What is the meaning of following code line in opencv?

Question

What does this code line means and how can I convert this code into javacv?

gray = Scalar::all(255);

This is whole code which related to this

Answer 1

The highlighted code line sets gray to 255. It is one of the methods avaliable in OpenCV to set a matrix to a value.

Other ways to do it are:

gray.setTo(255); // prior to 2.3.1 it was a buggy on multichannel images
gray = 255; // prior to 2.3.1 it was a buggy on multichannel images

gray.setTo(Scalar::all(255)); // it works regardless the OpenCV version.

But I think the question is why this source line after findfContours...

According to the docs, findContours modifies the image it is working on (it extracts a contour, deteles it, then proceed to the next one, until there are no more contours). The result is a garbage image (probably black).

So, the set-to-255 line clears it up for some other use.

The Mat::setTo() method should also be available in JavaCV, so you should not have problems converting it to Java