Issues while reading and displaying PNG with transparency layer opencv - c++

Question

I am doing a simple reading and displaying PNG image. I am reading a png image with background as transparent. I am converting the image in greyscale and then displaying it. But the converted image is looking something like this:
Original Image:
Greyscale image:
Here is the code. What am I doing wrong?:
Mat image = imread("3X3_a11.png",IMREAD_GRAYSCALE);
Mat output(image.size(),image.type());
// connectedComponents(image, output);
imshow("Output", image);
waitKey(0);
destroyAllWindows();

Answer 1

Your RGBA image is malformed, or at least is very wierd. On loading I see the warning:
libpng warning: iCCP: known incorrect sRGB profile
You can however get the binary version need for cv::connectedComponents with simple processing like:
#include <opencv2\opencv.hpp>
int main()
{
cv::Mat4b img4b = cv::imread("path_to_image", cv::IMREAD_UNCHANGED);
// Convert to grayscale getting rid of alpha channel
cv::Mat1b img(img4b.rows, img4b.cols, uchar(0));
for (int r = 0; r < img4b.rows; ++r) {
for (int c = 0; c < img4b.cols; ++c) {
if (img4b(r, c) == cv::Vec4b(255,255,255,255)) {
img(r, c) = uchar(255);
}
if (img4b(r, c)[3] == 0) {
img(r, c) = uchar(255);
}
}
}
cv::imshow("img", img);
cv::waitKey();
}
Result: