imgproc: fix BGRA2BGRA conversion

modules/imgproc/src/color.cpp

@@ -688,7 +688,7 @@ template<typename _Tp> struct RGB2RGB
             for( int i = 0; i < n; i += 4 )
             {
                 _Tp t0 = src[i], t1 = src[i+1], t2 = src[i+2], t3 = src[i+3];
-                dst[i] = t2; dst[i+1] = t1; dst[i+2] = t0; dst[i+3] = t3;
+                dst[i+bidx] = t0; dst[i+1] = t1; dst[i+(bidx^2)] = t2; dst[i+3] = t3;
             }
         }
     }
@@ -802,25 +802,25 @@ template<> struct RGB2RGB<uchar>
             for ( ; i <= n - 64; i += 64 )
             {
                 uint8x16x4_t v_src = vld4q_u8(src + i), v_dst;
-                v_dst.val[0] = v_src.val[2];
+                v_dst.val[0] = v_src.val[bidx];
                 v_dst.val[1] = v_src.val[1];
-                v_dst.val[2] = v_src.val[0];
+                v_dst.val[2] = v_src.val[bidx^2];
                 v_dst.val[3] = v_src.val[3];
                 vst4q_u8(dst + i, v_dst);
             }
             for ( ; i <= n - 32; i += 32 )
             {
                 uint8x8x4_t v_src = vld4_u8(src + i), v_dst;
-                v_dst.val[0] = v_src.val[2];
+                v_dst.val[0] = v_src.val[bidx];
                 v_dst.val[1] = v_src.val[1];
-                v_dst.val[2] = v_src.val[0];
+                v_dst.val[2] = v_src.val[bidx^2];
                 v_dst.val[3] = v_src.val[3];
                 vst4_u8(dst + i, v_dst);
             }
             for ( ; i < n; i += 4)
             {
                 uchar t0 = src[i], t1 = src[i+1], t2 = src[i+2], t3 = src[i+3];
-                dst[i] = t2; dst[i+1] = t1; dst[i+2] = t0; dst[i+3] = t3;
+                dst[i+bidx] = t0; dst[i+1] = t1; dst[i+(bidx^2)] = t2; dst[i+3] = t3;
             }
         }
     }

modules/imgproc/test/test_color.cpp

@@ -2190,3 +2190,28 @@ TEST(ImgProc_Bayer2RGBA, accuracy)
         }
     }
 }
+
+TEST(ImgProc_BGR2RGBA, regression_8696)
+{
+    Mat src(Size(80, 10), CV_8UC4);
+    src.setTo(Scalar(255, 0, 200, 100));
+
+    Mat dst;
+    cvtColor(src, dst, COLOR_BGR2BGRA);
+
+    EXPECT_DOUBLE_EQ(norm(dst - src, NORM_INF), 0.);
+}
+
+TEST(ImgProc_BGR2RGBA, 3ch24ch)
+{
+    Mat src(Size(80, 10), CV_8UC3);
+    src.setTo(Scalar(200, 0, 200));
+
+    Mat dst;
+    cvtColor(src, dst, COLOR_BGR2BGRA);
+
+    Mat expected(Size(80, 10), CV_8UC4);
+    expected.setTo(Scalar(80, 0, 200, 255));
+
+    EXPECT_DOUBLE_EQ(norm(expected - dst, NORM_INF), 0.);
+}