OpenCV call returns different results when wrapper DLL set to run in UI thread or any thread

[DavidU]

Hi, I'm writing a wrapper DLL to handle calls to the OpenCV 2.1 image processing library.

When using the OpenCV function 'cv::matchTemplate' I get different results depending on if the wrapper DLL is set to run in the UI thread or any thread.

When set to the UI thread I get incorrect results (see attachement resultUI.png).

When set to any thread I get correct results (see attachment resultAny.png).

I've created a minimal DLL which shows this behaviour:

#include <cv.h>
#include <highgui.h>

extern "C" __declspec(dllexport) int matchTest(int a);

BOOL APIENTRY DllMain( HMODULE hModule, DWORD  ul_reason_for_call, LPVOID lpReserved)
{
	switch (ul_reason_for_call)
	{
	case DLL_PROCESS_ATTACH:
	case DLL_THREAD_ATTACH:
	case DLL_THREAD_DETACH:
	case DLL_PROCESS_DETACH:
		break;
	}
	return TRUE;
}

extern "C" __declspec(dllexport) int matchTest(int a)
{
	cv::Mat srcImg, templateImg, result32Img, result8Img;
	cv::Size resultSize;

	srcImg= cv::imread("source.bmp", 0);
	templateImg = cv::imread("template.bmp", 0);

	resultSize.width = srcImg.size().width - templateImg.size().width + 1;
        resultSize.height = srcImg.size().height - templateImg.size().height + 1;
	
	result32Img = cv::Mat(resultSize, CV_32FC1);
	result8Img = cv::Mat(resultSize, CV_8UC1);

	cv::matchTemplate(srcImg, templateImg, result32Img, CV_TM_CCOEFF_NORMED);	//returns different results
	
        //convert to 8 bit and write to file
	result32Img.convertTo(result8Img, CV_8UC1, 256);
	cv::imwrite("result.png", result8Img);

	return a;
}

Why do I get different results?

Is there a way to get the 'Run in UI thread' version to return the correct results (I'm not sure that my wrapper DLL is/will be thread safe, I'm checking).

.

Thanks in advance,

Dave

[DavidU]

Update:

I realised that I had also set the execution system of the VI to 'other 1'.

If I set it to 'user interface' then it doesn't matter if the CLFN is set to run in UI thread or any thread, I get the incorrect result. I presume this is because there is only one thread in the 'user interface' execution system.

The dll I posted above is just to demonstrate the issue. The actual dll I use contains more functions and calls to openCV. I can't be sure that it is thread safe, so would like to force it to run in a single thread, but not the UI thread because of the matchTemplate issue. Looking at this post it looks like I have two options, the threadconfig suggestion or the NIComputeContext route. The Compute Context route seems neater since I don't have to worry about setting theadconfig if I use the code in multiple projects. So before I go diving into it, is that right, or is there a better way?

Thanks

Dave

[alpha1]

Hi Dave,

This sounds like a very specific and unique issue that not many of us have come across before. I wasn't able to find any useful information on the topic to help so far, but was wondering how you got on with the application?