/**
* SurfaceNets in JavaScript
*
* Written by Mikola Lysenko (C) 2012
*
* MIT License
*
* Based on: S.F. Gibson, 'Constrained Elastic Surface Nets'. (1998) MERL Tech Report.
* from https://github.com/mikolalysenko/isosurface/tree/master
*
*/
let surfaceNet = ( dims, potential, bounds ) => {
//Precompute edge table, like Paul Bourke does.
// This saves a bit of time when computing the centroid of each boundary cell
var cube_edges = new Int32Array(24) , edge_table = new Int32Array(256);
(function() {
//Initialize the cube_edges table
// This is just the vertex number of each cube
var k = 0;
for(var i=0; i<8; ++i) {
for(var j=1; j<=4; j<<=1) {
var p = i^j;
if(i <= p) {
cube_edges[k++] = i;
cube_edges[k++] = p;
}
}
}
//Initialize the intersection table.
// This is a 2^(cube configuration) -> 2^(edge configuration) map
// There is one entry for each possible cube configuration, and the output is a 12-bit vector enumerating all edges crossing the 0-level.
for(var i=0; i<256; ++i) {
var em = 0;
for(var j=0; j<24; j+=2) {
var a = !!(i & (1<<cube_edges[j]))
, b = !!(i & (1<<cube_edges[j+1]));
em |= a !== b ? (1 << (j >> 1)) : 0;
}
edge_table[i] = em;
}
})();
//Internal buffer, this may get resized at run time
var buffer = new Array(4096);
(function() {
for(var i=0; i<buffer.length; ++i) {
buffer[i] = 0;
}
})();
if(!bounds) {
bounds = [[0,0,0],dims];
}
var scale = [0,0,0];
var shift = [0,0,0];
for(var i=0; i<3; ++i) {
scale[i] = (bounds[1][i] - bounds[0][i]) / dims[i];
shift[i] = bounds[0][i];
}
var vertices = []
, faces = []
, n = 0
, x = [0, 0, 0]
, R = [1, (dims[0]+1), (dims[0]+1)*(dims[1]+1)]
, grid = [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
, buf_no = 1;
//Resize buffer if necessary
if(R[2] * 2 > buffer.length) {
var ol = buffer.length;
buffer.length = R[2] * 2;
while(ol < buffer.length) {
buffer[ol++] = 0;
}
}
//March over the voxel grid
for(x[2]=0; x[2]<dims[2]-1; ++x[2], n+=dims[0], buf_no ^= 1, R[2]=-R[2]) {
//m is the pointer into the buffer we are going to use.
//This is slightly obtuse because javascript does not have good support for packed data structures, so we must use typed arrays :(
//The contents of the buffer will be the indices of the vertices on the previous x/y slice of the volume
var m = 1 + (dims[0]+1) * (1 + buf_no * (dims[1]+1));
for(x[1]=0; x[1]<dims[1]-1; ++x[1], ++n, m+=2)
for(x[0]=0; x[0]<dims[0]-1; ++x[0], ++n, ++m) {
//Read in 8 field values around this vertex and store them in an array
//Also calculate 8-bit mask, like in marching cubes, so we can speed up sign checks later
var mask = 0, g = 0;
for(var k=0; k<2; ++k)
for(var j=0; j<2; ++j)
for(var i=0; i<2; ++i, ++g) {
var p = potential(
scale[0]*(x[0]+i)+shift[0],
scale[1]*(x[1]+j)+shift[1],
scale[2]*(x[2]+k)+shift[2]);
grid[g] = p;
mask |= (p < 0) ? (1<<g) : 0;
}
//Check for early termination if cell does not intersect boundary
if(mask === 0 || mask === 0xff) {
continue;
}
//Sum up edge intersections
var edge_mask = edge_table[mask]
, v = [0.0,0.0,0.0]
, e_count = 0;
//For every edge of the cube...
for(var i=0; i<12; ++i) {
//Use edge mask to check if it is crossed
if(!(edge_mask & (1<<i))) {
continue;
}
//If it did, increment number of edge crossings
++e_count;
//Now find the point of intersection
var e0 = cube_edges[ i<<1 ] //Unpack vertices
, e1 = cube_edges[(i<<1)+1]
, g0 = grid[e0] //Unpack grid values
, g1 = grid[e1]
, t = g0 - g1; //Compute point of intersection
if(Math.abs(t) > 1e-6) {
t = g0 / t;
} else {
continue;
}
//Interpolate vertices and add up intersections (this can be done without multiplying)
for(var j=0, k=1; j<3; ++j, k<<=1) {
var a = e0 & k
, b = e1 & k;
if(a !== b) {
v[j] += a ? 1.0 - t : t;
} else {
v[j] += a ? 1.0 : 0;
}
}
}
//Now we just average the edge intersections and add them to coordinate
var s = 1.0 / e_count;
for(var i=0; i<3; ++i) {
v[i] = scale[i] * (x[i] + s * v[i]) + shift[i];
}
//Add vertex to buffer, store pointer to vertex index in buffer
buffer[m] = vertices.length;
vertices.push(v);
//Now we need to add faces together, to do this we just loop over 3 basis components
for(var i=0; i<3; ++i) {
//The first three entries of the edge_mask count the crossings along the edge
if(!(edge_mask & (1<<i)) ) {
continue;
}
// i = axes we are point along. iu, iv = orthogonal axes
var iu = (i+1)%3
, iv = (i+2)%3;
//If we are on a boundary, skip it
if(x[iu] === 0 || x[iv] === 0) {
continue;
}
//Otherwise, look up adjacent edges in buffer
var du = R[iu]
, dv = R[iv];
//Remember to flip orientation depending on the sign of the corner.
if(mask & 1) {
faces.push([buffer[m], buffer[m-du], buffer[m-dv]]);
faces.push([buffer[m-dv], buffer[m-du], buffer[m-du-dv]]);
} else {
faces.push([buffer[m], buffer[m-dv], buffer[m-du]]);
faces.push([buffer[m-du], buffer[m-dv], buffer[m-du-dv]]);
}
}
}
}
//All done! Return the result
return { positions: vertices, cells: faces };
}
export { surfaceNet }