Develop Reference

Opencv Android : how to remove black background from thresholding output?

I am using OpenCV android library thresholding method for image segmentation, but the problem is that the output bitmap contains black background which I do not want please note that original image does not have any black background it is actually white. I am attaching the code for your reference, I am new to opencv and don't have much understanding about it also so kindly help me out.
private void Segmentation() {
Mat srcMat = new Mat();
gray = new Mat();
Utils.bitmapToMat(imageBmp, srcMat);
Imgproc.cvtColor(srcMat, gray, Imgproc.COLOR_RGBA2GRAY);
grayBmp = Bitmap.createBitmap(imageBmp.getWidth(), imageBmp.getHeight(), Bitmap.Config.RGB_565);
Utils.matToBitmap(gray, grayBmp);
grayscaleHistogram();
Mat threshold = new Mat();
Imgproc.threshold(gray, threshold, 0, 255, Imgproc.THRESH_BINARY_INV + Imgproc.THRESH_OTSU);
thresBmp = Bitmap.createBitmap(imageBmp.getWidth(), imageBmp.getHeight(), Bitmap.Config.RGB_565);
Utils.matToBitmap(threshold, thresBmp);
Mat closing = new Mat();
Mat kernel = Mat.ones(5, 5, CvType.CV_8U);
Imgproc.morphologyEx(threshold, closing, Imgproc.MORPH_CLOSE, kernel, new Point(-1, -1), 3);
closingBmp = Bitmap.createBitmap(imageBmp.getWidth(), imageBmp.getHeight(), Bitmap.Config.RGB_565);
Utils.matToBitmap(closing, closingBmp);
result = new Mat();
Core.subtract(closing, gray, result);
Core.subtract(closing, result, result);
resultBmp = Bitmap.createBitmap(imageBmp.getWidth(), imageBmp.getHeight(), Bitmap.Config.RGB_565);
Utils.matToBitmap(result, resultBmp);
Glide.with(ResultActivity.this).asBitmap().load(resultBmp).into(ivAfter);
}
enter image description here

What exactly do you want it to be then? Binary thresholding works like this:
if value < threshold:
value = 0
else:
value = 1
Of course you can convert it to a grayscale / RGB image and adjust the background to your liking. You can also invert your image (white background, black segmentation) by using the ~ operator.
segmented_image = ~ segmented_image
Edit: OpenCV has a dedicated flag to invert the results: CV_THRESH_BINARY_INV You are already using it, maybe try changing it to CV_THRESH_BINARY

Android OpenCV copy ROI to part of bigger image

I get a mat (320x480) from the camera of the phone. Then I need to process a part of that frame. I use ROI for that:
mat2 = new Mat(width, 175, CvType.CV_8SC3);
Rect roi = new Rect(75, 0, 175, 320);
mat2 = new Mat(mat1, roi);
Now I want to create a new mat with dimensions 320x480 with a black background. How do I do that?
Then I want to copy the processed ROI to the new mat at the same place it was on the first mat. How do I do that?
I am using OpenCV 3.4.6 and android studio.
Thank you in advance

You just need to submat() an ROI from the original Mat (your camera frame), and then process the submat as a normal Mat, but be careful to do not clone the sumbmat if you want the original get the effect.
Update according to the comment:
Mat originalMat = someMat;
Mat blackMat = Mat.zeros(size, CvType.CV_8UC1); // it is your black mat
// create a submat from the original mat, and clone it
Mat roiMat = originalMat.submat(rect).clone();
..... // do what you want with roiMat
// now copy the result to the original mat
Mat dst = originalMat.submat(rect); // do not clone this submat
roiMat.copyTo(dst);
// done!

Why is my copyTo function Not copying the mask to the correct Mat?

I started with the following image, named rgbaMat4Mask.bmp:
Then I converted it to HSV, and then did inRange() to find contours, and got the following Mat named maskedMat:
Then I went on to draw the first contour (the bigger one), on a newly created empty Mat named newMatWithMask, which has been given the same size as that of the first image I started with:
So far so good, but the problem starts now. I created a new Mat and gave it the same size as that of the first contour (the bigger one), and then set its background color to new Scalar(120, 255, 255). Then I copied the newMat4MaskFinished to it using copyTo function. But neither is the size of the resulting Mat same as that of the contour, nor is its background color set to new Scalar(120, 255, 255) which is blue.
It is rather an image with size same as that of the entire mask, and has a black background. why? What am I doing wrong?
public void doProcessing(View view) {
// READING THE RGBA MAT
Mat rgbaMat4Mask = Highgui.imread("/mnt/sdcard/DCIM/rgbaMat4Mask.bmp");
// CONVERTING TO HSV
Mat hsvMat4Mask = new Mat();
Imgproc.cvtColor(rgbaMat4Mask, hsvMat4Mask, Imgproc.COLOR_BGR2HSV);
Highgui.imwrite("/mnt/sdcard/DCIM/hsvMat4Mask.bmp", hsvMat4Mask);//check
// CREATING A FILTER/MASK FOR RED COLORED BLOB
Mat maskedMat = new Mat();
Core.inRange(hsvMat4Mask, new Scalar(0, 100, 100), new Scalar(10, 255, 255), maskedMat);
Highgui.imwrite("/mnt/sdcard/DCIM/maskedMat.bmp", maskedMat);// check
// COPYING THE MASK TO AN EMPTY MAT
// STEP 1:
List<MatOfPoint> contours = new ArrayList<MatOfPoint>();
Imgproc.findContours(maskedMat, contours, new Mat(), Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_NONE);
//STEP 2:
Mat newMat4Mask = new Mat(rgbaMat4Mask.rows(), rgbaMat4Mask.cols(), CvType.CV_8UC1);
newMat4Mask.setTo(new Scalar(0));
Imgproc.drawContours(newMat4Mask, contours, 0, new Scalar(255), -1);//TODO Using -1 instead of CV_FILLED.
Highgui.imwrite("/mnt/sdcard/DCIM/newMatWithMask.bmp", newMat4Mask);// check
//STEP 3
Log.i(TAG, "HAPPY rows:"+contours.get(0).rows()+" columns:"+contours.get(0).cols());
Mat newMatwithMaskFinished = new Mat(contours.get(0).rows(), contours.get(0).cols(), CvType.CV_8UC3);
newMatwithMaskFinished.setTo(new Scalar(120, 255, 255));
rgbaMat4Mask.copyTo(newMatwithMaskFinished, newMat4Mask);
Highgui.imwrite("/mnt/sdcard/DCIM/newMatwithMaskFinished.bmp", newMatwithMaskFinished);//check*/
}

Your newMatwithMaskFinished should have the same size as rgbaMat4Mask and newMat4Mask.
Mat newMatwithMaskFinished = new Mat(rgbaMat4Mask.rows(), rgbaMat4Mask.cols(), CvType.CV_8UC3);
If you want to have a Mat of the bigger circle only, with transparent background, then you need to:
1) create newMatwithMaskFinished with type CV_8UC4
Mat newMatwithMaskFinished = new Mat(rgbaMat4Mask.rows(), rgbaMat4Mask.cols(), CvType.CV_8UC4);
2) set a transparent background:
newMatwithMaskFinished.setTo(new Scalar(0, 0, 0, 0));
3) Compute the bounding box box of the contour you're interested in, with boundingRect.
4) Convert rgbaMat4Mask to 4 channels (unless it's already), with cvtColor(..., COLOR_BGR2BGRA), let's call this rgba
5) Copy rgba to newMatwithMaskFinished, with mask newMat4Mask.
6) Crop newMatwithMaskFinished on box, using submat method

Using OpenCV, how to find the values of HSV channels of the "masked region" in a masked image?

Using OpenCV4Android, how can I get the HSV channels of the first pixel of the masked region in a masked image (dilatedMat in the following snippet)? I know that we'd get the HSV channel values of first pixel by hsvMat.get(0,0) but I don't know how to apply this to the masked region only, rather than the entire Mat.
For example, following is a function to which a camera frame is passed as an argument, and I have generated a mask, but how should I proceed from there?
NOTE: Please keep in mind that the masked region is Not a rectangle, it has an irregular shape.
private void detectColoredBlob (Mat rgbaFrame) {
Mat hsvImage = new Mat();
Imgproc.cvtColor(rgbaFrame, hsvImage, Imgproc.COLOR_RGB2HSV_FULL);
Mat maskedImage = new Mat();
Scalar lowerThreshold = new Scalar(85, 50, 20);
Scalar upperThreshold = new Scalar(135, 255, 77);
Core.inRange(hsvImage, lowerThreshold, upperThreshold, maskedImage);
Mat dilatedMat= new Mat();
Imgproc.dilate(maskedImage, dilatedMat, new Mat() );
//****************WHAT NOW???**************
}

How to convert floating point image to 32bit single channel in OpenCv?

I'm doing a watershed segmentation and the marker image is derived from the source image put through a distance transform. The distance transform returns a floating point image (I have no idea about the bit-depth) and I have trouble putting it through the watershed method since it requires a 32 bit single channel image.
Can I use the mat's convertTo method to set the bit depth to 32?
I also have trouble trying to display the floating point image since the matToBitmap() method doesn't seem to accept them. (in Android)
Mat mImg = new Mat();
Mat mThresh = new Mat();
Mat mDist = new Mat();
ImageView imgView = (ImageView) findViewById(R.id.imageView);
Bitmap bmpIn = BitmapFactory.decodeResource(getResources(),
R.drawable.w1);
Utils.bitmapToMat(bmpIn, mImg);
Imgproc.cvtColor(mImg, mImg, Imgproc.COLOR_BGR2GRAY);
Imgproc.threshold(mImg, mThresh, 0, 255, Imgproc.THRESH_BINARY
| Imgproc.THRESH_OTSU);
//Marker image for watershed
Imgproc.distanceTransform(mThresh, mDist, Imgproc.CV_DIST_L2, Imgproc.CV_DIST_MASK_PRECISE);
//Conversions for watershed
Imgproc.cvtColor(mThresh, mThresh, Imgproc.COLOR_GRAY2BGR, 3);
//Floating-point image -> 32-bit single-channel
mDist.convertTo(...);
Imgproc.watershed(mThresh, mDist); //
Bitmap bmpOut = Bitmap.createBitmap(mThresh.cols(), mThresh.rows(),
Bitmap.Config.ARGB_8888);
Utils.matToBitmap(mThresh, bmpOut);
imgView.setImageBitmap(bmpOut);

Yes, you can use the convertTo function to convert any opencv matrix to another type. The type to convert to should be set in a destination matrix with the same size. convertTo has optional parameters scale and shift, so you can avoid clipping and quantization errors when converting to fixed point depths. So for your code:
Mat mDist32 = Mat(mDist.rows,mDist.cols,CV_32SC1); // 32 bit signed 1 channel, use CV_32UC1 for unsigned
mDist.convertTo(mDist32,CV_32SC1,1,0);
Imgproc.watershed(mThresh,mDist32);