cv::line draws wrong lines when coordinates are very far from image (Bug #2988)

Added by Adi Shavit almost 12 years ago. Updated over 9 years ago.

Status:Open Start date:2013-04-25
Priority:Normal Due date:
Assignee:Vadim Pisarevsky % Done:

0%
Category:core
Target version:-
Affected version:2.4.0 - 2.4.4 Operating System:
Difficulty: HW Platform:
Pull request:

Description

There seems to be a bug with cv::line related to clipping.
Sometimes, when it gets points outside and very far from the image it draws completely wrong lines.

Here's some code to demonstrate this:

{
      cv::Mat img = cv::Mat::zeros(cv::Size(480,360), CV_8UC3);   
// The points
      Point p2(262, 23),  q2(91618510, -54756195);
      Point p3(263, 214),  q3(80120779, 11846827);
// The bad lines
      line(img, p2, q2, cv::Scalar(255,255,255));
      line(img, p3, q3, cv::Scalar(255,255,255));
// Clip BEFORE drawing
      cv::clipLine(img.size(), p2, q2);
      cv::clipLine(img.size(), p3, q3);
// Good lines
      line(img, p2, q2, cv::Scalar(0,255,255));
      line(img, p3, q3, cv::Scalar(0,255,255));
imshow("img", img);
      waitKey();
}

This also happens with CV_AA.

History

#1
Updated by Gurpinder Sandhu almost 12 years ago

@Vadim I think this can solved simply by adding an assertion in the line function:


CV_Assert(pt1.x < img.cols && pt1.y < img.rows);
CV_Assert(pt2.x < img.cols && pt2.y < img.rows);

where pt1, pt2 are the input points and img is the input image.
Please give your comments

#2
Updated by Adi Shavit almost 12 years ago

@ Gurpinder Sandhu, I don't think it should assert that points are inside the image. For several reasons:
  1. First, it already works with internal clipping for many cases.
  2. Sometimes, you need to draw lines to points outside the image. Auto clipping is very helpful for making clean code.
  3. Since the Bresenham algorithm discretizes the line. A line to the clipped point will generally not be the same as the line to the point outside the image.

#3
Updated by Anna Kogan almost 12 years ago

Hello Gurpinder,
Thank you for reporting the issue. I agree with Adi that asserts are not needed in the case. If you could find another solution on your side, a pull request in our GitHub repo would be highly appreciated!

#4
Updated by Maksim Shabunin over 9 years ago

Issue has been transferred to GitHub: https://github.com/Itseez/opencv/issues/4555

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