How to get the image pixel at real locations in opencv?

Question

I want to retrieve the rgb of a pixel in the image. But the location is not integer location but real values (x,y). I want a bilinear interpolated value. How could I do it opencv?

Thanks a lot

Answer 1

There is no simple function for subpixel access but I can suggest you few options:

1. Use getRectSubPix and extract 1 pixel region:

   cv::Vec3b getColorSubpix(const cv::Mat& img, cv::Point2f pt)
   {
       cv::Mat patch;
       cv::getRectSubPix(img, cv::Size(1,1), pt, patch);
       return patch.at<cv::Vec3b>(0,0);
   }
   

2. Use more flexible but less precise remap with one-pixel map:

   cv::Vec3b getColorSubpix(const cv::Mat& img, cv::Point2f pt)
   {
       cv::Mat patch;
       cv::remap(img, patch, cv::Mat(1, 1, CV_32FC2, &pt), cv::noArray(),
           cv::INTER_LINEAR, cv::BORDER_REFLECT_101);
       return patch.at<cv::Vec3b>(0,0);
   }
   

3. Implement bilinear interpolation yourself, as it is not a rocket science:

   cv::Vec3b getColorSubpix(const cv::Mat& img, cv::Point2f pt)
   {
       assert(!img.empty());
       assert(img.channels() == 3);
   
       int x = (int)pt.x;
       int y = (int)pt.y;
   
       int x0 = cv::borderInterpolate(x,   img.cols, cv::BORDER_REFLECT_101);
       int x1 = cv::borderInterpolate(x+1, img.cols, cv::BORDER_REFLECT_101);
       int y0 = cv::borderInterpolate(y,   img.rows, cv::BORDER_REFLECT_101);
       int y1 = cv::borderInterpolate(y+1, img.rows, cv::BORDER_REFLECT_101);
   
       float a = pt.x - (float)x;
       float c = pt.y - (float)y;
   
       uchar b = (uchar)cvRound((img.at<cv::Vec3b>(y0, x0)[0] * (1.f - a) + img.at<cv::Vec3b>(y0, x1)[0] * a) * (1.f - c)
                              + (img.at<cv::Vec3b>(y1, x0)[0] * (1.f - a) + img.at<cv::Vec3b>(y1, x1)[0] * a) * c);
       uchar g = (uchar)cvRound((img.at<cv::Vec3b>(y0, x0)[1] * (1.f - a) + img.at<cv::Vec3b>(y0, x1)[1] * a) * (1.f - c)
                              + (img.at<cv::Vec3b>(y1, x0)[1] * (1.f - a) + img.at<cv::Vec3b>(y1, x1)[1] * a) * c);
       uchar r = (uchar)cvRound((img.at<cv::Vec3b>(y0, x0)[2] * (1.f - a) + img.at<cv::Vec3b>(y0, x1)[2] * a) * (1.f - c)
                              + (img.at<cv::Vec3b>(y1, x0)[2] * (1.f - a) + img.at<cv::Vec3b>(y1, x1)[2] * a) * c);
   
       return cv::Vec3b(b, g, r);
   }