Merge pull request #16085 from alalek:imgproc_threshold_to_zero_ipp_bug

* imgproc(IPP): wrong result from threshold(THRESH_TOZERO)

* imgproc(IPP): disable IPP code to pass THRESH_TOZERO test

modules/imgproc/src/thresh.cpp

@@ -774,6 +774,7 @@ thresh_32f( const Mat& _src, Mat& _dst, float thresh, float maxval, int type )
             }
             setIppErrorStatus();
             break;
+#if 0  // details: https://github.com/opencv/opencv/pull/16085
         case THRESH_TOZERO:
             if (0 <= CV_INSTRUMENT_FUN_IPP(ippiThreshold_LTVal_32f_C1R, src, (int)src_step*sizeof(src[0]), dst, (int)dst_step*sizeof(dst[0]), sz, thresh + FLT_EPSILON, 0))
             {
@@ -782,6 +783,7 @@ thresh_32f( const Mat& _src, Mat& _dst, float thresh, float maxval, int type )
             }
             setIppErrorStatus();
             break;
+#endif
         case THRESH_TOZERO_INV:
             if (0 <= CV_INSTRUMENT_FUN_IPP(ippiThreshold_GTVal_32f_C1R, src, (int)src_step*sizeof(src[0]), dst, (int)dst_step*sizeof(dst[0]), sz, thresh, 0))
             {

modules/imgproc/test/test_thresh.cpp

@@ -434,4 +434,13 @@ BIGDATA_TEST(Imgproc_Threshold, huge)
     ASSERT_EQ((uint64)nz, n / 2);
 }
 
+TEST(Imgproc_Threshold, regression_THRESH_TOZERO_IPP_16085)
+{
+    Size sz(16, 16);
+    Mat input(sz, CV_32F, Scalar::all(2));
+    Mat result;
+    cv::threshold(input, result, 2.0, 0.0, THRESH_TOZERO);
+    EXPECT_EQ(0, cv::norm(result, NORM_INF));
+}
+
 }} // namespace