Question

I am trying to process a bmp image obtained from Android’s camera intent through the following steps:

1. Obtain bmp image from intent
2. Convert bmp image to Mat object for OpenCV processing (problem starts here)
3. Do needed OpenCV processing (through sending Mat object, obj, as obj.getNativeObjAddr() to native processing, or perform it locally in java).
4. Convert Mat object back to bmp

The problem is indeed not novel. I have found countless similar questions online, none of which seem to resolve the situation however.

Results (problem)

The intention is to display the processed image (that undergoes the above 4 steps) in an ImageView object. After running, the ImageView however remains unchanged and logcat emits the following warning on reaching the line calling Utils.bitmapToMat()

 W/System.err(3872): java.lang.IllegalArgumentException: mat == null

Code

Below is the code’s outline that is used in the onActivityResult method (resultant_bmp is the acquired bmp from the camera intent. It displays successfully on its own).
filePath is the file path, name and extension included, of resultant_bmp.

The first 3 lines following the starting if condition are from here, where its use seems to work fine in the mentioned question.

Bitmap resultant_bmp /*image from camera*/,   
       bmp /*image after opencv processing*/;
Mat rgb_img, gray_img;      

if (OpenCVLoader.initDebug()) { 

     BitmapFactory.Options options = new BitmapFactory.Options();
     options.inPreferredConfig = Config.RGB_565;
     resultant_bmp= BitmapFactory.decodeFile(filePath, options);

     /****************** Problem starts HERE ******************
     Last point reachable before Logcat states:
       W/System.err(4460):java.lang.IllegalArgumentException: mat == null   

     After this point, the program doesn't crash, but expected results 
       in the imageView (last line of code) do not result.
     **********************************************************/  

     Utils.bitmapToMat(resultant_bmp, rgb_img);                                 
     Imgproc.cvtColor(rgb_img, gray_img, Imgproc.COLOR_RGBA2GRAY); 

     /*Do opencv processing (on gray_img) here*/

     Utils.matToBitmap(gray_img, bmp);

     imageView.setImageBitmap(bmp);

}

Previous attempts& research

• I found a similar problem here and tried, as recommended, to push the file libopencv_java.so to system/lib on the device, using adb in command line and received the error:

failed to copy 'libopencv_java.so' to 'system/lib/libopencv_java.so': Read-only file system

• This question considers the same problem domain in the current question; the code provided above follows a similar pattern as that suggested there, which still doesn’t work.

• Based on this answer (as well as this aforementioned one), I added the following lines to the code:

System.loadLibrary("opencv_java");
System.loadLibrary("libopencv_java");

• I have added the OpenCV library to the project properties; as well as the NDKROOT variable indicating where the NDK root path lies, as explained further here.

• I’ve tried the image processing required (in step 3 above) in another working sample of OpenCV’s, so the problem definitely lies within step 2 (above); conversion of a bmp to a Mat object.

There should be a simple solution around this, but I cannot seem to find it. Help would be much appreciated if possible.

Thank you for your time.

Answer 1

Have you initialized your mat variable ? I couldnt find it in provided code.

Add this before you try to convert to mat.

Mat rgb_img = new Mat();