Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site rochester.UUCP Path: utzoo!linus!philabs!cmcl2!seismo!rochester!sher From: sher@rochester.UUCP (David Sher) Newsgroups: net.graphics Subject: Re: FFT of image in sections ? Message-ID: <10900@rochester.UUCP> Date: Tue, 6-Aug-85 18:15:25 EDT Article-I.D.: rocheste.10900 Posted: Tue Aug 6 18:15:25 1985 Date-Received: Thu, 8-Aug-85 01:08:13 EDT References: <360@ur-laser.uucp> Reply-To: sher@rochester.UUCP (David Sher) Organization: U. of Rochester, CS Dept. Lines: 25 Keywords: FFT convolution Summary: FFT only requires a row or collumn at a time. In article <360@ur-laser.uucp> nitin@ur-laser.uucp (Nitin Sampat) writes: ... >We know that processing small images takes less time. Well, how does >one go about breaking up a large image and process it in sections, >and then most importantly, how does one put all these sections back >to get the FFT of the original image ? > > nitin > {seismo,allegra}!rochester!ur-laser!nitin.uucp It turns out that on a 2d image applying FFT only requires that a row or collumn of data need be processed at a time. If the image is reflected along a diagonal between the row and collumn processing the paging should be minimized fairly well and the entire transformation is applied. Of course there is still a log n factor that is lost from the nature of the FT. However I would suspect that for the small images (1Kx1K) that are worked on today it would cost more to do the gluing operation that glues together the results of FFT's on partitions of the image than to do the entire FFT. On the topic of FFT vs convolution, my latest TR on doing template matching on the WARP and the Butterfly addresses this issue for these architectures. However it is not classy to push ones own work so enough said. -David Sher sher@rochester seismo!rochester!sher