Losing alpha channel when passing from Unity to OpenCV?

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First here it's an opencv forum (read faq) it is difficult to help you with third party lib or wrappers and c#.

I have tested :

Mat pngimg = imread("c:/Users/Laurent.PC-LAURENT-VISI/Desktop/test.png",IMREAD_UNCHANGED);
cout<<"imread : "<<pngimg.channels() <<"\n";
imwrite("c:/Users/Laurent.PC-LAURENT-VISI/Desktop/test1.png",pngimg);
pngimg = imread("c:/Users/Laurent.PC-LAURENT-VISI/Desktop/test1.png", IMREAD_UNCHANGED);
cout << "imread after imwrite: " << pngimg.channels() << "\n";

result :

imread : 4
imread after imwrite: 4

On my disk image saved is good. Imread is called decoder->readData( *data ); exactly as like imdecode. may be I'm wrong but I don't think problem is in opencv

Than you for your time and for your answer. I performed a similar test and the result agree with your test. I still wonder if there is something wrong in my c++ code in the way that I construct data_mat_medium from buffer and in the decoding process. Also I think it's unfair for the question to be downvoted for mentioning the C# side of the code. I present the c# side to show that just before sending the image, this image really had an alpha channel and I think that this is important. I can remove the unity and c# stuff from the question if it is better for everyone. I apologize again and thank you one more time.

Opencv is a large lib 20000 commits for 8 years.May be 30 tags since 2.2 version and last one is 3.2.0-rc. I have tested with 3.2.0-rc. If you tested too and found same result problem is not in opencv.

There is no problem to help you for your problem if you can give a piece code in c++ java or python and give OS and opencv version.

I found a solution by replacing the lines

std::vector<uchar> vectordataImMedium(imMedium, imMedium + imMediumLength);

cv::Mat data_mat_medium(vectordataImMedium, true);

mediumImage = cv::imdecode(data_mat_medium, 1);

with these

cv::Mat imgBuf(cv::Size(1280, 960), CV_8UC4, imMedium);

mediumImage= cv::imdecode(imgBuf, CV_LOAD_IMAGE_UNCHANGED);

Thank you again for time and your interest.

Can you try without imdecode ?

mediumImage= imgBuf;

I tried that but the third party application crashed. After debugging the DLL visual studio gave me this message

Unhandled exception at 0x00007ffa401dc58a (opencv_imgcodecs310d.dll) in Unity.exe: 0xC0000005: Access violation reading location 0x000000002476d000.

at

line 831 : memcpy(png_ptr->row_buf + 1, row, row_info.rowbytes);

Try to do not use imdecode :

cv::Mat mediumImage(cv::Size(1280, 960), CV_8UC4, imMedium);

Sorry for the delay (too much paperwork), not using imdecode doesn't seem to work, here are 5 steps from the debugging process (it shows with imdecode - which works and without imdecode - which deosn't work with some info from the vs debugger) https://s30.postimg.org/j3hqd5601/Ste...https://s30.postimg.org/ndwe8qb35/Ste...https://s30.postimg.org/hr4mozxy9/Ste...https://s30.postimg.org/meaoqrlb5/Ste...https://s30.postimg.org/fp45aqzz5/Ste...

note at step4 the "prevAllocator=????" , it doesn't look right.

Something is wrong in your code. Try this small sample :

fstream fs;
fs.open("f:\\lib\\opencv\\samples\\data\\opencv-logo-white.png",ios::binary|ios::in);
fs.seekp(0, ios_base::end);
int nbByte=fs.tellp();
fs.seekp(0,ios_base::beg);
vector<char> tmp(nbByte);
fs.read(tmp.data(),nbByte);// in buffer I have got a png image 
Mat img=imdecode(tmp,IMREAD_UNCHANGED); // image is decode now
cout<<img.rows<<"\t"<<img.cols<<"\n";

You example should work like my example