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imgproc: fix matchTemplate TM_SQDIFF precision loss for CV_8U images

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When computing TM_SQDIFF for 8-bit images, crossCorr stores intermediate
results in CV_32F which loses precision for large patch sums (e.g. 25x25
patch of value 255 gives sum=40603125, exceeding float32's ~7 significant
digits). This causes the final SQDIFF result to have incorrect non-zero
values even when image and template are identical.

Fix: use a CV_64F intermediate buffer for crossCorr and common_matchTemplate
when depth==CV_8U and method is TM_SQDIFF or TM_SQDIFF_NORMED, then convert
back to CV_32F for the final output.

Fixes #21786
This commit is contained in:
Rishiii57
2026-05-22 02:39:41 +05:30
parent 723670c33d
commit 12eaf9bd4c
2 changed files with 36 additions and 5 deletions
Download Patch File Download Diff File Expand all files Collapse all files
modules/imgproc/src/templmatch.cpp
+21 -5
View File
@@ -975,13 +975,14 @@ static void common_matchTemplate( Mat& img, Mat& templ, Mat& result, int method,
for( i = 0; i < result.rows; i++ )
{
float* rrow = result.ptr<float>(i);
float* rrow = result.depth() == CV_32F ? result.ptr<float>(i) : nullptr;
double* drow = result.depth() == CV_64F ? result.ptr<double>(i) : nullptr;
int idx = i * sumstep;
int idx2 = i * sqstep;
for( j = 0; j < result.cols; j++, idx += cn, idx2 += cn )
{
double num = rrow[j], t;
double num = rrow ? (double)rrow[j] : drow[j], t;
double wndMean2 = 0, wndSum2 = 0;
if( numType == 1 )
@@ -1027,7 +1028,8 @@ static void common_matchTemplate( Mat& img, Mat& templ, Mat& result, int method,
num = method != cv::TM_SQDIFF_NORMED ? 0 : 1;
}
rrow[j] = (float)num;
if (rrow) rrow[j] = (float)num;
else drow[j] = num;
}
}
}
@@ -1120,6 +1122,11 @@ static bool ipp_matchTemplate( Mat& img, Mat& templ, Mat& result, int method)
if(templ.size().area()*4 > img.size().area())
return false;
// CV_8U SQDIFF/SQDIFF_NORMED suffer from float32 catastrophic cancellation
// in IPP's internal accumulators; fall through to the double-precision path instead.
if(img.depth() == CV_8U && (method == cv::TM_SQDIFF || method == cv::TM_SQDIFF_NORMED))
return false;
if(method == cv::TM_SQDIFF)
{
if(ipp_sqrDistance(img, templ, result))
@@ -1192,9 +1199,18 @@ void cv::matchTemplate( InputArray _img, InputArray _templ, OutputArray _result,
CV_IPP_RUN_FAST(ipp_matchTemplate(img, templ, result, method))
crossCorr( img, templ, result, Point(0,0), 0, 0);
bool use64f = (depth == CV_8U) && (method == cv::TM_SQDIFF || method == cv::TM_SQDIFF_NORMED);
Mat result64f;
Mat& workResult = use64f ? result64f : result;
if (use64f)
result64f.create(corrSize, CV_64F);
common_matchTemplate(img, templ, result, method, cn);
crossCorr( img, templ, workResult, Point(0,0), 0, 0);
common_matchTemplate(img, templ, workResult, method, cn);
if (use64f)
result64f.convertTo(result, CV_32F);
}
CV_IMPL void
modules/imgproc/test/test_templmatch.cpp
+15
View File
@@ -342,4 +342,19 @@ INSTANTIATE_TEST_CASE_P(/**/,
testing::Values(TM_SQDIFF, TM_SQDIFF_NORMED, TM_CCORR, TM_CCORR_NORMED, TM_CCOEFF, TM_CCOEFF_NORMED)));
TEST(Imgproc_MatchTemplate, bug_21786)
{
// CV_8U identical image/template with large patch sums triggers float32
// catastrophic cancellation in TM_SQDIFF. Result must be exactly zero.
Mat img(100, 100, CV_8U, Scalar(255));
Mat templ(25, 25, CV_8U, Scalar(255));
Mat result;
matchTemplate(img, templ, result, TM_SQDIFF);
EXPECT_NEAR(0.0, cv::norm(result, cv::NORM_INF), 1e-6);
matchTemplate(img, templ, result, TM_SQDIFF_NORMED);
EXPECT_NEAR(0.0, cv::norm(result, cv::NORM_INF), 1e-6);
}
}} // namespace