OpenCV (4.5.2., C++) Apply matrix (4*4) to image and write to file results in white file (or strange color result)

Question

I am getting an image from the mobile phone device camera and want to use OpenCV to apply color filters to the images. (Software/System is a Flutter App that uses Foreign Function Interface (FFI) to interact with the OpenCV Library in C++)

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I followed below question and tried to implement the solution, but so far I get only totally white images.

• StackOverflow: OpenCV help me with Sepia kernel

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I ended up with the following code trying to achieve it:

transform function (not working)

    void filter_1(char* inputImagePath, char* outputImagePath) {
        Mat input = imread(inputImagePath);
        Mat output = input.clone();

        Mat filter = (Mat_<float>(4,4) << 0.272, 0.534, 0.131, 0.0,
                                          0.349, 0.686, 0.168, 0.0,
                                          0.393, 0.769, 0.189, 0.0,
                                          0.0, 0.0, 0.0, 1.0);
        transform(output, input, filter);
        imwrite(outputImagePath, input);
    }

filter with 0~255 values instead of 0.0~1.0 (also not working)

       Mat filter = (Mat_<int>(4,4) <<69, 136, 33, 0,
                                      88, 174, 42, 0,
                                      99, 196, 48, 0,
                                      0, 0, 0, 255);

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So my question would be how to apply the filter and save the result successfully for further use? The sample filter here should result in a Sepia effect on the image.

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UPDATE (2021.07.21)

• Removed filter2d function and related text to avoid confusion.
• Removed link to filter2d related question in SO
  • (Just for notice: StackOverflow: OpenCV apply my own filter to a color image )
• Added sample for 0~255 values matrix

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