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# Terrain Collision

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What is the method used to determine the height of the ground is at a certain x,y point on a heightmap? I heard somewhere that you have to find what square in the grid an object is and then somehow average the height of those 4 points together to get the final height, is this correct?

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A gridcell consist out of 4 points. If you take the average height of each point, then you got the relative height of the gridcell itself.

A heightmap usually is a 8-bit bitmap, meaning every pixels can have a ranged value between 0 and 255. You could say to your engine, that value 127 is the sealevel for example and every value higher or lower is 1.0f or 100.0f in absolute z-coordinates higher or lower.
Every pixel in that bitmap could be a point in the gridfield.

I hope this is what would like to know, else feel free to ask.

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Basically yeah, that's the process. You get the 4 points for that cell and do a bilinear interpolation.

Here's my source for the meat of the algo:
	//the x and y coords are all either int(x) or int(x)+1	//use all 4 combos	//x0 <= x <= x1	//z0 <= z <= z1	int x0, z0, x1, z1;	x0 = int(x);	x1 = x0 + 1;	z0 = int(z);	z1 = z0 + 1;	//we are just slightly off the edge of the terrain if x1 or z1 is off the terrain	if( x0 < 0.0 || z0 < 0.0 )		return std::numeric_limits<float>::min();	if( x1 >= m_Width || z1 >= m_Height )		return std::numeric_limits<float>::min();	float h00, h01, h10, h11;	h00 = m_Heights[x0*m_Height+z0];	h01 = m_Heights[x0*m_Height+z1];	h10 = m_Heights[x1*m_Height+z0];	h11 = m_Heights[x1*m_Height+z1];	//NOTE: The faces are indexed like this:	//one tri is h00, h01, h10	//other tri is h01, h10, h11 --- NOT h00 in this tri	//This is important; most terrain engines appear to wind the verts along the other diagonal	//find distance from camera to x0,z0	float tx, ty;	tx = x - x0;	ty = z - z0;	//Now, how do we figure out the height from all this info?	//We could try a bilerp...	float txty = tx * ty;	float Height =  h00 * (1.0f - ty - tx + txty)					+ h10 * (tx - txty)					+ h11 * txty					+ h01 * (ty - txty);	return Height;

Fairly simple...get the four points, look up the heights, get the factors on x and z, and average them all together. If you work out the sum of the 4 factors, you'll find they add up to 1.

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Thanks guys, I got it now.

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