14.3具体的粒子系统：雪、火、粒子枪

现在让我们用cParticleSystem类开始一个具体的粒子系统，为了说明用意，这些系统的设计很简单，没有用到cParticleSystem类所提供的所有灵活性。我们实现雪、火、粒子枪系统。雪系统模拟下落的雪花，火系统模拟看上去像火焰的爆炸，粒子枪系统从照相机位置向对面发射出粒子（用键盘）。
14.3.1 例子程序：雪

雪系统类定义如下：
    class cSnow : public cParticleSystem
    {
    public:
        cSnow(cBoundingBox* bounding_box, int num_particles);
        virtual void reset_particle(sParticleAttribute* attr);
        virtual void update(float time_delta);
    };

构造函数提供一个点给边界盒结构，边界盒是粒子系统的成员。边界盒描述雪花在哪个范围内（体积范围）下落，如果雪花出了边界盒，它将被杀死并再生。这样，雪系统始终能保存有同样数量的激粒子，构造函数的实现：
    cSnow::cSnow(cBoundingBox* bounding_box, int num_particles)
    {
        m_bounding_box    = *bounding_box;
        m_size            = 0.25f;
        m_vb_num        = 2048;
        m_vb_offset        = 0;
        m_vb_batch_num    = 512;
        for(int i = 0; i < num_particles; i++)
            add_particle();
    }

同样注意：我们指定顶点缓存的尺寸，每一批的尺寸和开始的偏移。

reset_particle方法创建一个雪花，在x、z轴随机的位置并在边界盒的范围内。设置y轴高度为边界盒的顶部。我们给雪花一个速度，以便让雪花下落时稍稍向左倾斜。雪花是白色的。
    void cSnow::reset_particle(sParticleAttribute* attr)
    {
        attr->is_alive = true;
        // get random x, z coordinate for the position of the snow flake
            get_random_vector(&attr->position, &m_bounding_box.m_min, &m_bounding_box.m_max);
        // no randomness for height (y-coordinate). 
        // Snow flake always starts at the top of bounding box.
            attr->position.y = m_bounding_box.m_max.y;
        // snow flakes fall downwards and slightly to the left
            attr->velocity.x = get_random_float(0.0f, 1.0f) * (-3.0f);
        attr->velocity.y = get_random_float(0.0f, 1.0f) * (-10.0f);
        attr->velocity.z = 0.0f;
        // white snow flake
        attr->color = WHITE;
    }

update方法更新粒子和粒子间的位置，并且测试粒子是否在系统的边界盒之外，如果它已经跳出边界盒，就再重新创建。
    void cSnow::update(float time_delta)
    {
        for(list<sParticleAttribute>::iterator iter = m_particles.begin(); iter != m_particles.end(); iter++)
        {
            iter->position += iter->velocity * time_delta;
            // is the point outside bounds?
            if(! m_bounding_box.is_point_inside(iter->position))
                // recycle dead particles, so respawn it.
                    reset_particle(&(*iter));
        }
    }

执行程序：
    #include "d3dUtility.h"
    #include "camera.h"
    #include "ParticleSystem.h"
    #include <cstdlib>
    #include <ctime>
    #pragma warning(disable : 4100)
    const int WIDTH  = 640;
    const int HEIGHT = 480;
    IDirect3DDevice9*    g_device;
    cParticleSystem*    g_snow;
    cCamera                g_camera(AIR_CRAFT);
       
    bool setup()
    {   
        srand((unsigned int)time(NULL));
        // create snow system
        cBoundingBox bounding_box;
        bounding_box.m_min = D3DXVECTOR3(-10.0f, -10.0f, -10.0f);
        bounding_box.m_max = D3DXVECTOR3(10.0f, 10.0f, 10.0f);
        g_snow = new cSnow(&bounding_box, 5000);
        g_snow->init(g_device, "snowflake.dds");
        // setup a basic scnen, the scene will be created the first time this function is called.
            draw_basic_scene(g_device, 1.0f);
        // set the projection matrix
        D3DXMATRIX proj;
        D3DXMatrixPerspectiveFovLH(&proj, D3DX_PI/4.0f, (float)WIDTH/HEIGHT, 1.0f, 1000.0f);
        g_device->SetTransform(D3DTS_PROJECTION, &proj);
        return true;
    }
        ///
    void cleanup()
    {   
        delete g_snow;
        // pass NULL for the first parameter to instruct cleanup
        draw_basic_scene(NULL, 0.0f);
    }
        ///
    bool display(float time_delta)
    {
        // update the camera
        if( GetAsyncKeyState(VK_UP) & 0x8000f )
            g_camera.walk(4.0f * time_delta);
        if( GetAsyncKeyState(VK_DOWN) & 0x8000f )
            g_camera.walk(-4.0f * time_delta);
        if( GetAsyncKeyState(VK_LEFT) & 0x8000f )
            g_camera.yaw(-1.0f * time_delta);
        if( GetAsyncKeyState(VK_RIGHT) & 0x8000f )
            g_camera.yaw(1.0f * time_delta);
        if( GetAsyncKeyState('N') & 0x8000f )
            g_camera.strafe(-4.0f * time_delta);
        if( GetAsyncKeyState('M') & 0x8000f )
            g_camera.strafe(4.0f * time_delta);
        if( GetAsyncKeyState('W') & 0x8000f )
            g_camera.pitch(1.0f * time_delta);
        if( GetAsyncKeyState('S') & 0x8000f )
            g_camera.pitch(-1.0f * time_delta);   
        // update the view matrix representing the camera's new position/orientation
        D3DXMATRIX view_matrix;
        g_camera.get_view_matrix(&view_matrix);
        g_device->SetTransform(D3DTS_VIEW, &view_matrix);   
        g_snow->update(time_delta);
        // render now
        g_device->Clear(0, NULL, D3DCLEAR_TARGET | D3DCLEAR_ZBUFFER, 0x00000000, 1.0f, 0);
        g_device->BeginScene();
        D3DXMATRIX identity_matrix;
        D3DXMatrixIdentity(&identity_matrix);
        g_device->SetTransform(D3DTS_WORLD, &identity_matrix);
        draw_basic_scene(g_device, 1.0f);
        // order important, render snow last.
            g_device->SetTransform(D3DTS_WORLD, &identity_matrix);
        g_snow->render();
        g_device->EndScene();
        g_device->Present(NULL, NULL, NULL, NULL);
        return true;
    }
        ///
    LRESULT CALLBACK wnd_proc(HWND hwnd, UINT msg, WPARAM word_param, LPARAM long_param)
    {
        switch(msg)
        {
        case WM_DESTROY:
            PostQuitMessage(0);
            break;
        case WM_KEYDOWN:
            if(word_param == VK_ESCAPE)
                DestroyWindow(hwnd);
            break;
        }
        return DefWindowProc(hwnd, msg, word_param, long_param);
    }
        ///
    int WINAPI WinMain(HINSTANCE inst, HINSTANCE, PSTR cmd_line, int cmd_show)
    {
        if(! init_d3d(inst, WIDTH, HEIGHT, true, D3DDEVTYPE_HAL, &g_device))
        {
            MessageBox(NULL, "init_d3d() - failed.", 0, MB_OK);
            return 0;
        }
        if(! setup())
        {
            MessageBox(NULL, "Steup() - failed.", 0, MB_OK);
            return 0;
        }
        enter_msg_loop(display);
        cleanup();
        g_device->Release();
        return 0;
    }

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