Basic usage

Create Canvas

You can provide the width and height of the Canvas in pixels in the constructor.

from ipycanvas import Canvas

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

You can also create a multi-layer canvas. This is useful when you have a background that does not need to update much while other objects moves a lot on the screen.

from ipycanvas import MultiCanvas

# Create a multi-layer canvas with 4 layers
multi_canvas = MultiCanvas(4, width=200, height=200)
multi_canvas[0] #  Access first layer (background)
multi_canvas[3] #  Access last layer
multi_canvas

Note

Because the Canvas is an interactive widget (see https://ipywidgets.readthedocs.io/en/stable/) you can:

  • display it multiple times in the Notebook

  • observe some of its attributes and call functions when they change

  • link some of its attributes to other widget attributes

Resize Canvas

The Canvas and MultiCanvas have two sizes: the size of the color buffer in pixels, and the actual size displayed on the screen.

Color buffer size

The color buffer size can dynamically be updated through the width and height properties (value in pixels), note that this will clear the canvas.

canvas.width = 300
canvas.height = 600

Screen size

The size on the screen can be updated through the layout property, which comes from ipywidgets (see https://ipywidgets.readthedocs.io/en/latest/examples/Widget%20Styling.html#The-layout-attribute). The layout property is an object which contains CSS properties for the canvas.

The default value for the width and height of the layout is “auto”, this means the canvas will take the same screen size as the actual color buffer size: a Canvas of size 800x600 will take 800x600 pixels on the screen.

canvas.layout.width = 'auto'
canvas.layout.height = 'auto'

In order to get a “responsive” Canvas which takes as much space as available while still respecting the aspect ratio, you will need to set the width property to 100%, the height will automatically get computed:

canvas.layout.width = '100%'
canvas.layout.height = 'auto'

One can also set the screen size value in pixels:

canvas.layout.width = '200px'
canvas.layout.height = '500px'

Clear Canvas

The Canvas and MultiCanvas classes have a clear method which allows to clear the entire canvas.

from ipycanvas import Canvas

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

# Perform some drawings...

canvas.clear()

Optimizing drawings

By default, the Python Canvas object sends all the drawings commands like fill_rect and arc one by one through the widgets communication layer. This communication is limited to 1000 commands/s and it can be extremely slow to send commands one after the other. You can increase this limit via internal Jupyter parameters, however this is not recommended as it can lead to instability. Instead we provide a hold_canvas context manager which allows you to hold all the commands and send them in a single batch at the end. For optimal performance you should try to use hold_canvas as much as possible.

from ipycanvas import Canvas, hold_canvas

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

with hold_canvas(canvas):
    # Perform drawings...