opencv: how to draw arrows on orientation image

Question

I'm trying to perform orientation estimation on an input image in OpenCV. I used sobel function to get gradients of the image, and used another function called calculateOrientations, which I found on the internet, to calculate orientations.

The code is as follows:

void computeGradient(cv::Mat inputImg)
{
    // Gradient X
    cv::Sobel(inputImg, grad_x, CV_16S, 1, 0, 5, 1, 0, cv::BORDER_DEFAULT);
    cv::convertScaleAbs(grad_x, abs_grad_x);

    // Gradient Y
    cv::Sobel(inputImg, grad_y, CV_16S, 0, 1, 5, 1, 0, cv::BORDER_DEFAULT);
    cv::convertScaleAbs(grad_y, abs_grad_y);

    // convert from CV_8U to CV_32F
    abs_grad_x.convertTo(abs_grad_x2, CV_32F, 1. / 255);
    abs_grad_y.convertTo(abs_grad_y2, CV_32F, 1. / 255);

    // calculate orientations
    calculateOrientations(abs_grad_x2, abs_grad_y2);    
} 

void calculateOrientations(cv::Mat gradientX, cv::Mat gradientY)
{
    // Create container element
    orientation = cv::Mat(gradientX.rows, gradientX.cols, CV_32F);

    // Calculate orientations of gradients --> in degrees
    // Loop over all matrix values and calculate the accompagnied orientation
    for (int i = 0; i < gradientX.rows; i++){
        for (int j = 0; j < gradientX.cols; j++){
            // Retrieve a single value
            float valueX = gradientX.at<float>(i, j);
            float valueY = gradientY.at<float>(i, j);
            // Calculate the corresponding single direction, done by applying the arctangens function
            float result = cv::fastAtan2(valueX, valueY);
            // Store in orientation matrix element
            orientation.at<float>(i, j) = result;
        }
    }
}

Now, I need to make sure whether the obtained orientation is correct or not. For that I want to draw arrows for each block of size 5x5 on the orientation matrix. Could someone advice me on how to draw arrows on this? Thank you.

do you want to draw on a `` matrix or on some BGR image? Maybe you can draw an arrow on a black image, then for each orientation value, compute the rotation angle and warp the arrow to your output image location. Or create a function to draw a rotated arrow to some location... — Micka, Jan 15 '15 at 16:02
See: http://stackoverflow.com/questions/10161351/opencv-how-to-plot-velocity-vectors-as-arrows-in-using-single-static-image — beaker, Jan 15 '15 at 17:07

Vit · Accepted Answer · 2017-03-27T20:21:59.427

The simplest way for OpenCV to distinguish direction is to draw little circle or square at a start or end point of line. There are no function for arrows afaik. If you need arrow you have to write this (it is simple but takes time too). Once I did it this way (not openCV, but I hope you convert it):

  double arrow_pos = 0.5; // 0.5 = at the center of line
  double len = sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
  double co = (x2-x1)/len, si = (y2-y1)/len; // line coordinates are (x1,y1)-(x2,y2)
  double const l = 15, sz = linewidth*2; // l - arrow length 
  double x0 = x2 - len*arrow_pos*co;
  double y0 = y2 - len*arrow_pos*si;
  double x = x2 - (l+len*arrow_pos)*co;
  double y = y2 - (l+len*arrow_pos)*si;
  TPoint tp[4] = {TPoint(x+sz*si, y-sz*co), TPoint(x0, y0), TPoint(x-sz*si, y+sz*co), TPoint(x+l*0.3*co, y+0.3*l*si)};
  Polygon(tp, 3);
  Canvas->Polyline(tp, 2);

UPDATE: arrowedLine(...) function added since OpenCV 2.4.10 and 3.0

score 2 · Answer 2 · answered Mar 02 '22 at 11:29

2

The easiest way to draw an arrow in opencv is:

arrowedLine(img, pointStart, pointFinish, colorScalar, thickness, line_type, shift, tipLength);

thickness, line_type, shift and tipLength have already default values, so can be omitted

answered Mar 02 '22 at 11:29

vnapastiuk

609
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opencv: how to draw arrows on orientation image

2 Answers2