Merge pull request #23754 from dkurt:remap_linear_transparent

Keep inliers for linear remap with BORDER_TRANSPARENT #23754

Address https://github.com/opencv/opencv/issues/23562

### Pull Request Readiness Checklist

resolves https://github.com/opencv/opencv/issues/23562

I do think that this is a bug because with `INTER_CUBIC + BORDER_TRANSPARENT` the last column and row are preserved. So same should be done for `INTER_LINEAR`

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

modules/imgproc/src/imgwarp.cpp

@@ -757,13 +757,6 @@ static void remapBilinear( const Mat& _src, Mat& _dst, const Mat& _xy,
             }
             else
             {
-                if( borderType == BORDER_TRANSPARENT && cn != 3 )
-                {
-                    D += (X1 - dx)*cn;
-                    dx = X1;
-                    continue;
-                }
-
                 if( cn == 1 )
                     for( ; dx < X1; dx++, D++ )
                     {
@@ -774,6 +767,12 @@ static void remapBilinear( const Mat& _src, Mat& _dst, const Mat& _xy,
                         {
                             D[0] = cval[0];
                         }
+                        else if (borderType == BORDER_TRANSPARENT)
+                        {
+                            if (sx < ssize.width && sx >= 0 &&
+                                sy < ssize.height && sy >= 0)
+                                D[0] = S0[sy*sstep + sx];
+                        }
                         else
                         {
                             int sx0, sx1, sy0, sy1;
@@ -815,6 +814,13 @@ static void remapBilinear( const Mat& _src, Mat& _dst, const Mat& _xy,
                             for(int k = 0; k < cn; k++ )
                                 D[k] = cval[k];
                         }
+                        else if (borderType == BORDER_TRANSPARENT)
+                        {
+                            if (sx < ssize.width && sx >= 0 &&
+                                sy < ssize.height && sy >= 0)
+                                for(int k = 0; k < cn; k++ )
+                                    D[k] = S0[sy*sstep + sx*cn + k];
+                        }
                         else
                         {
                             int sx0, sx1, sy0, sy1;

modules/imgproc/test/test_imgwarp.cpp

@@ -1658,5 +1658,21 @@ TEST(Imgproc_warpPolar, identity)
 #endif
 }
 
+TEST(Imgproc_Remap, issue_23562)
+{
+    cv::RNG rng(17);
+    Mat_<float> mapx({3, 3}, {0, 1, 2, 0, 1, 2, 0, 1, 2});
+    Mat_<float> mapy({3, 3}, {0, 0, 0, 1, 1, 1, 2, 2, 2});
+    for (int cn = 1; cn <= 4; ++cn) {
+        Mat src(3, 3, CV_32FC(cn));
+        rng.fill(src, cv::RNG::UNIFORM, -1, 1);
+        Mat dst = Mat::zeros(3, 3, CV_32FC(cn));
+        Mat ref = src.clone();
+
+        remap(src, dst, mapx, mapy, INTER_LINEAR, BORDER_TRANSPARENT);
+        ASSERT_EQ(0.0, cvtest::norm(ref, dst, NORM_INF)) << "channels=" << cn;
+    }
+}
+
 }} // namespace
 /* End of file. */