=24, gap=6)
ctx.stroke()

make_image("/tmp/geometry-markers.png", draw, 600, 500)
```
If you found this article useful, you might be interested in the book Computer Graphics in Python or other books by the same author.

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