如何将C ++运算符转换为Java

Question

I'm trying to use the OpenCV library ( Java version). I found some code written in C++ and I'm trying to rewrite it to Java . However, I can't understand one construction.

Here is the C++ code:

void scaleDownImage(cv::Mat &originalImg,cv::Mat &scaledDownImage )
{      
    for(int x=0;x<16;x++)
    {  
        for(int y=0;y<16 ;y++)
        {
            int yd =ceil((float)(y*originalImg.cols/16));
            int xd = ceil((float)(x*originalImg.rows/16));
            scaledDownImage.at<uchar>(x,y) = originalImg.at<uchar>(xd,yd);

        }
    }
}

I can't understand how to translate this line:

scaledDownImage.at<uchar>(x,y) = originalImg.at<uchar>(xd,yd);

Answer 1

have a look at the Mat accessor functions here: http://docs.opencv.org/java/org/opencv/core/Mat.html#get(int,%20int)

so, to translate your example:

scaledDownImage.at<uchar>(r,c) = originalImg.at<uchar>(rd,cd); // c++

would be :

byte [] pixel = new byte[1]; // byte[3] for rgb
originalImg.get( rd, cd, pixel );
scaledDownImage.put( r,c, pixel );

note, that it's (row,col), not (x,y) !

Answer 2

uchar = unsigned char.

This statement:

scaledDownImage.at<uchar>(x,y)

returns unsigned char(I guess, pointer) at positions(x,y).

So, in java it will be like:

unsigned char scaledDownImageChar = scaledDownImage.charAt(x, y);
scaledDownImageChar = originalImg.charAt(x, y);

This is not real code, just an example.

Answer 3

It is called template, search generics for java.

Basically, the "at" method´s code uses some type T
(don´t know if it is called T or something else)
which could be int, float, char, any class...
just something which is unspecified at the moment

And in this case, you´re calling the method with T being an uchar