Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: $Revision: 1.6.2.16 $; site datacube.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!ihnp4!ucbvax!decvax!yale!datacube!shep From: shep@datacube.UUCP Newsgroups: net.graphics Subject: Re: Re: Texture mapping Message-ID: <6700018@datacube.UUCP> Date: Sat, 6-Jul-85 12:42:00 EDT Article-I.D.: datacube.6700018 Posted: Sat Jul 6 12:42:00 1985 Date-Received: Sat, 13-Jul-85 11:33:34 EDT References: <809@turtlevax.UUCP> Lines: 50 Nf-ID: #R:turtlevax:-80900:datacube:6700018:000:2664 Nf-From: datacube!shep Jul 6 12:42:00 1985 In article <221@cmu-cs-h.ARPA> rfb@cmu-cs-h.ARPA (Rick Busdiecker) writes: >Does anyone have an algorithm they'd be willing to share for mapping a >raster image to an arbitrary polygon in space? How about a pointer to a >published algorithm? And Ken Turkowski @ CADLINC, Menlo Park, CA replies: >Texture mapping is basically a resampling problem. First you need to >generate a mapping from the image to the polygon, then you need to >invert it. For every point in the target, map it back into the source, >and apply a digital filter to the neighborhood of the source, so that >the target image meets the Nyquist criterion (Shannon sampling >theorem). For mapping a 2-d raster scan image onto a 2-d polygon in three space the "backward mapping" Turk describes is efficient. Furthermore, many types of mappings, i.e. quadratic division, are separable, as was noted. A most common use of this is the separated perspective backward mapping used in the Ampex Digital Optics television special effects device. It uses separate horizontal and vertical FIR interpolators to interpolate from several source pixels a single target pixel. A good description of this is in the US patent assigned to Ampex by architect Steve Gabriel and engineer Phil Bennett. General purpose hardware to perform these backward mappings in real-time will be available by year's end. "General purpose" implies that for every target pixel, there is a address in a "comes from" address store that holds the source location in 2-space for the target pixel. Although this is optimal for many applications, it does have it's pitfalls: Bi-cubic patches, a computationally efficient way of describing a surface map, do NOT separate in the general case. Backward mapping has the problem of usually selecting only one point in source space. Thus making it useless for many mappings. Enter "forward mapping"; where each source pixel is littered at different amplitudes in the target space frame accumulator. Forward mapping has destroyed many good hardware engineers and at least one company has made a product that shows exactly how -NOT- to do it! This mapping/warping stuff is real computationally intensive, usually because of the interpolation, sometimes in the address generation. Array processors take seconds to "warp" a 512*512*8*3 image. Thus for many applications, dedicated hardware is required. Shep Siegel ihnp4!datacube!shep Datacube Inc. ima!inmet!mirror!datacube!shep 617-535-6644 decvax!cca!mirror!datacube!shep 4 Dearborn Rd. decvax!genrad!wjh12!mirror!datacube!shep Peabody, Ma. 01960 {mit-eddie,cyb0vax}!mirror!datacube!shep