This should be a chance to try out a drawing library and, importantly, figure out managing dependancies and such before Monday. I tried sudo apt install libcairo2-dev pkg-config but pip install pycairo still failed :/ Instead I did pip install cairocffi so I guess that's what I'm using. I added it to the requirements list in settings.ini.

Here's the first example I tried, which I export to core.py so that it can be used elsewhere. The main motivation here was to check that evreything works and to see how NBDev handles this sort of thing. You'll notice it hides the actual method by default in the html docs, but gives the docstring.

hw_cairo[source]

hw_cairo(text)

Write text as an angled line on a cairo ImageSurface and save to example.png, then display it.

hw_cairo('Hello World')

Inter-conversions

I know I will want to work with inputs (image files, numpy arrays, PIL) and convert my cairo surfaces to numpy arrays to mess with them and then convert back, etc. So let's mess with that a little bit.

surface_to_npy[source]

surface_to_npy(surface)

Cairo ImageSurface to numpy array for inline display

Creating a surface to mess with:

surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, 200, 100) # Creating a surface to play with
context = cairo.Context(surface)
context.set_source_rgb(0, 1, 0)  # green
context.move_to(50, 50)
context.show_text('Hello World')

Viewing with matplotlib

from matplotlib import pyplot as plt
plt.imshow(surface_to_npy(surface))

<matplotlib.image.AxesImage at 0x7f4ee7747810>

A nicer way to view the image without the matplotlib axes is to using PIL and IPython's display. We might as well make a surface_to_PIL function and a display_surface function since we will want to do this a lot.

surface_to_PIL[source]

surface_to_PIL(surface)

display_surface[source]

display_surface(surface)

display_surface(surface)

Animation

I like to watch things as they happen. ipywidgets should make for fun animations.

import ipywidgets as widgets

# Set up surface
surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, 200, 100) # Creating a surface to play with
context = cairo.Context(surface)

imw = widgets.Image(
    value=surface.write_to_png(),
    format='png',
    width=300,
    height=400,
)
display(imw)

import time
for i in range(10):
    context.set_source_rgb(0, 1, i/10)  # green
    context.move_to(50, 50+i*5)
    context.show_text(f'Hello {i}')
    imw.value = surface.write_to_png()
    time.sleep(0.1)

display_surface(surface) # Display the final surface so that it shows in the html docs

We first make out blank canvas and display that. Whenever we want to update the image, we simply modify the value of the widget. A neat trick here is that surface.write_to_png() returns an io.BytesIO thingee if we don't specify a target, which we can pass straight to the widget (before I was messing about trying to get this same thing to happen the long way around). This is pretty neat - we can set up a surface and then occasionally draw to it when we want to see updates. Let's demonstrate this with an example drawing a crazy amount of rectangles:

import random

# Set up surface
surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, 600, 400) # Creating a surface to play with
context = cairo.Context(surface)

imw = widgets.Image(
    value=surface.write_to_png(),
    format='png',
    width=600,
    height=400,
)
display(imw)

for i in range(100000):
    context.set_source_rgba(0.6, i/1e4, 1-i/1e5, 1-i/1e5)
    x, y = random.random()*600, random.random()*400
    context.rectangle(x, y, 2, 2)
    context.fill()
    if i%1e4 == 0: # Change this to alter the number of updates (this affects the speed of the loop)
        imw.value = surface.write_to_png()
        
display_surface(surface) # Show the final result for posterity (looses the alpha channel sadly...)

Conclusions

That's an hour of mostly logistics, but at the end of it we have easy ways to play with cairo in Jupyter, including converting surfaces to arrays, displaying them and, most satisfying, a good fast way to render animations for our viewing pleasure. Tomorrow I hope to start the actual fun stuff.