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Mail Archives: djgpp/1998/04/08/10:55:53

Date: Wed, 8 Apr 1998 16:55:17 +0200
From: Hans-Bernhard Broeker <broeker AT physik DOT rwth-aachen DOT de>
Message-Id: <199804081455.QAA29048@acp3bf.physik.rwth-aachen.de>
To: ShawnH AT Probe DOT co DOT uk (Shawn Hargreaves)
Cc: djgpp AT delorie DOT com
Subject: Re: What is Z-sorting?
Newsgroups: comp.os.msdos.djgpp
Organization: RWTH Aachen, III. physikalisches Institut B

> >> Lately I have been hering about somthing called Z-Sorting. What is
> >> it?

> > Z-buffering is an algorithm used to remove hidden surfaces when
> > displaying 3d surfaces.  The x and y coordinates (plotted on the 
[...]

> You just described the depth sort algorithm, also known as a z-sort: 
> this is a totally different thing to a zbuffer.

Isn't it funny how many interpretations such a seemingly innocent
question can have? Original poster asked about z-*sorting*. Got an
answer that deals with z-*buffering*, and that answer even happens to
mean the wrong thing by that name. Instead, what he called
'z-buffering' is claimed to be 'z-*sorting*' now. 

(And by my book, it's neither of them, but actually closer to
zbuffering with the order of operations mixed up)

> Depth sorting is a very crude algorithm, where all the polygons in
> a scene are sorted by distance from the camera and then drawn from
[...]
> but there are some cases that it just can't handle correctly, where
> polygons intersect or overlap in some interleaved patterns.

Just for the sake of correctness: this isn't the whole truth on
depth-sorting. Actually, you *can* make depth-sorting correct, it just
takes a bit more effort to get it right (been there, done that).  Take
Quake, as an example: AFAIK some of it uses BSP-trees, which can be
viewed just as a clever approach to depth-sorting.

And there are situation where you *need* one property of depth-sorting
that is nearly impossible to do with z-buffering: the result of
depth-sorting is still a set of polygons, not pixels, so you can use a
vector graphic device or file format (postscript, e.g.) for output.
That's why I (have to) stick to it in 'gnuplot'...

BTW: To make the difference between the 'naive' implementation and
full-fledged depth-sorting clear, the naive one is often called
'painter's algorithm', instead.

> A zbuffer is a 100% accurate sorting method, where you store the 
> distance to each pixel as it is drawn, so that subsequent pixels can

I have to raise a small objection to this: zbuffer is not '100%'
accurate.  It's exactly as accurate as the the pixel resolution you
happen to apply it to. And as z-values are only stored at limited
precision (often 16 bit fixed point), it may well be *less* precise
than a well-though-out depth sorting algorithm that uses the full
(floating point) precision of the geometric input.

> For a proper discussion of things like this, comp.graphics.algorithms
> is the place to go.

I second that.


--
Hans-Bernhard Broeker (broeker AT physik DOT rwth-aachen DOT de)
Even if all the snow were burnt, ashes would remain.

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