error occurred when binarization based ROI algorithm with OpenCV in JAVA

when implementing ROI algorithm to binarize images in JAVA by using OpenCV，exception threw. can anybody help me figure out bugs among my code snippets?

System.loadLibrary(org.opencv.core.Core.NATIVE_LIBRARY_NAME);
String filename = "path/to/image";
Mat image = Imgcodecs.imread(filename, Imgcodecs.IMREAD_COLOR);
int ch = 255, cw = 255; //---small block
int h = image.rows(), w = image.cols(); //---height vs width
Mat gray = new Mat();
Imgproc.cvtColor(image, gray, Imgproc.COLOR_BGR2GRAY); //--to gray

//-----handle every small block
for (int row = 0; row < h; row += ch) {
    for (int col = 0; col < w; col += cw) {
        Rect roi_rect = new Rect(col, row, cw, ch); //------small block:rectangle
        Mat roi = new Mat(gray, roi_rect);
        Imgproc.adaptiveThreshold(roi, roi, 255, Imgproc.ADAPTIVE_THRESH_GAUSSIAN_C, Imgproc.THRESH_BINARY, 127, 10); //----binarization
        roi.copyTo(gray.submat(roi_rect));//----replace original block
    }
}

HighGui.imshow("binary", gray);

exception messages as follows:

OpenCV(4.7.0-dev) Error: Assertion failed (0 <= _colRange.start && _colRange.start <= _colRange.end && _colRange.end <= m.cols) in cv::Mat::Mat, file C:\GHA-OCV-2\_work\ci-gha-workflow\ci-gha-workflow\opencv\modules\core\src\matrix.cpp, line 776

Exception in thread "main" CvException [org.opencv.core.CvException: cv::Exception: OpenCV(4.7.0-dev) C:\GHA-OCV-2\_work\ci-gha-workflow\ci-gha-workflow\opencv\modules\core\src\matrix.cpp:776: error: (-215:Assertion failed) 0 <= _colRange.start && _colRange.start <= _colRange.end && _colRange.end <= m.cols in function 'cv::Mat::Mat']

It threw exception at Mat roi = new Mat(gray, roi_rect). What's wrong with this?

Solution

The violated assertion is:

(
    0 <= _colRange.start &&
    _colRange.start <= _colRange.end &&
    _colRange.end <= m.cols
)

Possibilities:

Your ROI has negative size (start > end)
your ROI is out of bounds of the parent Mat
the parent Mat has zero size (and the ROI does not)

You fix that by figuring out which it is, and then figuring out why that is.