Focus measures for SFF-inspired relative depth estimation

Abstract

Shape from Focus (SFF) is a method which recovers the 3D geometry of the scene based on a sequence of images taken from different focus distances between the camera and the object. Generally SFF techniques require parallel projection of the scene on to the image plane so that the corresponding pixels in the set of images taken are easily identified. This can be achieved by using a lens which does parallel projection such as a telecentric lens. Moreover the SFF method is widely applied for extremely small objects due to the limited range of magnification that can be maintained. This again is another manifestation of the fact depth of objects produce perspective shift (generally called as structure-dependent pixel motion) in the image plane. All these facts are applicable for situations which utilizes SFF for complete reconstruction of the scene. Applications involving shape information extracted from focus as a secondary cue need not require a complete dense reconstructed information from SFF. Such applications might allow usage of wide angle lenses where the projection is basically a perspective projection of the scene on to the image plane. The research work utilizes a wide angle lens for SFF based scene reconstruction consisting of a macroscopic object. The paper is an attempt to present 24 different focus measures used for quantifying image focus from which depth is interpolated using a standard function. Since the images in the sequence suffer from changes in magnification, finding the correspondence itself is an issue worth addressing. The pixel motion is tackled by a powerful corner detector and a robust matching algorithm. The knowledge of the right focus measure is very important since it is after all from the focus measure depth of the scene is interpolated. The focus measures presented in the paper are a collection from various applications such as microscopy imaging, auto focussing, holographic reconstructions etc., but applied to an image sequence containing variations in focus and magnification.