I basically just wanted to know when to leave my house to catch the subway every morning. Transit data feeds are pretty heavy to pull down and parse, but a full Raspberry Pi seemed like overkill. Instead, I wrote a small server that fetched what I needed and just sent PNG's to the ESP32.

How things get rendered

Nowadays, we actually send frames in a WebP container, so animation is fully supported. And we have a rendering library that lets you just write simple Python to render things, frame by frame. For example, here's a simple "clock" app:

def main(config):
    # figure out the current time
    tz = config.get("tz", DEFAULT_TIMEZONE)
    now = time.now().in_location(tz)

    # build two frames so we have a blinking separator
    # between the hour and the minute
    formats = [
        "3:04 PM",
        "3 04 PM",
    ]

    return [
        build_frame(now.format(f))
        for f in formats
    ]


def build_frame(text):
    # draw a simple frame with the given text
    return r.Frame(
        root = r.Box(
            width = r.width(),
            height = r.height(),
            child = r.Text(
                color = "#fff",
                font = r.fonts["6x13"],
                content = text,
                height = 10,
            ),
        ),
    ) 

Actually to be entirely accurate, this looks like Python but is actually Starlark, which is a subset of Python. It's very fast to interpret, so we are able to embed it in a Go server. That way, each app can be rendered pretty quickly. This is what the basic clock app above looks like when it's rendered:

Your browser needs to support WebP to be able to see the image above.

Each ESP32 pings our backend server a few times a minute to tell us that it's online and report what it's currently displaying. If there's any new frames for it to render, they get pushed over.