arcLength vs ContourArea?

asked 2017-11-10 23:45:59 -0600

Santhosh1
318 ●11 ●15 ●37

updated 2018-01-22 21:53:17 -0600

sturkmen

6772 ●3 ●48 ●79 https://github.com/stu...

I have detected these two objects after processing an image. as seen below

image description

    arcLength                      contourArea
 1000.08743619918                      128         
 883.317884206771                      128

I do understand the arc length function of the openCV, but what is this CONTOUR AREA? Why is this so less than the arc length?

answered 2017-11-11 03:49:10 -0600

matman
1001 ●6 ●16

Your contours are not closed. In this case the contour area is calculated as a line. By the way: the arc length of a 1px wide line is around twice as long as it should be. This is because of the algorithm used in findContours.

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@matman Yes I have used cv2.RETR_TREE in cv2.findContour. But still doesn't explain how the Contour Area is calculated I'm still confused 🤔

Santhosh1 ( 2017-11-11 04:26:00 -0600 )edit

Area is calculated using green formula (contourArea doc).

LBerger ( 2017-11-11 04:36:37 -0600 )edit

@LBerger Yeah, green formula requires the contour to be a closed loop. My confusion arises when the loop is not close, then how is the contourArea calculated?

Santhosh1 ( 2017-11-14 01:22:54 -0600 )edit

@matman How come the arc length is twice as long as it should be?

John M ( 2018-01-14 10:17:10 -0600 )edit

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