I have an image and I want to perform OpenCV boxFilter on a ROI of this image.
image = cv::imread(argv[1], CV_LOAD_IMAGE_COLOR);
cv::Rect roi( 10, 10, 64, 64);
cv::Mat output;
cv::boxFilter(image(roi),output,-1,cv::Size(scale_size,scale_size));
I want to know what happen if the kernel is out of the ROI? Use the pixels out of the ROI but still within the image to do the filtering or just use the value specified by the BorderType?
Actually I want to use the former to do the filtering.
5
I think the accepted answer is wrong.
In the past I tested it with Sobel filter and I’m pretty sure I also found the original documentation or a semi-official answer from opencv that filters in subimages use the real border pixels from the original image.
EDIT: Found it (not a bug but a feature):
https://github.com/opencv/opencv/issues/5263
I now tested this for box-filter with this code:
int main()
{
try
{
cv::Mat image = cv::imread("Lenna.png");
cv::Rect roi(10, 10, 64, 64);
// box filter on roi which is a subimage of the full image
cv::Mat output_roi;
cv::boxFilter(image(roi), output_roi, -1, cv::Size(5, 5));
// box filter on a deep copy of the roi/subimage region
cv::Mat copy = image(roi).clone();
cv::Mat output_copy;
cv::boxFilter(copy, output_copy, -1, cv::Size(5, 5));
// highlight the differences between both results
cv::Mat diff;
cv::absdiff(output_roi, output_copy, diff);
cv::cvtColor(diff, diff, cv::COLOR_BGR2GRAY);
std::cout << "non-zero diff between clone and roi: " << cv::countNonZero(diff) << std::endl;
// now compute on full image and extract roi result:
cv::Mat output_full;
cv::boxFilter(image, output_full, -1, cv::Size(5, 5));
cv::Mat diff_full;
cv::absdiff(output_roi, output_full(roi), diff_full);
cv::cvtColor(diff_full, diff_full, cv::COLOR_BGR2GRAY);
std::cout << "non-zero diff between full(roi) and roi: " << cv::countNonZero(diff_full) << std::endl;
cv::imshow("diff>0", diff > 0);
cv::imshow("diff_full>0", diff_full > 0);
cv::waitKey(0);
}
catch(cv::Exception &e)
{
std::cout << e.what() << std::endl;
}
return 0;
}
Result is a diff between the clone and the roi but no diff between the the roi and the fullimage-result (in the roi region).
non-zero diff between clone and roi: 297
non-zero diff between full(roi) and roi: 0
here are the result images:
diff>0
diff_full>0
result images (hard to see the difference 😉 ):
roi:
copy:
full:
3
… the answer is actually incorrect (will be deleted when it is un-accepted) …
8