Flutter: How would one save a Canvas/CustomPainter to an image file?

You can capture the output of a CustomPainter with PictureRecorder. Pass your PictureRecorder instance to the constructor for your Canvas. The Picture returned by PictureRecorder.endRecording can then be converted to an Image with Picture.toImage. Finally, extract the image bytes using Image.toByteData.

Here’s an example: https://github.com/rxlabz/flutter_canvas_to_image