opencv
Pixel Access
Efficient pixel access using cv::Mat::ptr pointer
Example
If efficiency is important, a fast way to iterate over pixels in a cv::Mat object is to use its ptr<T>(int r) method to obtain a pointer to the beginning of row r (0-based index).
According to the matrix type, the pointer will have a different template.
- For
CV_8UC1:uchar* ptr = image.ptr<uchar>(r); - For
CV_8UC3:cv::Vec3b* ptr = image.ptr<cv::Vec3b>(r); - For
CV_32FC1:float* ptr = image.ptr<float>(r); - For
CV_32FC3:cv::Vec3f* ptr = image.ptr<cv::Vec3f>(r);
This ptr object can then be used to access the pixel value on row r and column c by calling ptr[c].
To illustrate this, here is an example where we load an image from disk and invert its Blue and Red channels, operating pixel by pixel:
#include <opencv2/core.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/highgui.hpp>
int main(int argc, char** argv) {
cv::Mat image = cv::imread("image.jpg", CV_LOAD_IMAGE_COLOR);
if(!image.data) {
std::cout << "Error: the image wasn't correctly loaded." << std::endl;
return -1;
}
// We iterate over all pixels of the image
for(int r = 0; r < image.rows; r++) {
// We obtain a pointer to the beginning of row r
cv::Vec3b* ptr = image.ptr<cv::Vec3b>(r);
for(int c = 0; c < image.cols; c++) {
// We invert the blue and red values of the pixel
ptr[c] = cv::Vec3b(ptr[c][2], ptr[c][1], ptr[c][0]);
}
}
cv::imshow("Inverted Image", image);
cv::waitKey();
return 0;
}
