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Drawing an arc

asked 2017-08-30 16:36:44 -0500

rudolfninja gravatar image

updated 2017-08-31 06:51:07 -0500

Hi guys, I have arc parameters from Autocad file and I need to draw it using opencv. However, using corresponding parameters in cv::ellipse function the arc is absolutely not the same as in Autocad. Here is my code:

cv::ellipse(image, center, cv::Size(radius, radius), angle, start_angle, end_angle, cv::Scalar(255, 255, 255));

The picture should be like image description

but in fact I've got image description

Here is parameters for every arc:

1) center.x = 495.419403
    center.y = 527.065674
    start angle = 0.0000
    end abgle = 56.88228
    full angle = 56.88228
    start.x = 501.41940
    start.y = 527.06567
    end.x = 498.69757
    end.y = 532.090979
    arc_center.x = 500.695245
    arc_center.y = 529.92321
    radius = 6.00000

2) center.x = 495.4194
    center.y = 486.31543
    start angle = 270.0000
    end abgle = 0.0000
    full angle = 90.0000
    start.x = 495.41940
    start.y = 480.31543
    end.x = 501.4194
    end.y = 486.31543
    arc_center.x = 499.6620
    arc_center.y = 482.07279
    radius = 6.00000

3) center.x = 454.6691
    center.y = 486.31543
    start angle = 213.11771
    end abgle = 270.000
    full angle = 56.88228
    start.x = 449.64386
    start.y = 483.03727
    end.x = 454.66916
    end.y = 480.31543
    arc_center.x = 451.8116
    arc_center.y = 481.03959
    radius = 6.00000

4) center.x = 419.4194
    center.y = 562.31543
    start angle = 56.88228
    end abgle = 213.1177
    full angle = 156.2354
    start.x = 429.2539
    start.y = 577.3913
    end.x = 404.3435
    end.y = 552.4809415
    arc_center.x = 406.69148
    arc_center.y = 575.04336
    radius = 18.00000

How can I draw correct arcs? I've got start and end points of the arc, radius, and center points (arc center and center from where radius goes)

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Comments

could you post whole code you tried. or values you get from autocad

sturkmen gravatar imagesturkmen ( 2017-08-30 17:12:34 -0500 )edit

Seems like you're not passing the right start and end angles. Can you calculate those angles in AutoCAD ?

Ziri gravatar imageZiri ( 2017-08-30 22:47:51 -0500 )edit

I updated my post with required info. Looks like the problem might be somewhere in that, that autocad coordinates start from left bottom corner while in openCV from left top corner. I changed Y value in every point to (H - autocad_point.y). Now the image has correct rotation, but still problems with arcs. Here is what I got now: picture

rudolfninja gravatar imagerudolfninja ( 2017-08-31 00:47:32 -0500 )edit

As @sturkmen asked, provide code please so that we can debug it locally. Could it be a radians/degrees issue?

StevenPuttemans gravatar imageStevenPuttemans ( 2017-08-31 03:45:09 -0500 )edit

Degrees. I can't provide full code because it uses 3rd partylibraries to work with autocad file. So you won't be able to compile this code. I just can provide coordinatess of all the primitives for you to try to draw them. I can provide autocad file as well.

rudolfninja gravatar imagerudolfninja ( 2017-08-31 03:50:34 -0500 )edit

1 answer

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answered 2017-08-31 04:19:36 -0500

updated 2017-08-31 06:40:11 -0500

i think we can use the code below to find the answer. lets try change something

code draws something like this

image description

#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>

#include <iostream>

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
    Mat img(600, 600, CV_8UC3, Scalar(0, 0, 0));

    Point2d center, start, end, arc_center;
    center.x = 495.419403;
    center.y = 527.065674;
    double start_angle = 0.0;
    double end_angle = 56.88228;
    double full_angle = 56.88228;

    start.x = 501.41940;
    start.y = 527.06567;
    end.x = 498.69757;
    end.y = 532.090979;
    arc_center.x = 500.695245;
    arc_center.y = 529.92321;
    double radius = 6.0;
    double angle = 0.0;

    Point2d lastline_end = end;
    Point2d line_start = start;
    end_angle = end_angle == 0.0 ? 360.0 : end_angle;
    cv::ellipse(img, center, cv::Size(radius, radius), angle, start_angle, end_angle, cv::Scalar(255, 0, 0));

    center.x = 495.4194;
    center.y = 486.31543;
    start_angle = 270.0000;
    end_angle = 0.0000;
    full_angle = 90.0000;
    start.x = 495.41940;
    start.y = 480.31543;
    end.x = 501.4194;
    end.y = 486.31543;
    arc_center.x = 499.6620;
    arc_center.y = 482.07279;
    radius = 6.00000;

    line(img, line_start, end, Scalar(255, 255, 255));
    line_start = start;
    end_angle = end_angle == 0.0 ? 360.0 : end_angle;
    cv::ellipse(img, center, cv::Size(radius, radius), angle, start_angle, end_angle, cv::Scalar(0, 0, 255));


    center.x = 454.6691;
    center.y = 486.31543;
    start_angle = 213.11771;
    end_angle = 270.000;
    full_angle = 56.88228;
    start.x = 449.64386;
    start.y = 483.03727;
    end.x = 454.66916;
    end.y = 480.31543;
    arc_center.x = 451.8116;
    arc_center.y = 481.03959;
    radius = 6.00000;

    line(img, line_start, end, Scalar(255, 255, 255));
    line_start = start;
    end_angle = end_angle == 0.0 ? 360.0 : end_angle;
    cv::ellipse(img, center, cv::Size(radius, radius), angle, start_angle, end_angle, cv::Scalar(0, 255, 0));

    center.x = 419.4194;
    center.y = 562.31543;
    start_angle = 56.88228;
    end_angle = 213.1177;
    full_angle = 156.2354;
    start.x = 429.2539;
    start.y = 577.3913;
    end.x = 404.3435;
    end.y = 552.4809415;
    arc_center.x = 406.69148;
    arc_center.y = 575.04336;
    radius = 18.00000;

    line(img, line_start, end, Scalar(255, 255, 255));
    line(img, lastline_end, start, Scalar(255, 255, 255));
    end_angle = end_angle == 0.0 ? 360.0 : end_angle;
    cv::ellipse(img, center, cv::Size(radius, radius), angle, start_angle, end_angle, cv::Scalar(255, 0, 255));
    flip(img, img, 0);
    imshow("Image", img);
    cv::waitKey(0);

    return 0;
}
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Comments

Why do you use angle with 0.0 value but not full angle (end angle - start angle)? This is the difference in my code

rudolfninja gravatar imagerudolfninja ( 2017-08-31 04:40:12 -0500 )edit

because i have no info about angle and first set it to 0 to test. and changed it to 180 for red angle

sturkmen gravatar imagesturkmen ( 2017-08-31 04:45:34 -0500 )edit

You've got end_angle and start angle. So you can calc full_angle. That waht I do. But looks like when use 0, it looks almost correct.

rudolfninja gravatar imagerudolfninja ( 2017-08-31 04:47:11 -0500 )edit

I've added additional check: end_angle = end_angle == 0.0 ? 360.0 : end_angle; and it solved the problem with red arc and now everything looks correct. Now the following question: autocad coordinates start from left bottom corner while openCV from left top. So, to transofrm autocad coordinates to openCV I use following formula: opencv.x = autocad.x; opencv.y = H - autocad.y; where H is matrix height. However in this case arcs has incorrect angles. How should I transform angles in this case?

rudolfninja gravatar imagerudolfninja ( 2017-08-31 05:03:06 -0500 )edit

maybe flipping image will help. see updated answer.

sturkmen gravatar imagesturkmen ( 2017-08-31 06:41:54 -0500 )edit

flipping change the position of the other points. But actually, it is not so importatnt for me.I still don't understand why we use zero angle instead of end_angle - start_angle. I've got another cad file with a lot of arcs where they doesn't shown correct using this way of drawing. Should I create new answer to discuss the problem with this cad file? I'd like to get common algorithm of drawing that works for every arc.

rudolfninja gravatar imagerudolfninja ( 2017-08-31 06:54:22 -0500 )edit
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Asked: 2017-08-30 16:36:44 -0500

Seen: 1,028 times

Last updated: Aug 31 '17