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Bilinear Interpollation

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Hi, I've written a function to find the height of a particle based on its position on a quad defined by 4 points. we know the xyz of all four points, and the xz of the point. Here's the code:
void GetTileHeight(float x, float z)
	char buffer[200];
	float heightO = Height(g_HeightMap, x, z );
	float points[4][3];		//four vercies making up the patch
	float ABDifference;		//Difference in height between AB and CD
	float CDDifference;
	int i;
	double intx;			//integer parts of each coordinate
	double intz;
	float floatx;			//and float parts of each coordinate
	float floatz;
	float u,v;				//Represent ratio of tile the point is at (float between 1 and 0)
	float interp1[2];		//stores the start and end height of the first interpollated line
	float interp1Diff;		//difference in height between the two heights of the first interp
	float Change;
	float FinalPoint;		//the final y value of the equation

	//Record the four points making up the patch as a set of coordinates
   ///¦    ¦  
   ///¦    ¦ 
	//Point A (Bottom Left)
	points[0][0] = x;
	points[0][1] = Height(g_HeightMap, x, z );
	points[0][2] = z;

	//Point B (Bottom Right)
	points[1][0] = x;
	points[1][1] = Height(g_HeightMap, x, z + STEP_SIZE);
	points[1][2] = z + STEP_SIZE;	

	//Point C (Top Left)
	points[2][0] = x + STEP_SIZE;
	points[2][1] = Height(g_HeightMap, x + STEP_SIZE, z );
	points[2][2] = z;	

	//Point D (Top Right)
	points[3][0] = x + STEP_SIZE;
	points[3][1] = Height(g_HeightMap, x + STEP_SIZE, z + STEP_SIZE );
	points[3][2] = z + STEP_SIZE;

	//Now we find the position on the tile as an expression between 0.0 and 1.0 in the x and z

	//Find the integer and float parts of the coordinates
	floatx = modf(x, &intx);
	floatz = modf(z, &intz);

	//Shrink the intx variable down to being in terms of a single tile
while(intx>16)intx -= STEP_SIZE;
	x = intx+floatx;  //Add to give the distance of the point from the origin of tile 

	//Ditto for z
while(intz>16)	intz -= STEP_SIZE;
	z = intz+floatz; 
	u = x/STEP_SIZE;	//u and v are floats between 0.0 and 1.0 that define where the point is
	v = z/STEP_SIZE;    //in terms of the width/height of the tile

//So now we find the difference in height between AB and CD 

	ABDifference = ( points[1][1] -  points[0][1]); //Change in y going from A to B
	CDDifference = ( points[3][1] -  points[2][1]); //Change in y going from C to D

	//Now we find the height at u by adding the starting point plus u* the difference
	//this gives us a point along lines AB and CD.  We use the above method with v to
	//find the height at point u,v

	interp1[0] = points[0][1] + (ABDifference *= u);
	interp1[1] = points[2][1] + (CDDifference *= u);

	//This gives the change in height between the two interp1 points
	interp1Diff = interp1[1] - interp1[0];

	FinalPoint = interp1[0] + (interp1Diff *= v);
	User->position[1] = FinalPoint;
I've followed the math through on paper and it looks sound, can anyone explain why this doesn't work?

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Have you tried stepping through the code line by line to see if you're getting correct values? The use of fmod and the accompanying code looks suspicious. I might try condensing all that into something like this:

u = (x - (int)(x / STEP_SIZE) * STEP_SIZE) / STEP_SIZE;
v = (z = (int)(z / STEP_SIZE) * STEP_SIZE) / STEP_SIZE;

The rest of code looks alright to me, so the problem could be somewhere else. Maybe the Height function isn't doing something right?

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I've done a run through with liberal message boxes....

Point 1:

Point 2:

Point 3:

Point 4:

Player Coords x, z = 5.76, 40.759998
xFloat = 0.76 xInt = 5
zFloat = 0.7998 zInt = 40

Coords in term of current tile: 5.76, 8.759998

U = 0.36, v = 0.5475

AB Diff = 1 CDDiff = 2

Interp1[0] = 124.36 Interp[1] = 118.72
interp1diff = -5.64

Final Point = 124.36 + (-5.64 * 0.5475)
= 121.2721

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