How do I initialize the t-variables in “A Fast Voxel Traversal Algorithm for Ray Tracing”?

Question

I am trying to implement the algorithm explained on this paper, used to traverse grid cells in order following a straight line, which is useful for ray tracing:

http://www.cse.yorku.ca/~amana/research/grid.pdf

The paper describes the algorithm as two parts: initialisation and iterative traversal. I can undersand the iterative traversal part, but I'm having trouble understanding how some of the variables in the initialisation part are calculated.

I need help initialising tMaxX, tMaxY, tDeltaX & tDeltaY. Their initialisation procedure is explained as follows:

Next, we determine the value of t at which the ray crosses the first vertical voxel boundary and store it in variable tMaxX. We perform a similar computation in y and store the result in tMaxY. The minimum of these two values will indicate how much we can travel along the ray and still remain in the current voxel.

Finally, we compute tDeltaX and tDeltaY. TDeltaX indicates how far along the ray we must move (in units of t) for the horizontal component of such a movement to equal the width of a voxel. Similarly, store in tDeltaY the amount of movement along the ray which has a vertical component equal to the height of a voxel.

I'm not able to deduce the code I need form the English description given above. Can someone translate it to a math/pseudocode expression for me?

Answer 1

Initialization for X-coordinate variables (the same for Y)

  DX = X2 - X1
  tDeltaX = GridCellWidth / DX
  tMaxX = tDeltaX * (1.0 - Frac(X1 / GridCellWidth)) 
  //Frac if fractional part of float, for example, Frac(1.3) = 0.3, Frac(-1.7)=0.3

Example:

  GridCellWidth, Height = 20
  X1 = 5, X2 = 105 
  Y1 = 5, Y2 = 55 
  DX = 100, DY  = 50
  tDeltaX = 0.2, tDeltaY = 0.4 
  tMaxX = 0.2 * (1.0 - 0.25) = 0.15  //ray will meet first vertical line at this param
  tMaxY = 0.4 * (1.0 - 0.25) = 0.3   //ray will meet first horizontal line at this param

We can see that first cell border will be met at parameter t = 0.15

Answer 2

The one that worked for me in 3D for both positive and negative directions (in CUDA C):

#define SIGN(x) (x > 0 ? 1 : (x < 0 ? -1 : 0))
#define FRAC0(x) (x - floorf(x))
#define FRAC1(x) (1 - x + floorf(x))

float tMaxX, tMaxY, tMaxZ, tDeltaX, tDeltaY, tDeltaZ;
int3 voxel;

float x1, y1, z1; // start point   
float x2, y2, z2; // end point   

dx = SIGN(x2 - x1);
if (dx != 0) tDeltaX = fmin(dx / (x2 - x1), 10000000.0f); else tDeltaX = 10000000.0f;
if (dx > 0) tMaxX = tDeltaX * FRAC1(x1); else tMaxX = tDeltaX * FRAC0(x1);
voxel.x = (int) x1;

int dy = SIGN(y2 - y1);
if (dy != 0) tDeltaY = fmin(dy / (y2 - y1), 10000000.0f); else tDeltaY = 10000000.0f;
if (dy > 0) tMaxY = tDeltaY * FRAC1(y1); else tMaxY = tDeltaY * FRAC0(y1);
voxel.y = (int) y1;

int dz = SIGN(z2 - z1);
if (dz != 0) tDeltaZ = fmin(dz / (z2 - z1), 10000000.0f); else tDeltaZ = 10000000.0f;
if (dz > 0) tMaxZ = tDeltaZ * FRAC1(z1); else tMaxZ = tDeltaZ * FRAC0(z1);
voxel.z = (int) z1;

while (true) {
    if (tMaxX < tMaxY) {
        if (tMaxX < tMaxZ) {
            voxel.x += dx;
            tMaxX += tDeltaX;
        } else {
            voxel.z += dz;
            tMaxZ += tDeltaZ;
        }
    } else {
        if (tMaxY < tMaxZ) {
            voxel.y += dy;
            tMaxY += tDeltaY;
        } else {
            voxel.z += dz;
            tMaxZ += tDeltaZ;
        }
    }
    if (tMaxX > 1 && tMaxY > 1 && tMaxZ > 1) break;
    // process voxel here
}

Note, grid cell's width/height/depth are all equal to 1 in my case, but you can easily modify it for your needs.