cannot Segment a RGB image by pointer based accessing pixel intensities - c++

Question

I define a function void segRgb(Mat &src, Mat &dst, Rect roi), using which I try to segment the region of region (ROI) of an input RGB image by simply thresholding a lumped pixel intensities derived from R, G and B channels. Here below is the code of the function:
void segRgb(Mat &src, Mat &dst, Rect roi)
{
uchar *bgrdata = src.data;
uchar *outdata = dst.data;
int ystart = roi.y;
int yend = roi.y + roi.height;
int xstart = roi.x;
int xend = roi.x+roi.width;
int step1 = src.cols-roi.width;
int step3 = 3*step1;
int start1 = roi.y*src.cols+roi.x;
int start3 = 3*start1;
bgrdata += start3;
outdata += start1;
uchar r, g, b;
double score=0.0;
for(int i=ystart; i<yend; i++)
{
qDebug()<<"Rows: "<<i;
for(int j=xstart; j<xend; j++)
{
b = *bgrdata++;
g = *bgrdata++;
r = *bgrdata++;
score = 0.21*r+0.72*g+0.07*b; //a simple rule to lump RGB values
if(score>100)
{
*outdata = 255;
}
else
{
*outdata = 0;
}
outdata++;
}
outdata+=step1;
bgrdata+=step3;
}
}
Following is my test code for the function:
Rect cvRect = Rect(10,50,256,256);
Mat dst;
segRgb(im, dst, cvRect); //im is a loaded Matrix of 427*640*3, CV_8UC3
namedWindow("Thresholded");
imshow("Thresholed", dst);
I run the codes above. The function segRgb does not work for some reason. No image is shown. Actually, the loop inside the segRgb does not proceed. Anyone can point to the problem, debug my codes bit? Thanks!

Answer 1

void segRgb(Mat &src, Mat &dst, Rect roi)
{
uchar *bgrdata = src.data;
uchar *outdata = dst.data;
int ystart = roi.y;
int yend = roi.y + roi.height;
int xstart = roi.x;
int xend = roi.x + roi.width;
int step1 = src.cols - roi.width;
int step3 = 3 * step1;
int start1 = roi.y*src.cols + roi.x;
int start3 = 3 * start1;
bgrdata += start3;
outdata += start1;
uchar r, g, b;
double score = 0.0;
for (int i = ystart; i < yend; i++)
{
cout << "Rows: " << i;
for (int j = xstart; j < xend; j++)
{
b = *bgrdata++;
g = *bgrdata++;
r = *bgrdata++;
score = 0.21*r + 0.72*g + 0.07*b; //a simple rule to lump RGB values
if (score > 100)
{
*outdata = 255;
}
else
{
*outdata = 0;
}
outdata++;
}
outdata += step1;
bgrdata += step3;
}
}
int main() {
Mat im = imread("urimage");
Rect cvRect = Rect(10, 50, 256, 256);
// you have to allocate a size for the dst Mat otherwise the uchar* output you point to above will be garbage
Mat dst(im.size(),im.type());
segRgb(im, dst, cvRect); //im is a loaded Matrix of 427*640*3, CV_8UC3
//Resize you dst or you can change a bit in your function paramters to get it directly
dst=Mat(dst, cvRect);
namedWindow("Thresholded");
imshow("Thresholed", dst);
waitKey(0);
}