Question

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);

Answer 1

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);