How to convert users signature to base64 encoding in Dart

I'm using the syncfusion_flutter_signaturepad package to let the user input their signature. Now, what I'm trying to achieve is to convert that signature to base64 format for our API. Here's what I have.

GlobalKey<SfSignaturePadState> _signaturePadKey = GlobalKey();
.....

final signatureData = await _signaturePadKey.currentState!.toImage();
final image = await signatureData.toByteData(format: ui.ImageByteFormat.png);

if (image != null) {
  final imageBytes = image.buffer.asUint8List();
  final decodedImage = img.decodeImage(imageBytes);
  //String encodedSignature = base64.encode(decodedImage); //decodedImage error The argument type 'Image?' can't be assigned to the parameter type 'List<int>'.
  print('Base64 encoded string: $decodedImage');
}

Printed result is

Base64 encoded string: Instance of 'Image'

I also tried this way but it's still not working.

ui.Image signatureData =
    await _signaturePadKey.currentState!.toImage();
ByteData? byteData = await signatureData.toByteData(
    format: ui.ImageByteFormat.png);
String imageEncoded =
    base64.encode(byteData!.buffer.asUint8List());

print("Encoded: $imageEncoded");

Result is :

Encoded: iVBORw0KGgoAAAANSUhEUgAAAYkAAAKQCAYAAABq7IZzAAAAAXNSR0IArs4c6QAAAARzQklUCAgICHwIZIgAABUGSURBVHic7d3NdRPZvofhf99159KJQCICmwhsRndoEYHlCGxHYDkCRAQWEbh62D2hiAB1BFZHQBGB7oAlHZvuH9gg+YvnWYt1uuljqcRAL3vv2rt+Wy6XywKAf/rzfx76CgB4vEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEAoBIJACIRAKASCQAiEQCgEgkAIhEA

That when I try to test it in online converter, it's not working.

EDIT Added mimetype:

String imageEncoded = "data:image/png;base64," +
                  base64.encode(byteData!.buffer.asUint8List());

But it's only a blank page, the signature is not there at all.

RESULT:

Encoded: data:image/png;base64,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

Am I on the right track on trying to get the desired result? How can I attain my goal using the syncfusion_flutter_signaturepad flutter package?

Solution

Oh! Might be helpful to anyone, conversion is working fine, my default here is that I relied on the printdebug value. This one did not display all converted data so I have to add a temporary textfield to display generated value.

TextEditingController _textEditingController = TextEditingController();

//under save button
// ......
String imageEncoded = await "data:image/png;base64," +
                      base64.encode(byteData!.buffer.asUint8List());
_textEditingController.text = imageEncoded;

TextField(
  controller: _textEditingController,
  //this one holds all the necessary encryped data
),

Hope this one might help someone!