Copy part of image to another image with EmguCV

问题

I want to copy a center part (Rectangle) of my image to a completely white Mat (to the same position).

Code:

        Mat src = Image.Mat;
        Mat dst = new Mat(src.Height, src.Width, DepthType.Cv8U, 3);
        dst.SetTo(new Bgr(255, 255, 255).MCvScalar);

        Rectangle roi = new Rectangle((int)(0.1 * src.Width), (int)(0.1 * src.Height), (int)(0.8 * src.Width), (int)(0.8 * src.Height));

        Mat srcROI = new Mat(src, roi);

        Mat dstROI = new Mat(dst, roi);

        srcROI.CopyTo(dstROI);

        //I have dstROI filled well. CopyTo method is doing well. 
        //However I have no changes in my dst file.

However I'm getting only white image as a result - dst. Nothing inside.

What i'm doing wrong?

using EmguCV 3.1

EDIT

I have a dstROI Mat filled well. But there is a problem how to apply changes to original dst Mat now.

Changing CopyTo like this:

  srcROI.CopyTo(dst);

causes that dst is filled now with my part of src image but not in the centre like i wanted

EDIT 2

 src.Depth = Cv8U

As you suggested I check a value of IsSubmatrix property.

   Console.WriteLine(dstROI.IsSubmatrix);         
   srcROI.CopyTo(dstROI);
   Console.WriteLine(dstROI.IsSubmatrix);

gives output:

   true
   false

What can be wrong then?

回答1:

According to the operator precedence rules of C# a type cast has higher priority than multiplication.

Hence (int)0.8 * src.Width is equivalent to 0 * src.Width, and the same applies to the other parameters of the roi rectangle. Therefore the line where you create the roi is basically

Rectangle roi = new Rectangle(0,0,0,0);

Copying a 0-size block does nothing, so you're left with the pristine white image you created earlier.

Solution

Parenthesize your expressions properly.

Rectangle roi = new Rectangle((int)(0.1 * src.Width)
    , (int)(0.1 * src.Height)
    , (int)(0.8 * src.Width)
    , (int)(0.8 * src.Height));

回答2:

Ancient question, I know, but it came up when I searched so an answer here might still be being hit in searches. I had a similar issue and it may be the same problem. If src and dst have different numbers of channels or different depths, then a new Mat is created instead. I see that they both have the same depth, but in my case, I had a single channel going into a 3 channel Mat. If your src is not a 3 channel Mat, then this may be the issue (it might be 1 (gray) or 4 channel (BGRA) for example).

来源：https://stackoverflow.com/questions/37335774/copy-part-of-image-to-another-image-with-emgucv

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OpenCV

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