Path: chuka.playstation.co.uk!news
From: Robert Swan <rs108@mdx.ac.uk>
Newsgroups: scee.yaroze.programming.3d_graphics
Subject: Re: 3D Terrain, Again
Date: Thu, 13 Aug 1998 14:15:06 +0100
Organization: I wish!
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Nathan Miller wrote:
> 
> ...with a focus on outside uneven terrains.
> Your help, again, will be greatly appreciated.

Ive got some collision detection going in my game based on an uneven
terrain, but you can really help yourself by limiting yourself to
certain things when creating the level (for example in mine the world is
basicaly a grid with each vertex raised or lowered. Then its a simple
enough case to find out the height at any particular point (included
code from adventure game - )

this assumes that my grid is made up of cells, each of which is 512x512
units large
Also >> 9 is division by 512 (which effectively takes a coordinate in
the world units, and turns it into a coordinate in cell units.


u_long calculteheight(u_long tx, u_long ty) // notice only handles
positive :) thats deliberate in my world
	{
	long	tCornerHeight[4];
	long	tHeight1, tHeight2;
	u_long	tCellX = tx>>9;
	u_long	tCellZ = (19-ty)>>9;
// 19-ty because the array holding the ground data (20,20) incidentally
starts from top left corner (when viewed from overhead) but the model
starts from the bottom left (when viewed from overhead) so merely -ve y
value

	HEIGHTSTRUCT	*start = (HEIGHTSTRUCT) Addressofheightarray;
// HEIGHTSTRUCT in my case is an array [21][21] large (there are 20x20
cells, but a height assigned at each corner, so 21x21 for height array

	tCornerHeight[0] = (start[20-tCellZ][tCellX]) << 4;
	tCornerHeight[1] = (start[20-tCellZ][tCellX+1]) << 4;
	tCornerheight[2] = (start[19-tCellZ][tCellX]) << 4;
	tCornerHeight[3] = (start[19-tCellZ][tCellX+1]) << 4;

	tHeight1 = CalcHeightBetween2points(tCornerHeight[0], tCornerHeight[1],
tX%512);
	tHeight2 = CalcHeightBetween2points(tCornerHeight[2], tCornerHeight[3],
tX%512);
	return -CalcHeightbetween2poitns(tHeight2,  tHeight1, tz%512);
	}

long	CalcHeightbetween2points(u_long tHeight1, u_long tHeight2, u_long
tOffset)
	{
	u_long tHeightMultiplier = (tHeight1 * (511-tOffset) + (tHeight2 *
tOffset);
	return (long) (theightMultiplier >> 9);
	}


Having read through that, it does look messy I know, but does work.
Basically it finds out the height at each corner of the current cell,
and returns a value which is based on all four corners, weighted more
heavily to those that you are closer. Its a pretty quick routine i
think...

Robert Swan
rs108@mdx.ac.uk