AI in demo sequences goes wrong

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Watch the demo attract sequences of some games - the AI will often get stuck, 
crash, die, shoot at nothing etc., which doesn't occur on real hardware.

I've noticed this particularly on:

Sonic Adventure :- On the Casino level particularly, Sonic will repeatedly 
commit suicide by jumping off a side ledge until running out of lives, where 
the game over screen appears (during a demo!). At other times the AI will get 
stuck on scenery and the demo won't end.

Red Dog :- Often the AI will try to drive through walls and shoot at nothing, 
while getting hammered by the enemies.

Xtreme Sports :- Often the AI crashes and stops dead once remounted, or gets 
stuck on scenery and the demo won't end.

There's a video on this subject for Border Down:
http://www.youtube.com/watch?v=6WU_X2zD0LY
It specifically mentions Makaron and Chankast, but a comment also mentions 
incorrect behaviour in nullDC.

Tested on r78, GDI versions.

Original issue reported on code.google.com by matbur...@gmail.com on 14 Oct 2010 at 1:02

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Sadly, there's not much we can do about this :/

Original comment by drkIIRaziel on 14 Oct 2010 at 8:27

• Added labels: Type-Defect
• Removed labels: Type-Enhancement

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Well, i had the same problem with SA2 demo. Sonic's AI was wrong in City Escape

Original comment by proletar...@gmail.com on 14 Oct 2010 at 2:37

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Raz, can you explain a little why it happens, and why it can't be fixed please? 
This is an interesting one.

Original comment by matbur...@gmail.com on 14 Oct 2010 at 7:53

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In most games it is a precision error on the FPU. The dreamcast fpu is not as 
accurate as the x86 one on many operation. As these small precision errors 
add-up a big difference happens somewhere.

Other causes are also possible, however this is the most likely one for games 
that use the fpu ...

Making an 100% accurate fpu is over 30x slowdown

Original comment by drkIIRaziel on 14 Oct 2010 at 8:03

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Interesting, so modern CPUs are *too* accurate!

Thanks for the explanation.

Original comment by matbur...@gmail.com on 14 Oct 2010 at 10:15

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Original comment by drkIIRaziel on 16 Oct 2010 at 11:14

• Added labels: emu-sh4

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I suppose this is the video meant at first:
http://www.youtube.com/watch?v=ZOKZZqCGQnY

Here's a new one I just finished uploading:
http://www.youtube.com/watch?v=nyQZu-teHN8

Original comment by KrossX3 on 1 Dec 2010 at 7:27