Merge pull request #7122 from haikusw:UIImageAlphaToMatFix

modules/imgcodecs/src/ios_conversions.mm

@@ -106,6 +106,8 @@ void UIImageToMat(const UIImage* image,
         bitmapInfo = kCGImageAlphaNone;
         if (!alphaExist)
             bitmapInfo = kCGImageAlphaNone;
+        else
+            m = cv::Scalar(0);
         contextRef = CGBitmapContextCreate(m.data, m.cols, m.rows, 8,
                                            m.step[0], colorSpace,
                                            bitmapInfo);
@@ -116,6 +118,8 @@ void UIImageToMat(const UIImage* image,
         if (!alphaExist)
             bitmapInfo = kCGImageAlphaNoneSkipLast |
                                 kCGBitmapByteOrderDefault;
+        else
+            m = cv::Scalar(0);
         contextRef = CGBitmapContextCreate(m.data, m.cols, m.rows, 8,
                                            m.step[0], colorSpace,
                                            bitmapInfo);