From: mike@zogwarg.etl.army.mil (Mike McDonnell)
Newsgroups: comp.infosystems.gis
Subject: Re: Ray tracing and imagery enhanced terrain modeling.
Date: 16 Nov 92 15:11:35 GMT
Organization: Engineer Topographic Labs, Ft. Belvoir, VA

aearl@afit.af.mil (Arthur J. Earl) writes:

>Subject: Ray tracing and imagery enhanced terrain modeling.

>Problem:
>I now what to map images on to a terrain model.  Are there any puplic 
>domain programs or references on performing ray tracing on to 3d surfaces?  
>Has anyone else approached a similar problem.  (aearl@afit.af.mil)

There is a better solution.  I have written code implementing Philip
K. Robertson's scanline perspective technique.  A standalone demo
including data can be ftp'ed from eeyore.tec.army.mil.  This code is
very fast on general computers (i.e. without "graphics accelerator"
hardware) and is public domain. The code is available in
pub/drape.tar.Z.  This code needs a Unix system with X Windows.
That's all.

>Purpose:  During Desert Storm I was in charge of an ad hoc cell that analyzed
>imagery for the Commander of the 1st Infantry Division.  There was a time lag 
>in acquiring mosaic photos of the battlefield.  However, we received video tapes
>quickly.  I decided that creating a mosaic from the video tapes would solve 
>the time delay problem in subsequent conflicts.  My goal is to provide timely
>and accurate imagery intelligence to front line commanders using currently available imagery assets. A spin off from this research will be to add imagery 
>to computer simulations making them more realistic.

I work for the Army.  Let's talk.

>Backgound:
>I am working on projecting video images on to a terrain model.
>I have a program that creates a polygonal terrain model from digital terrain
>elevation data.  I also have a video tape of the terrain model area.  I have
>grabbed the images from the video tape.  I have all the information to map
>each pixel to the ground coordinates.  The video was produced by a 
>stabilized camera in an Remotely Piloted Vehicle.  Since the field of 
>view of the camera system is small, the altitude the aircraft flies is
>low, and most images are low obliques, I can assume away the curvature of
>the earth.

There are scanline methods for image rectification too.  See George
Wolberg's book "Digital Image Warping" available from IEEE press.

>Hardware & Software:  I am primarily using Silicon Graphics Systems. 

But they don't exist in the field.  Last time I benchmarked my code it
was faster than SGI's method and I don't use any graphics hardware support.

>Research Thus Far:
>So far I've researched the following subject areas to 
>find a solution: Photogrammetry, Computer Graphics, Computer Simulation, and Terrain Modeling.

>Development Thus Far: 
>I can capture the images from video tape and convert them to almost any
>format.  I have attempted to rectify the images (convert them to an overhead
>view) using the texture mapping and transformations.  However, this assumes the earth is a flat surface and I have run into a bunch of other assorted problems (e.g. Correcting for the change in scale as I progress radially from the 
>center of the image to the outside edges).  I now believe Ray Tracing is 
>the way to go.

No.  You need a true orthorectification using photogrammetry.  This is
not easy to understand or to calculate, but it is the only solution.
Ray tracing is too compute intensive for this application, and is
unnecessary anyway.

>*The real world terrain is not flat it is a polygonal representation of the
>Earth's surface.

>I am a masters student at the Air Force Institute of Technology.
>My thesis work requires me to create an imagery enhanced terrain model.

>THANKS FOR ANY ASSISTANCE YOU CAN RENDER.

Get in touch.

>Art Earl, Captain U.S. Army, AFIT Graduate Student, Department of Engineering Sciences, aearl@afit.af.mil
Comments: Gated by NETNEWS@AUVM.AMERICAN.EDU