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Drawing simple shapes#

Note

Note that because we are exposing the Web Canvas API, you can find more tutorials and documentation following this link: https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API

There are some API differences though:

  • The Canvas widget is directly exposing the CanvasRenderingContext2D API

  • All the API is written in snake_case instead of camelCase, so for example canvas.fillStyle = 'red' in JavaScript becomes canvas.fill_style = 'red' in Python

Before we can start drawing, we need to talk about the canvas grid. The origin of this grid is positioned in the top left corner at coordinate (0,0). All elements are placed relative to this origin. So the position of the top left corner of the blue square becomes x pixels from the left and y pixels from the top, at coordinate (x,y).

_images/grid.png

Drawing rectangles#

There are six methods that draw rectangles on the canvas:

  • fill_rect(x, y, width, height=None):

    Draws a filled rectangle. If height is None, it is set to the same value as width.

  • stroke_rect(x, y, width, height=None):

    Draws a rectangular outline. If height is None, it is set to the same value as width.

  • fill_rects(x, y, width, height=None):

    Draws filled rectangles. Where x, y, width and height are either integers, lists of integers or NumPy arrays. If height is None, it is set to the same value as width.

  • stroke_rects(x, y, width, height=None):

    Draws rectangular outlines. Where x, y, width and height are either integers, lists of integers or NumPy arrays. If height is None, it is set to the same value as width.

  • fill_styled_rects(x, y, width, height, color, alpha):

    Same as fill_rects but with additional (n x 3) color ndarray and (n) alpha ndarray.

  • stroke_styled_rects(x, y, width, height, color, alpha):

    Same as stroke_rects but with additional (n x 3) color ndarray and (n) alpha ndarray.

You can also clear a certain canvas rectangle area:

  • clear_rect(x, y, width, height=None):

    Clears the specified rectangular area, making it fully transparent. If height is None, it is set to the same value as width.

from ipycanvas import Canvas

canvas = Canvas(width=200, height=200)

canvas.fill_rect(25, 25, 100, 100)
canvas.clear_rect(45, 45, 60, 60)
canvas.stroke_rect(50, 50, 50, 50)

canvas
_images/rect.png

fill_rects and stroke_rects are blazingly fast ways of drawing up to a million rectangles at once:

import numpy as np

from ipycanvas import Canvas

n_particles = 100_000

x = np.array(np.random.rayleigh(250, n_particles), dtype=np.int32)
y = np.array(np.random.rayleigh(250, n_particles), dtype=np.int32)
size = np.random.randint(1, 3, n_particles)

canvas = Canvas(width=800, height=500)

canvas.fill_style = "green"
canvas.fill_rects(x, y, size)

canvas
_images/rects.png

Drawing polygons#

You can draw a polygon by providing a list of points, either a Python list, or a NumPy array. It’s the fastest way to draw a polygon with ipycanvas.

  • fill_polygon(points):

    Fill a polygon from a list of points [(x1, y1), (x2, y2), ..., (xn, yn)].

  • stroke_polygon(points): Draw polygon outline from a list of points [(x1, y1), (x2, y2), ..., (xn, yn)].

  • fill_polygons(points, points_per_polygon=None):

    Fill multiple polygons at once.

  • stroke_polygons(points, points_per_polygon=None):

    Stroke multiple polygons at once. See Polygons / line-segments for details.

  • fill_styled_polygons(points, color, alpha, points_per_polygon=None):

    Fill multiple polygons at once where each polygon can have its own color. See Polygons / line-segments for details.

  • stroke_styled_polygons(points, color, alpha, points_per_polygon=None):

    Stroke multiple polygons at once where each polygon can have its own color. See Polygons / line-segments for details.

from ipycanvas import Canvas

canvas = Canvas(width=200, height=200)

canvas.fill_style = "#63934e"
canvas.stroke_style = "#4e6393"
canvas.line_width = 5
canvas.fill_polygon([(20, 20), (180, 20), (100, 150)])
canvas.stroke_polygon([(20, 20), (180, 20), (100, 150)])

canvas
_images/polygon.png 
from math import pi
import numpy as np
from ipycanvas import Canvas


def polygon(canvas, x, y, radius, n_points):
    angles = (2 * pi / n_points) * np.arange(n_points)

    v_x = x + np.cos(angles) * radius
    v_y = y + np.sin(angles) * radius

    points = np.stack((v_x, v_y), axis=1)

    canvas.fill_polygon(points)


background_color = "#89c64f"
polygon_color = "#c6574f"

canvas = Canvas(width=200, height=200)

canvas.fill_style = background_color
canvas.fill_rect(0, 0, canvas.width, canvas.height)

canvas.fill_style = polygon_color
polygon(canvas, 100, 100, 70, 6)

canvas
_images/polygon_numpy.png

Drawing arcs and circles#

There are methods that draw arcs/circles on the canvas:

  • fill_arc(x, y, radius, start_angle, end_angle, anticlockwise=False):

    Draw a filled arc centered at (x, y) with a radius of radius.

  • stroke_arc(x, y, radius, start_angle, end_angle, anticlockwise=False):

    Draw an arc outline centered at (x, y) with a radius of radius.

  • fill_arcs(x, y, radius, start_angle, end_angle, anticlockwise=False):

    Draw filled arcs centered at (x, y) with a radius of radius. Where x, y, radius and other arguments are NumPy arrays, lists or scalar values.

  • stroke_arcs(x, y, radius, start_angle, end_angle, anticlockwise=False):

    Draw an arc outlines centered at (x, y) with a radius of radius. Where x, y, radius and other arguments are NumPy arrays, lists or scalar values.

  • fill_styled_arcs( x, y, radius, start_angle, end_angle, color, alpha, anticlockwise=False):

    Same as fill_arcs but with additional (n x 3) color ndarray and (n) alpha ndarray.

  • stroke_styled_arcs( x, y, radius, start_angle, end_angle, color, alpha, anticlockwise=False):

    Same as stroke_arcs but with additional (n x 3) color ndarray and (n) alpha ndarray.

  • fill_circle(x, y, radius):

    Draw a filled circle centered at (x, y) with a radius of radius.

  • stroke_circle(x, y, radius):

    Draw an circle outline centered at (x, y) with a radius of radius.

  • fill_circles(x, y, radius):

    Draw filled circles centered at (x, y) with a radius of radius. Where x, y, radius are NumPy arrays, lists or scalar values.

  • stroke_circles(x, y, radius):

    Draw a circle outlines centered at (x, y) with a radius of radius. Where x, y, radius are NumPy arrays, lists or scalar values.

  • fill_styled_circles( x, y, radius color, alpha) :

    Same as fill_circles but with additional (n x 3) color ndarray and (n) alpha ndarray.

  • stroke_styled_circles( x, y, radius, color, alpha) :

    Same as stroke_circles but with additional (n x 3) color ndarray and (n) alpha ndarray.

from math import pi

from ipycanvas import Canvas

canvas = Canvas(width=200, height=200)

canvas.fill_style = "red"
canvas.stroke_style = "blue"

canvas.fill_arc(60, 60, 50, 0, pi)
canvas.stroke_circle(60, 60, 40)

canvas
_images/arc.png

Drawing lines#

There are two commands for drawing a straight line from one point to another:

  • stroke_line(x1, y1, x2, y2):

    Draw a line from (x1, y1) to (x2, y2).

  • stroke_lines(points):

    Draw a path of consecutive lines from a list of points [(x1, y1), (x2, y2), ..., (xn, yn)].

  • stroke_styled_line_segments(points, points_per_line_segment=None):

    Draw multiple disconnected line-segments at once. See Polygons / line-segments for details.

  • stroke_styled_line_segments(points, color, alpha, points_per_line_segment=None):

    Draw multiple disconnected line-segments at once. See Polygons / line-segments for details.

from ipycanvas import Canvas

canvas = Canvas(width=200, height=200)

canvas.stroke_style = "blue"
canvas.stroke_line(0, 0, 150, 150)

canvas.stroke_style = "red"
canvas.stroke_line(200, 0, 0, 200)

canvas.stroke_style = "green"
canvas.stroke_line(150, 150, 0, 200)

canvas
_images/lines.png 
import numpy as np

from ipycanvas import Canvas

canvas = Canvas(width=200, height=200)

n = 50
x = np.linspace(0, 200, n)
y = np.random.randint(200, size=n)

points = np.stack((x, y), axis=1)

canvas.stroke_lines(points)

canvas
_images/stroke_lines.png

Vectorized methods#

Most methods like fill_rect/stroke_rect and fill_circle/stroke_circle have vectorized counterparts: fill_rects/stroke_rects and fill_circles/stroke_circles. It is essential to use those methods when you want to draw the same shape multiple times with the same style.

For example, it is way faster to run:

from ipycanvas import Canvas

canvas = Canvas(width=300, height=300)

canvas.global_alpha = 0.01

size = [i for i in range(300)]
position = [300 - i for i in range(300)]

canvas.fill_rects(position, position, size)

canvas

instead of running:

from ipycanvas import Canvas

canvas = Canvas(width=300, height=300)

canvas.global_alpha = 0.01

for i in range(300):
    size = i
    position = 300 - i

    canvas.fill_rect(position, position, size)

canvas

Styled vectorized methods#

Ipycanvas provides methods to draw the same shape multiple times but with different colors:

  • fill_styled_rects / stroke_styled_rects

  • fill_styled_circles / stroke_styled_circles

  • fill_styled_arcs / stroke_styled_arcs

  • fill_styled_polygons / stroke_styled_polygons

  • fill_styled_line_segments / stroke_styled_line_segments

Rects#

import numpy as np
from ipycanvas import Canvas, hold_canvas

canvas = Canvas(width=400, height=300)
n_rects = 300
x = np.random.randint(0, canvas.width, size=(n_rects))
y = np.random.randint(0, canvas.width, size=(n_rects))
width = np.random.randint(10, 40, size=(n_rects))
height = np.random.randint(10, 40, size=(n_rects))
colors_fill = np.random.randint(0, 255, size=(n_rects, 3))
colors_outline = np.random.randint(0, 255, size=(n_rects, 3))
alphas = np.random.random(n_rects)
with hold_canvas():
    canvas.fill_styled_rects(x, y, width, height, color=colors_fill, alpha=alphas)
    canvas.line_width = 2
    canvas.stroke_styled_rects(x, y, width, height, color=colors_outline, alpha=alphas)
canvas
_images/draw_styled_rects.png

Circles#

import numpy as np
from ipycanvas import Canvas, hold_canvas

canvas = Canvas(width=300, height=300)
n_circles = 100
x = np.random.randint(0, canvas.width, size=(n_circles))
y = np.random.randint(0, canvas.width, size=(n_circles))
r = np.random.randint(10, 20, size=(n_circles))
colors_fill = np.random.randint(0, 255, size=(n_circles, 3))
colors_outline = np.random.randint(0, 255, size=(n_circles, 3))
alphas = np.random.random(n_circles)
with hold_canvas():
    canvas.fill_styled_circles(x, y, r, color=colors_fill, alpha=alphas)
    canvas.line_width = 2
    canvas.stroke_styled_circles(x, y, r, color=colors_outline)
canvas
_images/draw_styled_circles.png

Arcs#

import numpy as np
from ipycanvas import Canvas, hold_canvas
import math

canvas = Canvas(width=300, height=300)
n_circles = 100
x = np.random.randint(0, canvas.width, size=(n_circles))
y = np.random.randint(0, canvas.width, size=(n_circles))
r = np.random.randint(10, 20, size=(n_circles))
start_angle = np.random.randint(0, 360, size=(n_circles))
end_angle = np.random.randint(0, 360, size=(n_circles))
start_angle = 0
end_angle = math.pi
start_angle = np.random.random(n_circles) * math.pi
end_angle = np.random.random(n_circles) * math.pi
alphas = np.random.random(n_circles)
with hold_canvas():
    canvas.fill_style = "cyan"
    canvas.fill_arcs(x, y, r, start_angle, end_angle)
    canvas.line_width = 1
    canvas.stroke_style = "black"
    canvas.stroke_arcs(x, y, r, start_angle, end_angle)
canvas
_images/draw_styled_arcs.png

Polygons / line-segments#

Case 1: All polygons / line-segments have the same number of points#

import numpy as np
from ipycanvas import Canvas, hold_canvas

canvas = Canvas(width=300, height=300)
n_polygons = 50

# each polygon has 4 points
n_points_per_polygon = 4

polygons = np.zeros([n_polygons, n_points_per_polygon, 2])

polygons[:, 0, 0] = 0.0
polygons[:, 0, 1] = 0.0

polygons[:, 1, 0] = 1.0
polygons[:, 1, 1] = 0.0

polygons[:, 2, 0] = 1.0
polygons[:, 2, 1] = 1.0

polygons[:, 3, 0] = 0.0
polygons[:, 3, 1] = 1.0

colors_fill = np.random.randint(0, 255, size=(n_polygons, 3))
colors_outline = np.random.randint(0, 255, size=(n_polygons, 3))

# scale each polygon
polygons *= np.linspace(1.0, 200.0, num=n_polygons)[:, None, None]

# translate each polygon
polygons += np.linspace(1.0, 100.0, num=n_polygons)[:, None, None]

points_per_polygon = np.ones([n_polygons]) * n_points_per_polygon
with hold_canvas():
    canvas.stroke_styled_polygons(polygons, color=colors_fill)
canvas
_images/draw_styled_polygons.png 
import numpy as np
from ipycanvas import Canvas, hold_canvas

canvas = Canvas(width=300, height=300)

n_line_segments = 20

n_points_per_line_segment = 500

line_segments = np.zeros([n_line_segments, n_points_per_line_segment, 2])

x = np.linspace(0, canvas.width, num=n_points_per_line_segment)[None, :]
line_segments[:, :, 0] = np.linspace(0, canvas.width, num=n_points_per_line_segment)[
    None, :
]
line_segments[:, :, 1] = (30.0 * np.sin(x * 0.1))[None, :]

colors_outline = np.random.randint(0, 255, size=(n_polygons, 3))

# translate line segments in y direction
line_segments[:, :, 1] += np.linspace(1.0, canvas.height, num=n_line_segments)[:, None]

with hold_canvas():
    canvas.stroke_styled_line_segments(line_segments, color=colors_fill)
canvas
_images/draw_styled_line_segments.png

Case 2: Polygons / line-segments can have different number of Points.#

Polygons can be given as a list of ndarrays:

import numpy as np
from ipycanvas import Canvas, hold_canvas

canvas = Canvas(width=400, height=400)

triangle = [(0, 0), (0, 40), (30, 40)]  # triangle
rectangle = [(100, 100), (300, 100), (300, 200), (100, 200)]  # rectangle
irregular = np.random.randint(0, 400, size=(5, 2))  # irregular with 5 sides
polygons = [triangle, rectangle, irregular]
colors = [(255, 0, 0), (0, 255, 0), (0, 0, 255)]

with hold_canvas():
    canvas.fill_styled_polygons(polygons, color=colors)
canvas
_images/draw_styled_polygons_from_list.png

Polygons can be given as a flat ndarray:

import numpy as np
from ipycanvas import Canvas, hold_canvas

canvas = Canvas(width=400, height=400)
n_polygons = 20
points_per_polygon = np.random.randint(3, 6, size=n_polygons)
total_points = np.sum(points_per_polygon)
polygons = np.random.randint(0, 400, size=[total_points, 2])
alpha = np.random.random(n_polygons)
colors_fill = np.random.randint(0, 255, size=(n_polygons, 3))
colors_outline = np.random.randint(0, 255, size=(n_polygons, 3))

with hold_canvas():
    # the filling
    canvas.fill_styled_polygons(
        polygons, points_per_polygon=points_per_polygon, color=colors_fill, alpha=alpha
    )

    # draw outlines ontop where each line has the same style
    canvas.stroke_style = "black"
    canvas.line_width = 2
    canvas.stroke_polygons(polygons, points_per_polygon=points_per_polygon)
canvas
_images/draw_styled_polygons_flat_array.png
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