initial commit

samples/cpp/tutorial_code/snippets/imgcodecs_imwrite.cpp

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+#include <opencv2/imgcodecs.hpp>
+
+using namespace cv;
+using namespace std;
+
+static void createAlphaMat(Mat &mat)
+{
+    CV_Assert(mat.channels() == 4);
+    for (int i = 0; i < mat.rows; ++i)
+    {
+        for (int j = 0; j < mat.cols; ++j)
+        {
+            Vec4b& bgra = mat.at<Vec4b>(i, j);
+            bgra[0] = UCHAR_MAX; // Blue
+            bgra[1] = saturate_cast<uchar>((float (mat.cols - j)) / ((float)mat.cols) * UCHAR_MAX); // Green
+            bgra[2] = saturate_cast<uchar>((float (mat.rows - i)) / ((float)mat.rows) * UCHAR_MAX); // Red
+            bgra[3] = saturate_cast<uchar>(0.5 * (bgra[1] + bgra[2])); // Alpha
+        }
+    }
+}
+
+int main()
+{
+    // Create mat with alpha channel
+    Mat mat(480, 640, CV_8UC4);
+    createAlphaMat(mat);
+    vector<int> compression_params;
+    compression_params.push_back(IMWRITE_PNG_COMPRESSION);
+    compression_params.push_back(9);
+
+    bool result = false;
+    try
+    {
+        result = imwrite("alpha.png", mat, compression_params);
+    }
+    catch (const cv::Exception& ex)
+    {
+        fprintf(stderr, "Exception converting image to PNG format: %s\n", ex.what());
+    }
+    if (result)
+        printf("Saved PNG file with alpha data.\n");
+    else
+        printf("ERROR: Can't save PNG file.\n");
+    return result ? 0 : 1;
+}