幽灵粒子(Ghost Particles)

1、流体仿真

案例位置：examples/Cuda/SPH/GL_GhostSPH

功能介绍：SPH方法存在虚假的数值表面张力伪影、不满足质量守恒约束，无法获取液体与固体的真实内力等问题。Ghost SPH方案通过一种新的粒子采样算法解决了这一问题。该算法在周围的空气和固体中创建了一个狭窄的Ghost粒子层作为边界条件，以更加真实的模拟物理现象。

案例说明： 案例展示了GhostParticles的流体仿真功能。如下图所示流体粒子从Ghost粒子层滑落，没有发生穿透现象。

2、程序实现

接下来介绍案例的实现过程：

  • 创建Ghost Particles:
std::shared_ptr<GhostParticles<DataType3f>> createGhostParticles()
{
	auto ghost = std::make_shared<GhostParticles<DataType3f>>();

	std::vector<Vec3f> host_pos;
	std::vector<Vec3f> host_vel;
	std::vector<Vec3f> host_force;
	std::vector<Vec3f> host_normal;
	std::vector<Attribute> host_attribute;

	Vec3f low(-0.2, -0.015, -0.2);
	Vec3f high(0.2, -0.005, 0.2);

	Real s = 0.005f;
	int m_iExt = 0;

	float omega = 1.0f;
	float half_s = -s / 2.0f;

	int num = 0;

	for (float x = low.x - m_iExt * s; x <= high.x + m_iExt * s; x += s) {
		for (float y = low.y - m_iExt * s; y <= high.y + m_iExt * s; y += s) {
			for (float z = low.z - m_iExt * s; z <= high.z + m_iExt * s; z += s) {
				Attribute attri;
				attri.setFluid();
				attri.setDynamic();

				host_pos.push_back(Vec3f(x, y, z));
				host_vel.push_back(Vec3f(0));
				host_force.push_back(Vec3f(0));
				host_normal.push_back(Vec3f(0, 1, 0));
				host_attribute.push_back(attri);
			}
		}
	}

	ghost->statePosition()->resize(num);
	ghost->stateVelocity()->resize(num);
	ghost->stateForce()->resize(num);

	ghost->stateNormal()->resize(num);
	ghost->stateAttribute()->resize(num);

	ghost->statePosition()->assign(host_pos);
	ghost->stateVelocity()->assign(host_vel);
	ghost->stateForce()->assign(host_force);
	ghost->stateNormal()->assign(host_normal);
	ghost->stateAttribute()->assign(host_attribute);

	host_pos.clear();
	host_vel.clear();
	host_force.clear();
	host_normal.clear();
	host_attribute.clear();

	return ghost;
}
  • 创建场景图及边界:
	std::shared_ptr<SceneGraph> scn = std::make_shared<SceneGraph>();
	scn->setUpperBound(Vec3f(0.5, 1, 0.5));
	scn->setLowerBound(Vec3f(-0.5, 0, -0.5));
	auto boundary = scn->addNode(std::make_shared<StaticBoundary<DataType3f>>());
	boundary->loadCube(Vec3f(-0.1f, 0.0f, -0.1f), Vec3f(0.1f, 1.0f, 0.1f), 0.005, true);
  • 创建基于Ghost GhostParticles的流体粒子:
	auto fluid = scn->addNode(std::make_shared<ParticleSystem<DataType3f>>());
	fluid->loadParticles(Vec3f(-0.1, 0.0, -0.1), Vec3f(0.105, 0.1, 0.105), 0.005);

	auto ghost = scn->addNode(createGhostParticles());

	auto incompressibleFluid = scn->addNode(std::make_shared<GhostFluid<DataType3f>>());
	fluid->connect(incompressibleFluid->importFluidParticles());
	ghost->connect(incompressibleFluid->importBoundaryParticles());
	incompressibleFluid->connect(boundary->importParticleSystems());
  • 创建渲染节点:
	{
		auto calculateNorm = std::make_shared<CalculateNorm<DataType3f>>();
		auto colorMapper = std::make_shared<ColorMapping<DataType3f>>();
		colorMapper->varMax()->setValue(5.0f);

		fluid->stateVelocity()->connect(calculateNorm->inVec());
		calculateNorm->outNorm()->connect(colorMapper->inScalar());

		fluid->graphicsPipeline()->pushModule(calculateNorm);
		fluid->graphicsPipeline()->pushModule(colorMapper);

		auto ptRender = std::make_shared<GLPointVisualModule>();
		ptRender->setColor(Vec3f(1, 0, 0));
		ptRender->setColorMapMode(GLPointVisualModule::PER_VERTEX_SHADER);

		fluid->statePointSet()->connect(ptRender->inPointSet());
		colorMapper->outColor()->connect(ptRender->inColor());

		fluid->graphicsPipeline()->pushModule(ptRender);

		// A simple color bar widget for node
		auto colorBar = std::make_shared<ImColorbar>();
		colorBar->varMax()->setValue(5.0f);
		calculateNorm->outNorm()->connect(colorBar->inScalar());
		// add the widget to app
		fluid->graphicsPipeline()->pushModule(colorBar);
	}
	
	{
		auto ghostRender = std::make_shared<GLPointVisualModule>();
		ghostRender->setColor(Vec3f(1, 0.5, 0));
		ghostRender->setColorMapMode(GLPointVisualModule::PER_OBJECT_SHADER);

		ghost->statePointSet()->connect(ghostRender->inPointSet());

		ghost->graphicsPipeline()->pushModule(ghostRender);
	}