warp.hash_grid_point_id

warp.hash_grid_point_id(id: uint64, index: int32) → int

Kernel

Return the original point index stored at a given position in the warp.HashGrid’s spatially-sorted order.

The grid sorts its points by cell so that points sharing a cell are adjacent. Given a position index in that sorted order (typically the thread index), this returns the corresponding index into the original points array. Looking points up in this order makes neighboring threads access nearby points, improving memory locality.

Parameters:

• id – The warp.HashGrid identifier

• index – A position in the grid’s sorted order, in [0, number_of_points)

Returns:

The corresponding index into the original points array, or -1 if the warp.HashGrid has not been built/reserved.

Example

@wp.kernel
def gather(grid_id: wp.uint64, pts: wp.array[wp.vec3], out: wp.array[wp.vec3]):
    i = wp.tid()
    # visit points in the grid's cell-sorted order for memory locality;
    # hash_grid_point_id maps sorted position i -> the original point index
    out[i] = pts[wp.hash_grid_point_id(grid_id, i)]

points = wp.array([[0,0,0],[0.1,0,0],[0.5,0,0],[2.0,0,0]], dtype=wp.vec3)
grid = wp.HashGrid(dim_x=8, dim_y=8, dim_z=8)
grid.build(points=points, radius=0.3)

out = wp.zeros(4, dtype=wp.vec3)
wp.launch(gather, dim=4, inputs=[grid.id, points], outputs=[out])
# every point is visited exactly once, so the gather is a permutation of the input
print(sorted(out.numpy().tolist()) == sorted(points.numpy().tolist()))

True