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Merge pull request #30690 from vrabaud:imgwarp

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imgproc: handle empty input image in findContours and findTRUContours - #30690

Since cv::copyMakeBorder now asserts that the source matrix is non-empty, calling cv::findContours or cv::findTRUContours on an empty image threw an exception instead of returning an empty contour list.

Guard against empty input images by clearing the output contours (and hierarchy) and returning early, matching cv::findContoursLinkRuns.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake
This commit is contained in:
Vincent Rabaud
2026-11-05 12:15:49 +01:00
committed by GitHub
parent 4f4d6125f7
commit 3b42da9cda
3 changed files with 38 additions and 1 deletions
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5
modules/imgproc/src/contours_new.cpp
View File

@@ -700,7 +700,10 @@ void cv::findContours(InputArray _image,
// preprocess
Mat image;
copyMakeBorder(_image, image, 1, 1, 1, 1, BORDER_CONSTANT | BORDER_ISOLATED, Scalar(0));
if (_image.empty())
image = Mat::zeros(2, 2, _image.type());
else
copyMakeBorder(_image, image, 1, 1, 1, 1, BORDER_CONSTANT | BORDER_ISOLATED, Scalar(0));
if (image.type() != CV_32SC1)
threshold(image, image, 0, 1, THRESH_BINARY);

5
modules/imgproc/src/contours_truco.cpp
View File

@@ -648,6 +648,11 @@ void findTRUContours(InputArray _src, OutputArrayOfArrays _contours, int minSize
CV_INSTRUMENT_REGION();
Mat src = _src.getMat();
CV_Assert(src.type() == CV_8UC1);
if (src.empty())
{
_contours.clear();
return;
}
// Buffer handling
cv::Mat padded;

29
modules/imgproc/test/test_contours.cpp
View File

@@ -125,6 +125,35 @@ TEST(Imgproc_FindContours, border)
ASSERT_EQ(0, cvtest::norm(img, img_draw_contours, NORM_INF));
}
TEST(Imgproc_FindContours, empty_input)
{
Mat empty;
std::vector<std::vector<Point> > contours(1, std::vector<Point>(1, Point(1, 2)));
std::vector<Vec4i> hierarchy(1, Vec4i(0, 0, 0, 0));
// General path (with hierarchy)
findContours(empty, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE);
EXPECT_TRUE(contours.empty());
EXPECT_TRUE(hierarchy.empty());
// Fast path (RETR_LIST without hierarchy -> findTRUContours)
contours.assign(1, std::vector<Point>(1, Point(1, 2)));
findContours(empty, contours, RETR_LIST, CHAIN_APPROX_SIMPLE, Point(3, 4));
EXPECT_TRUE(contours.empty());
// RETR_FLOODFILL with empty CV_32SC1
Mat empty32s(0, 0, CV_32SC1);
contours.assign(1, std::vector<Point>(1, Point(1, 2)));
hierarchy.assign(1, Vec4i(0, 0, 0, 0));
findContours(empty32s, contours, hierarchy, RETR_FLOODFILL, CHAIN_APPROX_SIMPLE);
EXPECT_TRUE(contours.empty());
EXPECT_TRUE(hierarchy.empty());
// Empty input still goes through type/mode validation
EXPECT_THROW(findContours(Mat(0, 0, CV_32FC1), contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE), cv::Exception);
EXPECT_THROW(findContours(empty, contours, hierarchy, RETR_FLOODFILL, CHAIN_APPROX_SIMPLE), cv::Exception);
}
TEST(Imgproc_FindContours, regression_4363_shared_nbd)
{
// Create specific test image