Why does number of contours differ on picture resolution

findContours

asked 2016-01-28 10:40:00 -0600

Polyquin
21 ●3

updated 2020-10-10 14:47:12 -0600

sturkmen

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

Hi all,

Quick question on findContours. I ran findContours on a picture with resolution of 640 x 960 and findContours returned more contours. I resized the same picture to 320x480 and it returned lesser contours.

I'm not sure why is this happening. I would have thought that the resolution is immaterial to findContour.

Any ideas?

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it depends on the object & the noise in the image & how you are applying threshold. Can you show us your code & results?

Balaji R ( 2016-01-28 11:17:26 -0600 )edit

You can use drawContours to see which contours are found. Often, when you look at the results the source of the problem becomes obvious. guy

Guyygarty ( 2016-01-28 13:31:35 -0600 )edit

Ok, I'll show the code and results shortly.

Polyquin ( 2016-01-28 21:17:15 -0600 )edit

you must show your code and the image.

sturkmen ( 2016-02-01 15:55:43 -0600 )edit

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answered 2016-02-01 15:20:16 -0600

Polyquin
21 ●3

Hi all,

I've figured out why the contours decrease when the image resolution is reduced. The reason is due to image interpolation.

If cv2.resize was used, there is a argument for the type of interpolation to be used:

interpolation method:

INTER_NEAREST - a nearest-neighbor interpolation
INTER_LINEAR - a bilinear interpolation (used by default)
INTER_AREA - resampling using pixel area relation. It may be a preferred method for image decimation, as it gives moire’-free results. But when the image is zoomed, it is similar to the INTER_NEAREST method.
INTER_CUBIC - a bicubic interpolation over 4x4 pixel neighborhood
INTER_LANCZOS4 - a Lanczos interpolation over 8x8 pixel neighborhood

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why you did not post the code and the image you tried?

sturkmen ( 2016-02-01 15:55:12 -0600 )edit

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Why does number of contours differ on picture resolution

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