From 4733a19babec760ba237b8c277bb1de664a641c1 Mon Sep 17 00:00:00 2001
From: Alexander Alekhin <alexander.a.alekhin@gmail.com>
Date: Thu, 19 Dec 2019 13:20:42 +0300
Subject: [PATCH] Merge pull request #16194 from alalek:fix_16192

* imgproc(test): resize(LANCZOS4) reproducer 16192

* imgproc: fix resize LANCZOS4 coefficients generation
---
 modules/imgproc/src/resize.cpp        | 23 +++++++++++++----------
 modules/imgproc/test/test_imgwarp.cpp | 13 +++++++++++++
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/modules/imgproc/src/resize.cpp b/modules/imgproc/src/resize.cpp
index cc967cf..02f7881 100644
--- a/modules/imgproc/src/resize.cpp
+++ b/modules/imgproc/src/resize.cpp
@@ -920,20 +920,23 @@ static inline void interpolateLanczos4( float x, float* coeffs )
     static const double cs[][2]=
     {{1, 0}, {-s45, -s45}, {0, 1}, {s45, -s45}, {-1, 0}, {s45, s45}, {0, -1}, {-s45, s45}};
 
-    if( x < FLT_EPSILON )
-    {
-        for( int i = 0; i < 8; i++ )
-            coeffs[i] = 0;
-        coeffs[3] = 1;
-        return;
-    }
-
     float sum = 0;
     double y0=-(x+3)*CV_PI*0.25, s0 = std::sin(y0), c0= std::cos(y0);
     for(int i = 0; i < 8; i++ )
     {
-        double y = -(x+3-i)*CV_PI*0.25;
-        coeffs[i] = (float)((cs[i][0]*s0 + cs[i][1]*c0)/(y*y));
+        float y0_ = (x+3-i);
+        if (fabs(y0_) >= 1e-6f)
+        {
+            double y = -y0_*CV_PI*0.25;
+            coeffs[i] = (float)((cs[i][0]*s0 + cs[i][1]*c0)/(y*y));
+        }
+        else
+        {
+            // special handling for 'x' values:
+            // - ~0.0: 0 0 0 1 0 0 0 0
+            // - ~1.0: 0 0 0 0 1 0 0 0
+            coeffs[i] = 1e30f;
+        }
         sum += coeffs[i];
     }
 
diff --git a/modules/imgproc/test/test_imgwarp.cpp b/modules/imgproc/test/test_imgwarp.cpp
index 400426a..232f374 100644
--- a/modules/imgproc/test/test_imgwarp.cpp
+++ b/modules/imgproc/test/test_imgwarp.cpp
@@ -1708,6 +1708,19 @@ TEST(Resize, Area_half)
     }
 }
 
+TEST(Resize, lanczos4_regression_16192)
+{
+    Size src_size(11, 17);
+    Size dst_size(11, 153);
+    Mat src(src_size, CV_8UC3, Scalar::all(128));
+    Mat dst(dst_size, CV_8UC3, Scalar::all(255));
+
+    cv::resize(src, dst, dst_size, 0, 0, INTER_LANCZOS4);
+
+    Mat expected(dst_size, CV_8UC3, Scalar::all(128));
+    EXPECT_EQ(cvtest::norm(dst, expected, NORM_INF), 0) << dst(Rect(0,0,8,8));
+}
+
 TEST(Imgproc_Warp, multichannel)
 {
     static const int inter_types[] = {INTER_NEAREST, INTER_AREA, INTER_CUBIC,
-- 
2.7.4