OpenCV get pixels that are within a certain color range

Question

I want to get the pixels within a specific color range in android using OpenCV.

This is how i initialize the imageReader (i am using RGBA):

    imageReader = ImageReader.newInstance(screenWidth, screenHeight, PixelFormat.RGBA_8888, 2);

This is how i process an image from the imageReader:

    Image image = reader.acquireLatestImage();

    //Create a Mat using 4 channels (since RGBA uses 4 channels) and fill it with the image-data.
    Mat rgba = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8UC4); 
    ByteBuffer buffer = image.getPlanes()[0].getBuffer();
    byte[] bytes = new byte[buffer.remaining()];
    buffer.get(bytes);
    rgba.put(0, 0, bytes);

    //Range of colors to be detected:
    Scalar lower = new Scalar(10, 10, 100);
    Scalar upper = new Scalar(100, 100, 255);

    //Create a Mat using 3 channels (since HSV uses 3 channels)
    Mat hsv = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8UC3);

    //Convert from source RGBA to destination HSV, the 3 specifes the channels for the destination Mat.
    Imgproc.cvtColor(rgba, hsv, Imgproc.COLOR_RGB2HSV, 3);

    //Do the filtering
    Core.inRange(hsv, lower, upper, hsv);

    //Convert back to RGBA (now i use 4 channels since the destination is RGBA)
    Imgproc.cvtColor(hsv, rgba, Imgproc.COLOR_HSV2RGB, 4); 

    image.close();           

But at:

Imgproc.cvtColor(hsv, rgba, Imgproc.COLOR_HSV2RGB, 4); 

I get the error:

cv::Exception: OpenCV(4.1.2) ...

    > Invalid number of channels in input image:
    >     'VScn::contains(scn)'
    > where
    >     'scn' is 1

hsv is used as the input-argument at that line, but when converting I have always been explicit in how many channels im using, and for hsv I always use three.

Why am i getting this error?

Answer 1

After

//Do the filtering
Core.inRange(hsv, lower, upper, hsv);

the matrix hsv is of type CV_8UC1 , and represents the binary single channel mask created by the inRange function.

So:

//Do the filtering
Mat mask = new Mat(hsv.rows(), hsv.cols(), CvType.CV_8U, new Scalar(0));
Core.inRange(hsv, lower, upper, mask);

// Set to (0,0,0) all pixels that are 0 in the mask, i.e. not in range
Core.bitwise_not( mask, mask);
hsv.setTo(new Scalar(0,0,0), mask);

//Convert back to RGBA (now i use 4 channels since the destination is RGBA)
Imgproc.cvtColor(hsv, rgba, Imgproc.COLOR_HSV2RGB, 4);