how to modify values of Mat according to some condition in opencv?

Question

In Matlab a(a>50)=0 can replace all elements of a that are greater than 50 to 0. I want to do same thing with Mat in OpenCV. How to do it?

Pretypify

a>50 gives you a matrix with 0 elements everywhere where a <= 50 and 255 everywhere where your condition holds. So (0 * (a>50)).copyTo(a, a>50) would do this for example. Or just use a.setTo(cv::Scalar(0), a>50) – Headmaster

Answer 1

What you want is to truncate the image with cv::threshold.

The following should do what you require:

cv::threshold(dst, dst, 50, 0, CV_THRESH_TOZERO_INV);

this is the function definition

double threshold(InputArray src, OutputArray dst, double thresh, double maxval, int type)

http://docs.opencv.org/modules/imgproc/doc/miscellaneous_transformations.html?highlight=threshold#threshold

Civil

I think the OP is actually looking for THRESH_TOZERO_INV, which sets values greater than the threshold to 0. THRESH_TRUNC sets values greater than the threshold to the threshold. – Quite

Sorry, OP is "Original Post" or "Original Poster"... either the question or the person who asked it. – Quite

definition of threshold says not only values greater than threshold will be changed to maxvalue but it will set all values less than threshold to 0. but I want them values less than threshold as it is. – Pretypify

Answer 2

Naah. to do that, just one line:

cv::Mat img = imread('your image path');
img.setTo(0,img>50);

as simple as that.

October

Exactly what I needed!! – Machmeter

I generally use ther thresholding method... that's also just one statement but this is wayy more flexible and I love it – Haws

Answer 3

Sometimes threshold doesn't work because you can have a diferent kind of Mat. If your Mat type supports double, threshold will crash (at least in my android studio).

You can find compare here: http://docs.opencv.org/2.4/modules/core/doc/operations_on_arrays.html

So I use the function compare:

Mat mask = new Mat(yourMat.rows(), yourMat.cols(), CvType.CV_64FC1);
Mat ones = org.opencv.core.Mat.ones(yourMat.rows(), yourMat.cols(), CvType.CV_64FC1);
Scalar alpha = new Scalar(50, CvType.CV_64FC1);
Core.multiply(ones, alpha, ones);

Core.compare(yourMat, zeros, mask, Core.CMP_LT);

Here I am creating a matrix with only 50 in all points. After that i am comparing it with yourMat, using CMP_LT (Less than). So all pixels less than 50 will turn in 255 in your mask and 0 if bigger. It is a mask. So you can just:

yourMat.copyTo(yourMat, mask);

Now all the pixels bigger than 50 will be zero and all the other will have their own values.

Santiago