Face alignment. The face alignment procedure for an example image. Based on the detected eye coordinates the position of the mouth is estimated (A). After rotating the facial image into an upright position (B), such that both eyes lie on a horizontal line, the left and the right corner of the mouth is estimated (C). Based on these four points a projective transformation is applied (D). This ensures that facial features like eyes, nose, and mouth are located approximately at the same positions throughout the entire dataset, which is a prerequisite for accurate identification.