Python OpenCV仅在外部边框上绘制轮廓

Question

When drawing a contour using OpenCV's drawContours the borders is drawn centered to the contour, I want to draw the border only on the outside of the contour.

This image (taken from the SketchUp documentation) explains it best:

drawContours draws the contour like in the first circle (the contour is in the middle of the drawn border). I need to have the border only on the outside of the contour, like in the last circle.

Anyone have an idea as to how can I achieve this kind of behaviour?

Thanks.

Answer 1

use the code as

  _ret, contours, hierarchy = cv2.findContours(threshold, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE) 
  cv2.drawContours(img,contours , -1, (255,0,0), 1)

here cv2.RETR_EXTERNAL gives only the external detected contour.

Answer 2

Assuming that the color of the inside core is always homogeneous, and you know the value of the core color beforehand, we can simply do it as:

#First you draw the contour on both the sides of the border.
contour_id = 0
border_thickness = 10
border_color = (185, 115, 72)
cv2.drawContours(img, contours, contour_id, border_color, border_thickness)

#Now you again draw contour but with thickness = -1 and color = Core color
border_thickness = -1
core_color = (225, 141, 98)
cv2.drawContours(img, contours, contour_id, core_color, border_thickness)