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<

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

From Android 6+

Beginning in Android 6.0 (API level 23), users grant permissions to apps while the app is running, not when they install the app...

And you need to add something like:

  if (ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) 
  {
        ActivityCompat.requestPermissions(
                this,
                new String[] { Manifest.permission.WRITE_EXTERNAL_STORAGE },
                PERMISSION_REQUEST_WRITE_EXTERNAL_STORAGE);
  }

https://developer.android.com/training/permissions/requesting.html

Answer 3

Take care with versions +4.4 (KitKat) because android.permission.WRITE_EXTERNAL_STORAGE doesn't allow to write on the external SD card:

The WRITE_EXTERNAL_STORAGE permission must only grant write access to the primary external storage on a device. Apps must not be allowed to write to secondary external storage devices, except in their package-specific directories as allowed by synthesized permissions. Restricting writes in this way ensures the system can clean up files when applications are uninstalled.

Take a look to: Writing to SD Card always failing

Answer 4

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");
   }