Strange behaviour with GetOpenFileName dialog and opencv - c++

Question

I'm trying to use the open file dialog to allow the user of my program to select an image which will then be processed using opencv.
I have a function in which i open the dialog and try to load the image:
Mat load_image()
{
OPENFILENAME ofn; // common dialog box structure
TCHAR szFile[260]; // buffer for file name
HWND hwnd; // owner window
HANDLE hf; // file handle
// Initialize OPENFILENAME
ZeroMemory(&ofn, sizeof(ofn));
ofn.lStructSize = sizeof(ofn);
ofn.hwndOwner = hwnd;
ofn.lpstrFile = szFile;
// Set lpstrFile[0] to '\0' so that GetOpenFileName does not
// use the contents of szFile to initialize itself.
ofn.lpstrFile[0] = '\0';
ofn.nMaxFile = sizeof(szFile);
ofn.lpstrFilter = L"All\0*.*\0Text\0*.TXT\0";
ofn.nFilterIndex = 1;
ofn.lpstrFileTitle = NULL;
ofn.nMaxFileTitle = 0;
ofn.lpstrInitialDir = NULL;
ofn.Flags = OFN_PATHMUSTEXIST | OFN_FILEMUSTEXIST;
// Display the Open dialog box.
if (GetOpenFileName(&ofn)==TRUE)
hf = CreateFile(ofn.lpstrFile,
GENERIC_READ,
0,
(LPSECURITY_ATTRIBUTES) NULL,
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL,
(HANDLE) NULL);
char filepath[260];
wcstombs(filepath, ofn.lpstrFile, 260);
cout << filepath << endl;
Mat src = imread(filepath);
//imshow("src", src);
return src;
}
The code you see here works, opens the dialog box and allows the user to choose a file. The filepath is then converted to string (to be passed into imread). The problem occurs when I try to interact with the image src.
For example, if I uncomment the 'imshow' line, GetOpenFileName will return 0 and the dialog box doesnt even open. This happens more than not, I cant actually access the image because everything I try causes this to happen.
In order for the function to work it is called like this:
load_image();
But if I try to assign an image ie:
img = load_image();
the same problem occurs. Help! What could possibly be causing this?

Answer 1

As it is mentioned in the previous comment, the CreateFile method locks the image file so cv::imread() is not able to access/read its content.
Try to avoid using of CreateFile, I see no reason of using it in your code, the following code is working well:
cv::Mat load_image()
{
cv::Mat outMat;
OPENFILENAME ofn; // common dialog box structure
TCHAR szFile[260]; // buffer for file name
// Initialize OPENFILENAME
ZeroMemory(&ofn, sizeof(ofn));
ofn.lStructSize = sizeof(ofn);
ofn.hwndOwner = NULL;
ofn.lpstrFile = szFile;
ofn.lpstrFile[0] = '\0';
ofn.nMaxFile = sizeof(szFile);
ofn.lpstrFilter = L"All\0*.*\0Text\0*.TXT\0";
ofn.nFilterIndex = 1;
ofn.lpstrFileTitle = NULL;
ofn.nMaxFileTitle = 0;
ofn.lpstrInitialDir = NULL;
ofn.Flags = OFN_PATHMUSTEXIST | OFN_FILEMUSTEXIST;
// Display the Open dialog box.
if (GetOpenFileName(&ofn) == TRUE)
{
char filepath[260];
wcstombs(filepath, ofn.lpstrFile, 260);
cout << filepath << endl;
outMat = cv::imread(filepath);
}
return outMat;
}
int main()
{
cv::Mat image = load_image();
cv::imshow("image", image);
cv::waitKey(-1);
return 0;
}

Answer 2

These are just things I notice in your code...so I don't expect this to work but maybe it will help.
imread is probably failing. You opened the file using CreateFile and locked it as a result of opening it (No Sharing Attributes. See CreateFile for more info). imread tries to open the file anyways so you don't need to create your own handle! Just use the file path received and call imread instead of using CreateFile.
Mat src;
if (GetOpenFileName(&ofn)==TRUE)
{
// Succeeded in choosing a file
char filepath[260];
wcstombs(filepath, ofn.lpstrFile, 260);
cout << filepath << endl;
src = imread(filepath);
}
Also if you don't have an hwnd then you should just set ofn.hwndOwner=NULL instead of arbitrarily setting it.
If you so choose to use CreateFile (maybe you wish to open it for manual read access for your own purposes outside of OpenCV), then make sure to call CloseHandle( HANDLE h ) on the file handle before you open the file again.