How to change the icon size of Google Maps marker in Flutter?

TL;DR: As long as are able to encode any image into raw bytes such as Uint8List, you should be fine using it as a marker.

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As of now, you can use Uint8List data to create your markers with Google Maps. That means that you can use raw data to paint whatever you want as a map marker, as long as you keep the right encode format (which in this particular scenario, is a png).

I will go through two examples where you can either:

1. Pick a local asset and dynamically change its size to whatever you want and render it on the map (a Flutter logo image);
2. Draw some stuff in canvas and render it as marker as well, but this can be any render widget.

Besides this, you can even transform a render widget in an static image and thus, use it as marker too.

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1. Using an asset

First, create a method that handles the asset path and receives a size (this can be either the width, height, or both, but using only one will preserve ratio).

import 'dart:ui' as ui;

Future<Uint8List> getBytesFromAsset(String path, int width) async {
  ByteData data = await rootBundle.load(path);
  ui.Codec codec = await ui.instantiateImageCodec(data.buffer.asUint8List(), targetWidth: width);
  ui.FrameInfo fi = await codec.getNextFrame();
  return (await fi.image.toByteData(format: ui.ImageByteFormat.png)).buffer.asUint8List();
}

Then, just add it to your map using the right descriptor:

final Uint8List markerIcon = await getBytesFromAsset('assets/images/flutter.png', 100);
final Marker marker = Marker(icon: BitmapDescriptor.fromBytes(markerIcon));

This will produce the following for 50, 100 and 200 width respectively.

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2. Using canvas

You can draw anything you want with canvas and then use it as a marker. The following will produce some simple rounded box with a Hello world! text in it.

So, first just draw some stuff using the canvas:

Future<Uint8List> getBytesFromCanvas(int width, int height) async {
  final ui.PictureRecorder pictureRecorder = ui.PictureRecorder();
  final Canvas canvas = Canvas(pictureRecorder);
  final Paint paint = Paint()..color = Colors.blue;
  final Radius radius = Radius.circular(20.0);
  canvas.drawRRect(
      RRect.fromRectAndCorners(
        Rect.fromLTWH(0.0, 0.0, width.toDouble(), height.toDouble()),
        topLeft: radius,
        topRight: radius,
        bottomLeft: radius,
        bottomRight: radius,
      ),
      paint);
  TextPainter painter = TextPainter(textDirection: TextDirection.ltr);
  painter.text = TextSpan(
    text: 'Hello world',
    style: TextStyle(fontSize: 25.0, color: Colors.white),
  );
  painter.layout();
  painter.paint(canvas, Offset((width * 0.5) - painter.width * 0.5, (height * 0.5) - painter.height * 0.5));
  final img = await pictureRecorder.endRecording().toImage(width, height);
  final data = await img.toByteData(format: ui.ImageByteFormat.png);
  return data.buffer.asUint8List();
}

and then use it the same way, but this time providing any data you want (eg. width and height) instead of the asset path.

final Uint8List markerIcon = await getBytesFromCanvas(200, 100);
final Marker marker = Marker(icon: BitmapDescriptor.fromBytes(markerIcon));

and here you have it.