-215 opencv assertion_error ai_generated true

cv::error: (-215:Assertion failed) count >= 0 in function 'cv::approxPolyDP'

ID: opencv/contour-approximation-empty

Also available as: JSON · Markdown · 中文
85%Fix Rate
82%Confidence
1Evidence
2023-03-22First Seen

Version Compatibility

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

Root Cause

approxPolyDP received an empty or invalid contour (zero points) because the input contour was not properly extracted or was filtered out.

generic

中文

approxPolyDP 接收到空或无效的轮廓（零个点），因为输入轮廓未被正确提取或被过滤掉。

Official Documentation

https://docs.opencv.org/4.x/dd/d49/tutorial_py_contour_features.html

Workarounds

  1. 95% success Add a guard clause before calling approxPolyDP: if len(contour) == 0: continue. Example: for cnt in contours: if cnt.shape[0] < 3: continue; approx = cv2.approxPolyDP(cnt, epsilon, True) 
    Add a guard clause before calling approxPolyDP: if len(contour) == 0: continue. Example: for cnt in contours: if cnt.shape[0] < 3: continue; approx = cv2.approxPolyDP(cnt, epsilon, True)
  2. 90% success Use cv2.contourArea to filter out contours with zero area before approximation: if cv2.contourArea(cnt) > 0: approx = cv2.approxPolyDP(cnt, 0.02 * cv2.arcLength(cnt, True), True) 
    Use cv2.contourArea to filter out contours with zero area before approximation: if cv2.contourArea(cnt) > 0: approx = cv2.approxPolyDP(cnt, 0.02 * cv2.arcLength(cnt, True), True)
  3. 80% success Ensure findContours is called with the correct mode (e.g., RETR_TREE or RETR_LIST) and that the hierarchy is not corrupt; sometimes contour[0] can be empty if the image is all black 
    Ensure findContours is called with the correct mode (e.g., RETR_TREE or RETR_LIST) and that the hierarchy is not corrupt; sometimes contour[0] can be empty if the image is all black

中文步骤

  1. 在调用 approxPolyDP 前添加守卫检查：if len(contour) == 0: continue。示例：for cnt in contours: if cnt.shape[0] < 3: continue; approx = cv2.approxPolyDP(cnt, epsilon, True)
  2. 使用 cv2.contourArea 过滤面积为零的轮廓：if cv2.contourArea(cnt) > 0: approx = cv2.approxPolyDP(cnt, 0.02 * cv2.arcLength(cnt, True), True)
  3. 确保 findContours 使用正确的模式（如 RETR_TREE 或 RETR_LIST），并且层次结构未损坏；如果图像全黑，轮廓[0]可能为空

Dead Ends

Common approaches that don't work:

  1. 40% fail

    Increasing the epsilon parameter to make the approximation less strict, which only masks the issue but doesn't fix empty contours

  2. 30% fail

    Re-running findContours with different retrieval modes (e.g., RETR_EXTERNAL) without checking if contours exist

  3. 50% fail

    Assuming the contour is always valid and skipping the empty check entirely