Sphere clouds

Particles of differing sizes, kept as a center and a radius each in arrays of the program’s own: the signed distance to their union, and the particle closest to a point, through a SphereCloudBVH (see Point clouds). The source is at Examples/SphereClouds/main.cpp.

The code that runs on either side

The query functor ClosestParticle holds the cloud by value and maps a point to the closest particle, through an EBGEOMETRY_HOST_DEVICE call operator. A cloud owns no memory – it describes the spheres and the BVH in a Pool – so it is trivially copyable, and a kernel can receive it by value once it describes memory the device can read.

Building the cloud

The cloud is built from the program’s arrays of centers and radii (or from one array of SphereCloud::Sphere), and keeps its own copy in the pool, which must outlive it:

EBGeometry::Pool pool(EBGeometry::hostMemoryResource());

const SphereCloudBVH<T> cloud(pool, centers, radii);

A query reports a particle by its index in those arrays, so data the cloud does not hold (here a material per particle) stays in the program’s own arrays, indexed the same way.

Using it on the host

signedDistance(p) is the smallest \(|p - c| - r\) over the particles: negative inside a particle, and the distance to the nearest one outside them all. closestSphere(p) names that particle and gives the same distance. The program prints both at a few points, and the value at a particle’s center.

The same query on a GPU

The pool is frozen and mirrored into GPU::memoryResource(), and rebasedView() gives a cloud that reads the copy; GPU::transform() runs ClosestParticle once per point in a kernel. In a plain C++ build the same lines run on the host.

Looking at it

The example ends with Render::view(cloud, pool, argc, argv); X cuts the cloud open.

The window needs SDL2 and is built with make VIEWER=SDL2 or cmake -DEBGEOMETRY_VIEWER=SDL2 (see The viewer window (EBGEOMETRY_VIEWER)); otherwise, and with --no-view or without a display, view() prints why and returns. Drag to orbit and scroll to zoom; the window lists its other keys (see The viewer’s keys).

Building and running

See Building for how to compile it with CMake, GNU Make, or a direct compiler invocation. Built for a GPU (see Running on a GPU), the queries and the window run on the device.

cd Examples/SphereClouds
./SphereClouds.ex