# cv::error: (-215:Assertion failed) npoints >= 0 && npoints == std::max(1, (int)objectPoints.size()) in function 'cv::fisheye::estimateExtrinsics'

- **ID:** `opencv/calib3d-fisheye-estimate-extrinsics-invalid`
- **Domain:** opencv
- **Category:** assertion_error
- **Verification:** ai_generated
- **Fix Rate:** 86%

## Root Cause

The number of image points (npoints) passed to fisheye::estimateExtrinsics does not match the number of object points, or objectPoints vector is empty.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| opencv-4.5.5 | active | — | — |
| opencv-4.8.0 | active | — | — |
| opencv-4.9.0 | active | — | — |

## Workarounds

1. **Ensure imagePoints and objectPoints have the same size: assert(imagePoints.size() == objectPoints.size()); Then call: cv::fisheye::estimateExtrinsics(objectPoints, imagePoints, K, D, rvec, tvec);** (90% success)
   ```
   Ensure imagePoints and objectPoints have the same size: assert(imagePoints.size() == objectPoints.size()); Then call: cv::fisheye::estimateExtrinsics(objectPoints, imagePoints, K, D, rvec, tvec);
   ```
2. **If using a chessboard, use cv::findChessboardCorners to get imagePoints and generate objectPoints with cv::fisheye::calcChessboardCorners. Example: std::vector<cv::Point3f> objp; cv::fisheye::calcChessboardCorners(cv::Size(9,6), 1.0f, objp); Then match sizes.** (85% success)
   ```
   If using a chessboard, use cv::findChessboardCorners to get imagePoints and generate objectPoints with cv::fisheye::calcChessboardCorners. Example: std::vector<cv::Point3f> objp; cv::fisheye::calcChessboardCorners(cv::Size(9,6), 1.0f, objp); Then match sizes.
   ```
3. **Debug by printing sizes before the call: std::cout << 'obj size: ' << objectPoints.size() << ' img size: ' << imagePoints.size() << std::endl; If they differ, pad or trim the larger vector to match.** (80% success)
   ```
   Debug by printing sizes before the call: std::cout << 'obj size: ' << objectPoints.size() << ' img size: ' << imagePoints.size() << std::endl; If they differ, pad or trim the larger vector to match.
   ```

## Dead Ends

- **Use cv::fisheye::calibrate instead of estimateExtrinsics** — calibrate is for full calibration, not for single-view extrinsic estimation. It requires multiple views and returns different outputs. (85% fail)
- **Set npoints to a large constant like 100** — npoints must be exactly the size of the imagePoints vector. A hardcoded value will always mismatch unless coincidentally correct. (95% fail)
- **Convert objectPoints to a vector of cv::Point3f instead of cv::Point3d** — The assertion checks size, not type. Type mismatch causes a different error, but size mismatch persists. (70% fail)