added test to save image with webp with alpha channel

modules/highgui/src/grfmt_webp.cpp

@@ -143,7 +143,15 @@ bool WebPDecoder::readData(Mat &img)
         uchar* out_data = img.data;
         unsigned int out_data_size = m_width * m_height * 3 * sizeof(uchar);
 
-        uchar *res_ptr = WebPDecodeBGRInto(data.data, data.total(), out_data, out_data_size, m_width * 3);
+        uchar *res_ptr = 0;
+        if (channels == 3)
+        {
+            res_ptr = WebPDecodeBGRInto(data.data, data.total(), out_data, out_data_size, img.step);
+        }
+        else if (channels == 4)
+        {
+            res_ptr = WebPDecodeBGRAInto(data.data, data.total(), out_data, out_data_size, img.step);
+        }
 
         if(res_ptr == out_data)
         {

modules/highgui/test/test_grfmt.cpp

@@ -391,7 +391,7 @@ TEST(Highgui_WebP, encode_decode_lossy_webp)
     cv::Mat img = cv::imread(input);
     ASSERT_FALSE(img.empty());
 
-    for(int q = 100; q>=0; q-=10)
+    for(int q = 100; q>=0; q-=20)
     {
         std::vector<int> params;
         params.push_back(IMWRITE_WEBP_QUALITY);
@@ -405,4 +405,25 @@ TEST(Highgui_WebP, encode_decode_lossy_webp)
     }
 }
 
+TEST(Highgui_WebP, encode_decode_with_alpha_webp)
+{
+    cvtest::TS& ts = *cvtest::TS::ptr();
+    std::string input = std::string(ts.get_data_path()) + "/../cv/shared/lena.png";
+    cv::Mat img = cv::imread(input);
+    ASSERT_FALSE(img.empty());
+
+    std::vector<cv::Mat> imgs;
+    cv::split(img, imgs);
+    imgs.push_back(cv::Mat(imgs[0]));
+    imgs[imgs.size() - 1] = cv::Scalar::all(128);
+    cv::merge(imgs, img);
+
+    string output = cv::tempfile(".webp");
+
+    EXPECT_NO_THROW(cv::imwrite(output, img));
+    cv::Mat img_webp = cv::imread(output);
+    remove(output.c_str());
+    EXPECT_FALSE(img_webp.empty());
+}
+
 #endif