Path: chuka.playstation.co.uk!news
From: alex@teeth.demon.co.uk (Alex Amsel)
Newsgroups: scee.yaroze.programming.3d_graphics
Subject: OT explanation: was Re: GsSortOt() function
Date: Tue, 29 Apr 1997 11:47:35 GMT
Organization: Into Beyond
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On 29 Apr 1997 01:58:52 GMT, "Michael Enoch"
<100413.2514@compuserve.com> did quoth at me:

>Hi, could someone explain how to use GsSortOt() better than the manuals
>explain it, I wan't to force an object to be drawn last.

Mike,

I've only just got up (!) so would probably explain everything
completely wrong, but if you are unsure just whack me an email and
I'll try and explain it when I'm more awake :)

Take that back...I'll try now, though I really have just stepped out
of bed - been installing CodeWright overnight!

To force an object to be drawn last, draw that object into a separate
OT defined as normal. So you have your main OT (used by DrawOT) and
then this special one. 

Then you need to 'insert' this OT into the main one. This is done by
assigning a mock Z value to OT.point in the GsOT structure. As you
know, each OT is basically like a set of marks on a ruler. Except
instead of one centimeter, they may mean 1 meter, or 1 mile, or who
knows what. If your OT is 256 long (OT_LENGTH = 8), then to insert
this OT as the last item you need to set OT.point as 255. DrawOT will
draw from the end of the OT to the start, and bob's your uncle. Or
something like that.

Also note that you should call GsSortOt to do this as the second last
command. The last being the ClearScreen one. Otherwise you could end
up with other distant objects (where their z = 255) being drawn over
your object.

It may help to know that OT's are basically just linked lists. I'll
explain...

An OT of length 4 units (OT_LENGTH = 2).

Unit 0

ptr->primitive 1
ptr->primitive 2
ptr->primitive 3
END_OT_TAG (something like a ptr to 7ffffff)

Unit 1
ptr->primitive 4
ptr->unit 0 

Unit 2
ptr->unit 1

Unit 3
ptr->primitive 5 [e.g. clear screen which is probably a square fill)
ptr->unit 2

DrawOT starts at the top of unit 3, and follows the list of pointers.
When a new primitive is inserted at any point, it adds it at the TOP
of the Unit. So the objects added to the OT later get a higher
priority for polygons at the same Z value was previous objects since
they are placed 'higher' in the linked list.

Sorry if this explanation is a bit cack. I'll try again if anyone
wants.

Anyway, this info is pretty useful to know when you start suffering
from polygons being flipped behind scenery (since you know which order
to draw your objects in if you know how the OT works).

P.S. Do Sony want to give me a tech support job?! Heard of PS-X jobs
available there ;) Except no office in Sheffield :(


* Alex Amsel * Into Beyond Web Design & JAVA Programming *
* http://www.intobeyond.com  *  WWFC Utter Rubbish 1996-7*
MM - "Steve Corica is every bit as good as that Kinkladze"