Opencv c++ resize function: new Width should be multiplied by 3

Question

I am programming in Qt environment and I have a Mat image with size 2592x2048 and I want to resize it to the size of a "label" that I have. But when I want to show the image, I have to multiply the width by 3, so the image is shown in its correct size. Is there any explanation for that?

This is my code:

//Here I get image from the a buffer and save it into a Mat image.
//img_width is 2592 and img_height is 2048
Mat image = Mat(cv::Size(img_width, img_height), CV_8UC3, (uchar*)img, Mat::AUTO_STEP);
Mat cimg;
double r; int n_width, n_height;
//Get the width of label (lbl) into which I want to show the image
n_width = ui->lbl->width();
r = (double)(n_width)/img_width;
n_height = r*(img_height);
cv::resize(image, cimg, Size(n_width*3, n_height), INTER_AREA);

Thanks.

Answer 1

The resize function works well, because if you save the resized image as a file is displayed correctly. Since you want to display it on QLabel, I assume you have to transform your image to QImage first and then to QPixmap. I believe the problem lies either in the step or the image format.

If we ensure the image data passed in

Mat image = Mat(cv::Size(img_width, img_height), CV_8UC3, (uchar*)img, Mat::AUTO_STEP);

are indeed an RGB image, then below code should work:

ui->lbl->setPixmap(QPixmap::fromImage(QImage(cimg.data, cimg.cols, cimg.rows, *cimg.step.p, QImage::Format_RGB888 )));

Finally, instead of using OpenCV, you could construct a QImage object using the constructor

QImage((uchar*)img, img_width, img_height, QImage::Format_RGB888)

and then use the scaledToWidth method to do the resize. (beware thought that this method returns the scaled image, and does not performs the resize operation to the image per se)