Thursday, September 22, 2022

Returning an image from an Azure Function App in Python

I wrote a tiny icon-writer package this week that uses pillow to generate simple text-only icons, and used that to generate images to replace the fancy logo'd icons on my Mac OS X dock.

After generating the icons on my own machine, I wanted to make a website that others could use to generate them, so I started off by writing an Azure Function App.

I considered a few options for what the function could return:

  1. It could store the image in an Azure Storage container and return the URL to the created image.
  2. It could return a base-64 encoded string, and I could serve that in a webpage img tag using a data URI.
  3. It could return an actual binary image with the mime-type set appropriately, to be used directly as the src of the img.

I went for option three, since I liked the idea of being able to test the API in the browser and instantly see an image, plus I wanted to make sure it could be done.

Here's the relevant code in the function:

img = icon_writer.write_icon(text, size=size, bgcolor=bgcolor, fontcolor=fontcolor)
img_byte_arr = io.BytesIO(), format='PNG')
img_byte_arr = img_byte_arr.getvalue()
return func.HttpResponse(img_byte_arr, mimetype='image/png')

It creates a new pillow Image using my write_icon function and saves that into a BytesIO object as a PNG. It converts that object into a bytes array using BytesIO.getvalue(). Finally, it returns an HttpResponse with the bytes array as the body and a mime-type of 'image/png'.

See the full function code on Github.

Then I can call the function in a webpage just like this:

const image = document.createElement("img");
image.src = `${params}`;

That src attribute actually uses an API Management service URL, which protects the Funtions App endpoint with a CORS policy.

You can try the website here.

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