Negative row and column in terrain following algorithm - c++

Question

I'm trying to do terrain following, and I get a negative camera position in the xz plane. Now I get an out of boundary exception, because the row or the col is negative. How would I transform the cell of the grid to the origin correctly, giving negative camera coordinates.
Here is the two functions
int cGrid::getHeightmapEntry(int row, int col)
{
return m_heightmap[row * 300 + col];
}
float cGrid::getHeight(float x, float z, float _width, float _depth, int _cellSpacing)
{
// Translate on xz-plane by the transformation that takes
// the terrain START point to the origin.
x = ((float)_width / 2.0f) + x;
z = ((float)_depth / 2.0f) - z;
// Scale down by the transformation that makes the
// cellspacing equal to one. This is given by
// 1 / cellspacing since; cellspacing * 1 / cellspacing = 1.
x /= (float)_cellSpacing;
z /= (float)_cellSpacing;
// From now on, we will interpret our positive z-axis as
// going in the 'down' direction, rather than the 'up' direction.
// This allows to extract the row and column simply by 'flooring'
// x and z:
float col = ::floorf(x);
float row = ::floorf(z);
if (row < 0 || col<0)
{
row = 0;
}
// get the heights of the quad we're in:
//
// A B
// *---*
// | / |
// *---*
// C D
float A = getHeightmapEntry(row, col);
float B = getHeightmapEntry(row, col + 1);
float C = getHeightmapEntry(row + 1, col);
float D = getHeightmapEntry(row + 1, col + 1);
//
// Find the triangle we are in:
//
// Translate by the transformation that takes the upper-left
// corner of the cell we are in to the origin. Recall that our
// cellspacing was nomalized to 1. Thus we have a unit square
// at the origin of our +x -> 'right' and +z -> 'down' system.
float dx = x - col;
float dz = z - row;
// Note the below compuations of u and v are unneccessary, we really
// only need the height, but we compute the entire vector to emphasis
// the books discussion.
float height = 0.0f;
if (dz < 1.0f - dx) // upper triangle ABC
{
float uy = B - A; // A->B
float vy = C - A; // A->C
// Linearly interpolate on each vector. The height is the vertex
// height the vectors u and v originate from {A}, plus the heights
// found by interpolating on each vector u and v.
height = A + Lerp(0.0f, uy, dx) + Lerp(0.0f, vy, dz);
}
else // lower triangle DCB
{
float uy = C - D; // D->C
float vy = B - D; // D->B
// Linearly interpolate on each vector. The height is the vertex
// height the vectors u and v originate from {D}, plus the heights
// found by interpolating on each vector u and v.
height = D + Lerp(0.0f, uy, 1.0f - dx) + Lerp(0.0f, vy, 1.0f - dz);
}
return height;
}
float height = m_Grid.getHeight(position.x, position.y, 49 * 300, 49 * 300, 6.1224489795918367f);
if (height != 0)
{
position.y = height + 10.0f;
}
m_Camera.SetPosition(position.x, position.y, position.z);
bool cGrid::readRawFile(std::string fileName, int m, int n)
{
// A height for each vertex
std::vector<BYTE> in(m*n);
std::ifstream inFile(fileName.c_str(), std::ios_base::binary);
if (!inFile)
return false;
inFile.read(
(char*)&in[0], // buffer
in.size());// number of bytes to read into buffer
inFile.close();
// copy BYTE vector to int vector
m_heightmap.resize(n*m);
for (int i = 0; i < in.size(); i++)
m_heightmap[i] = (float)((in[i])/255)*50.0f;
return true;
}
m_Grid.readRawFile("castlehm257.raw", 50, 50);

Answer 1

I infer that you’re storing a 50 by 50 matrix inside a 300 by 300 matrix, to represent a grid of 49 by 49 cells. I also infer that m_Grid is an object of type cGrid. Your code appears to contain the following errors:
Argument(2) of call m_Grid.getHeight is not a z value.
Argument(3) of call m_Grid.getHeight is inconsistent with argument(5).
Argument(4) of call m_Grid.getHeight is inconsistent with argument(5).
Implicit cast of literal float to int in argument(5) of call m_Grid.getHeight - the value will be truncated.
Try changing your function call to this:
float height = m_Grid.getHeight(position.x, position.z, 49 * cellspacing, 49 * cellspacing, cellspacing);
-- where cellspacing is as defined in your diagram.
Also, try changing parameter(5) of cGrid::getHeight from int _cellSpacing to float _cellSpacing.
(I have edited this answer a couple of times as my understanding of your code has evolved.)