How do I draw a green rectangle with opencv?

Question

Really new to C++ so my apologies for such a question. Been trying out with these but they dont seem to work.

(I'm executing a template matching function in opencv - https://docs.opencv.org/3.4/de/da9/tutorial_template_matching.html )

Edit: Here is my code for my image, template and mask i used!

cv::Mat image = cv::Mat(height,width, CV16UC1, image); // image in short
cv::Mat temp;
image.convertTo(temp, CV32_F); // convert image to 32 bits

cv::image_template = cv::Mat(t_width, t_height, CV_32F, t_image); // template
cv::mask_template = cv::Mat(t_width, t_height, CV_32F, m_image); // mask

cv:: Mat img_display, result;
temp.copyTo(img_display); // image to display
int result_cols = temp.cols - image_template.cols + 1;
int result_rows = temp.rows - image_template.rows + 1;
result.create(result_rows, result_cols, CV32FC1);

// all the other code
matchTemplate(temp, image_template, result, 0, mask_template);
normalize( result, result, 0, 1, cv::NORM_MINMAX, -1, cv::Mat());

// localize the minimum and maximum values in the result matrix
double minVal;
double maxVal;
cv::Point minLoc;
cv::Point maxLoc;
cv::Point matchLoc;
minMaxLoc(result, &minVal, &maxVal, &minLoc, &maxLoc, cv::Mat());
// for match_method TM_SQDIFF we take lowest values
matchLoc = minLoc;


// display source image and result matrix , draw rectangle around highest possible matching area
cv::rectangle( img_display, matchLoc, cv::Point( matchLoc.x + image_template.cols, matchLoc.y + image_template.rows), cv::Scalar::all(255), 2, 8, 0);
cv::rectangle( result, matchLoc, cv::Point(matchLoc.x + image_template.cols, matchLoc.y + image_template.rows), cv::Scalar::all(255), 2, 8, 0);

This is the given code:

cv::rectangle( img_display, matchLoc, cv::Point( matchLoc.x + templ.cols , matchLoc.y + templ.rows ), cv::Scalar::all(0), 2, 8, 0 );

Tried to change it with the following code snippets but doesn't seem to work.

cv::rectangle( img_display, matchLoc, cv::Point( matchLoc.x + templ.cols , matchLoc.y + templ.rows ), cv::Scalar(0,255,0) , 2, 8, 0 );

This doesn't work either

rectangle(ref, maxloc, Point(maxloc.x + tpl.cols, maxloc.y + tpl.rows), CV_RGB(0,255,0), 2);

Do let me know where I am wrong!

Answer 1

First of all, you are trying to scale your pixels 0 to 255 but you cant do that because your image is a float type image(32FC1) float type image pixel values can be scaled 0.0 to 1.0.

You need to convert your image to 8UC to be able to colorize easily. But this way will also have several problems which mentioned here . OpenCV matchTemplate function always gives result in 32FC1 format so it is difficult to make some colorized things on this type image.

In your source image you can draw your rectangles with your desired color but not in float type. You can also check this link

Answer 2

Simply add

#define CV_RGB(r, g, b) 

on top of your code so that OpenCV knows it will use RGB color space instead of the default BGR.

And then draw your green rectangle this way.

rectangle(frame, Point(startX, startY), Point(endX, endY), CV_RGB(0, 255, 0), 2);