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

[DavidU]

Hello,

I originally posted this on the NI forums, here, but it hasn't had a response so I'm posting it here too.

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 Run in any thread I get correct results (see attachment resultAny.png).

I've created a minimal DLL which shows this behaviour, see below.

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 or indeed the OpenCV dlls are thread safe).

Thanks in advance,

Dave

#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;}  

source.zip