How to fix error with cv::boundingRect in OpenCV - c++

Question

I am using OpenCV version 4.0.0. I am trying to stitch some images together and trim the resulting image and while I am able to stitch the images, I am not able to trim the resulting image.
My program keeps aborting with the following error:
libc++abi.dylib: terminating with uncaught exception of type cv::Exception: OpenCV(4.0.0) /Users/RAR/opencv/modules/core/src/umatrix.cpp:545: error: (-215:Assertion failed) 0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= m.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= m.rows in function 'UMat'
Abort trap: 6
The error occurs at the line stitched = stitched(cv::boundingRect(c)); in the code below.
while (cv::countNonZero(sub) > 0) {
cv::erode(minRect, minRect, cv::Mat()); // Erode the minimum rectangular mask
cv::subtract(minRect, thresh, sub); // Subtract the thresholded image from the minmum rectangular mask (count if there are any non-zero pixels left)
std::vector<std::vector<cv::Point>> cnts4;
cv::findContours(minRect.clone(), cnts4, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_SIMPLE);
c = cnts4[0];
for (auto iter = cnts4.begin(); iter != cnts4.end(); ++iter) {
if (cv::contourArea(*iter) > cv::contourArea(c)) { // Finds the largest contour (the contour/outline of the stitched image)
c = *iter;
}
}
stitched = stitched(cv::boundingRect(c)); // Extract the bounding box and use the bounding box coordinates to extract the final stitched images
}
Why am I getting this error?

Answer 1

From OP's comments:
stitched: cols: 4295 rows: 2867 bounding rect[4274 x 2845 from (11, 12)]
stitched: cols: 4274 rows: 2845 bounding rect[4272 x 2843 from (12, 13)]
In the first case, the rectangle is trying to extract a size of (4274, 2845) from (11, 12) in the stitched image. This means that it is taking pixels from (11, 12) to (4285, 2857), which is within the bounds of the stitched image since the stitched image has a size of (4295, 2867). No problem.
In the second case, the rectangle is trying to extract a size of (4272, 2843) from (12, 13) in the stitched image. This means that it is taking pixels from (12, 13) to (4284, 2856), which is out of bounds of the stitched image since the stitched image has a size of (4274, 2845). Problem.
The sub-image you are trying to extract is much bigger than the bigger image.
(-215:Assertion failed) 0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= m.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= m.rows
The error message also indicates this. roi in the error message refers to the sub-image you are trying to extract using cv::boundingRect(c) and m is the stitched image. The coordinates of this rectangle are beyond the size of the stitched image.
You can test this by setting the coordinates of the rectangle manually.
You should not get an error with stitched(cv::Rect(11, 12, cv::Size(4274, 2845)
You will get the error with stitched(cv::Rect(12, 13, cv::Size(4272, 2843)

Answer 2

Last iteration is the problem, as it won't find any contours.
Maybe you can try something like so :
int nonZeroCount = 1;
while (nonZeroCount)
{
cv::erode(minRect, minRect, cv::Mat());
cv::subtract(minRect, thresh, sub);
nonZeroCount = cv::countNonZero(sub);
if (nonZeroCount)
{
std::vector< std::vector<cv::Point> > cnts4;
cv::findContours(minRect.clone(), cnts4, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_SIMPLE);
c = cnts4[0];
for (auto iter = cnts4.begin(); iter != cnts4.end(); ++iter)
{
if (cv::contourArea(*iter) > cv::contourArea(c))
{
c = *iter;
}
}
stitched = stitched(cv::boundingRect(c));
}
}