Android: OpenCV: imwrite always returns false and fails to write

Question

This code used to work but on Android 4.2 and OpenCV 2.4.4 it fails, but I don't know why. Can anyone shed any light on it for me?

Thanks for any help.

Baz

public void SaveImage (Mat mat) {
  Mat mIntermediateMat = new Mat();

  Imgproc.cvtColor(mRgba, mIntermediateMat, Imgproc.COLOR_RGBA2BGR, 3);

  File path = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES);
  String filename = "barry.png";
  File file = new File(path, filename);

  Boolean bool = null;
  filename = file.toString();
  bool = Highgui.imwrite(filename, mIntermediateMat);

  if (bool == true)
    Log.d(TAG, "SUCCESS writing image to external storage");
  else
    Log.d(TAG, "Fail writing image to external storage");
  }

bool comes back false every time. The file path/name is as it should be ("storage/emulated/0/Pictures/barry.png") for User #0 and the directory Pictures is there.

What can make imwrite return false?

I could convert the mat to a bmp and save it myself (which I do elsewhere in a non-OpenCV app) but since Highgui.imwrite is there, and this code used to work when I was writing to 2.4.0 and testing on Android 3.x, I'd like to use it if I can.

Many thanks

Baz

Answer 1

I can't believe it. [facepalm].

Add this to your manifest file Barry:

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

Would somebody come and slap me please? I managed to remove the write permission from the manifest so really there shouldn't be any surprise that it wouldn't write...

Doh.

Barry

Answer 2

There are actually two possible problems:

1. Android permissions. To fix this problem add the permission to the manifest

   "android.permission.WRITE_EXTERNAL_STORAGE"

2. You are trying to save file to SD card and your device is connected to PC in "Disk drive" mode. It means that you share the access rights with PC. Connect your device in mode "Charge only". To check the connection mode use

   Environment.getExternalStorageState(), it will return shared for "Disk mode" and mounted for "Charge only".

   I modified your code and now it looks like

   public void SaveImage (Mat mat) {
      Mat mIntermediateMat = new Mat();
      Imgproc.cvtColor(mRgba, mIntermediateMat, Imgproc.COLOR_RGBA2BGR, 3);
   
      File path = new File(Environment.getExternalStorageDirectory() + "/Images/");
      path.mkdirs();
      File file = new File(path, "image.png");
   
      filename = file.toString();
      Boolean bool = Highgui.imwrite(filename, mIntermediateMat);
   
      if (bool)
       Log.i(TAG, "SUCCESS writing image to external storage");
      else
       Log.i(TAG, "Fail writing image to external storage");
   }