can't index 108th element

[oliver9523]

Hi all,

This has been driving me nuts. There's probably something that already exists to do this function but I'm trying to do it manually to further my understanding.

I have two gray scale images coming from two webcams. I've aligned them using SURF features and perspective warped one to overlay onto the other.

I want to overlay the warped one onto the reference image.
The warped image has black areas that I don't want to overlay...so I'm trying this....

int nCols = LeftTrans.cols; //320
int nRows = LeftTrans.rows; //240

Mat OverLay = Mat::zeros(nRows,nCols,CV_8UC1);

int val = 0;
for (int col=0; col < nCols ; col++) {
	for (int row=0; row < nRows ; row++) {
		val = (int)LeftTrans.at<cv::Vec3b>(col,row)[0];

		if (val == 0) {
		//std::cout << "row = "<< row << " col = " << col <<" Val = " << val << std::endl;
		OverLay.at<cv::Vec3b>(row,col)[0] = img_scene.at<cv::Vec3b>(row,col)[0];
		} else {
		//std::cout << "row = "<< row << " col = " << col << " Val = " << val <<std::endl;
		OverLay.at<cv::Vec3b>(row,col)[0] = LeftTrans.at<cv::Vec3b>(row,col)[0];
		}
	}
}

The problem that's driving me nuts is I get "Unhandled exception error"
on the line
val = (int)LeftTrans.at<cv::Vec3b>(col,row)[0];
but only when col == 107 or row == 107

Why can't I index anything from 107 or above?

[Peter87]

I have never used OpenCV but some quick googling suggests that CV_8UC1 is just 1 byte data. cv::Vec3b is probably 3 bytes so when you try to access col 107 it probably access data on byte column 3 * 107 = 321 which is more than nCols. I guess you have to use something else than CV_8UC1.

[oliver9523]

ah cheers for that, I've now replaced
<cv::Vec3b>
with
<uchar>
it works a treat now.

Getting used to C++ when coming from a Matlab background is tough, I'm not used to having to worry about all these little details!