Itseez/opencv — pull request #2873 by jjgignac (2014-06-17 21:50:52+00:00)
Title: Facilitate computing SIFT descriptors one by one.

Some applications might want to only process as many SIFT features as
time permits before a new video frame becomes available. This change
enables a program to detect the keypoints, then compute descriptors one
by one without recalculating the image pyramid repeatedly.