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From: AER6101@TECHNION.BITNET
Newsgroups: comp.graphics
Subject: Re: Z Buffer for Amiga
Message-ID: <90305.160417AER6101@TECHNION.BITNET>
Date: 2 Nov 90 08:13:53 GMT
References: <1157@iceman.jcu.oz> <1990Oct30.201240.24479@assari.tut.fi>
Organization: TECHNION - Israel Institute of Technology.
Lines: 43

I am presently working with 3-D scene reconstruction from a sequence of

images. The method I am using is based on corner matching between a pair

of consecutive images. The output is the estimated depth at the corner

pixels. The images are rendered by a perspective projection of 3-D blocks

whose vertices are supplied by me as input to the program. However, the

detected corners are not necessarily close to those vertices. In order to

obtain a measurement of the accuracy of the algorithm I am using, the actual

depth at that pixel is needed and I tried to recover it from the z-buffer.

I thought that (my station is a SilliconGraphics 4D-GT) the z-buffer values

(between 0 and 0x7fffff) were linearly mapped to the world z-coordinates

between the closest and farthest planes used in the perspective projection

procedure available in the Sillicon's graphic library.

The results however don't match the above hypothesis. I tested the values

of the z-buffer obtained when viewing a plane at a known depth and it was

clear that the relation was not linear. Can someone enlighten me about

how the z-buffer values are related to actual depth? I know there is

a clipping transformation that transforms the perspective pyramid into

a -1<x,y,z<1 cube, but perhaps I am missing something else. If anybody

has an opinion or reference that could help me I would be very pleased

to receive it in my E-mail (aer6101@technion.bitnet). I would summarize

the received answers and post a message with the conclusions.

Thanking you in advance,        jacques-