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I came across with this comment in the compute code:

/*
 * Compute the descriptors for a set of keypoints in an image.
 * image        The image.
 * keypoints    The input keypoints. Keypoints for which a descriptor cannot be computed are removed.
 * descriptors  Copmputed descriptors. Row i is the descriptor for keypoint i.
 */
CV_WRAP void compute( const Mat& image, CV_OUT CV_IN_OUT vector<KeyPoint>& keypoints, CV_OUT Mat& descriptors ) const;

Pay attention to the line after keypoints. It says keypoints are removed if no descriptor is found. I think this answer the question of why they are zeroed.

I came across with this comment in the compute compute code:

/*
 * Compute the descriptors for a set of keypoints in an image.
 * image        The image.
 * keypoints    The input keypoints. Keypoints for which a descriptor cannot be computed are removed.
 * descriptors  Copmputed descriptors. Row i is the descriptor for keypoint i.
 */
CV_WRAP void compute( const Mat& image, CV_OUT CV_IN_OUT vector<KeyPoint>& keypoints, CV_OUT Mat& descriptors ) const;

Pay attention to the line after keypoints. It says keypoints are removed if no descriptor is found. I think this answer the question of why they are zeroed.

I came across with this comment in the compute code:

/*
 * Compute the descriptors for a set of keypoints in an image.
 * image        The image.
 * keypoints    The input keypoints. 
 *              Keypoints for which a descriptor cannot be computed are removed.
 * descriptors  Copmputed descriptors. Row i is the descriptor for keypoint i.
 */
CV_WRAP void compute( const Mat& image, CV_OUT CV_IN_OUT vector<KeyPoint>& keypoints,  CV_OUT Mat& descriptors ) const;

Pay attention to the line after keypoints. It says keypoints are removed if no descriptor is found. I think this answer the question of why they are zeroed.