-215 opencv assertion_error ai_generated true

cv::error: (-215:Assertion failed) hierarchy.checkVector(3, 4) >= 0 in function 'drawContours'

ID: opencv/contour-not-found-in-hierarchy

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80%Fix Rate
85%Confidence
1Evidence
2023-04-15First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
4.5.5 active — — —
4.6.0 active — — —
4.7.0 active — — —
4.8.0 active — — —

Root Cause

Contour hierarchy vector is empty or malformed when calling drawContours with RETR_TREE or RETR_CCOMP mode after findContours.

generic

中文

在调用 findContours 后使用 RETR_TREE 或 RETR_CCOMP 模式调用 drawContours 时，轮廓层次向量为空或格式错误。

Official Documentation

https://docs.opencv.org/4.x/d3/dc0/group__imgproc__shape.html#ga17ed9f5d79ae97bd4c7cf18403e1689a

Workarounds

  1. 85% success Check if hierarchy is not None and has at least 4 elements before calling drawContours. Example: `if hierarchy is not None and len(hierarchy) > 0: cv2.drawContours(img, contours, -1, (0,255,0), 2)` 
    Check if hierarchy is not None and has at least 4 elements before calling drawContours. Example: `if hierarchy is not None and len(hierarchy) > 0: cv2.drawContours(img, contours, -1, (0,255,0), 2)`
  2. 75% success Use RETR_LIST mode which returns no hierarchy, then draw without hierarchy parameter. 
    Use RETR_LIST mode which returns no hierarchy, then draw without hierarchy parameter.
  3. 90% success Ensure contours list is non-empty before drawing: `if len(contours) > 0: cv2.drawContours(img, contours, -1, (0,255,0), 2, hierarchy=hierarchy)` 
    Ensure contours list is non-empty before drawing: `if len(contours) > 0: cv2.drawContours(img, contours, -1, (0,255,0), 2, hierarchy=hierarchy)`

中文步骤

  1. Check if hierarchy is not None and has at least 4 elements before calling drawContours. Example: `if hierarchy is not None and len(hierarchy) > 0: cv2.drawContours(img, contours, -1, (0,255,0), 2)`
  2. Use RETR_LIST mode which returns no hierarchy, then draw without hierarchy parameter.
  3. Ensure contours list is non-empty before drawing: `if len(contours) > 0: cv2.drawContours(img, contours, -1, (0,255,0), 2, hierarchy=hierarchy)`

Dead Ends

Common approaches that don't work:

  1. 60% fail

    Setting RETR_EXTERNAL only retrieves outer contours, but hierarchy is still required for drawing with RETR_TREE. The error persists because hierarchy is empty.

  2. 50% fail

    Changing CHAIN_APPROX_NONE to other approximations doesn't fix hierarchy emptiness; it only changes point storage.

  3. 40% fail

    Increasing threshold values may reduce contour count but doesn't guarantee hierarchy is non-empty.