c – 使用GL_TRIANGLE_STRIP或索引GL_TRIANGLES绘制动态数量的四

我在C中开发了一个简单的基于精灵的2D游戏,它使用OpenGL进行硬件加速渲染,使用SDL进行窗口管理和用户输入处理.由于它是2D游戏,我只需要绘制四边形,但由于精灵的数量是动态的,我永远不能依赖于有恒定数量的四边形.因此,我需要每帧通过我的VBO重新缓冲所有顶点数据(因为可能有比最后一帧更多或更少的四边形,因此缓冲区可能是不同的大小).

到目前为止我的原型程序创建了一个窗口,允许用户使用向上和向下箭头键在对角线中添加和删除四边形.现在我正在绘制的四边形是简单,无纹理的白色方块.这是我正在使用的代码(在OS X 10.6.8下编译并正常工作,在OpenGL 2.1下使用Ubuntu 12.04)：

#if defined(__APPLE__)
    #include <OpenGL/OpenGL.h>
#endif
#if defined(__linux__)
    #define GL_GLEXT_PROTOTYPES
    #include <GL/glx.h>
#endif

#include <GL/gl.h>
#include <SDL.h>
#include <iostream>
#include <vector>
#include <string>


struct Vertex
{   
    //vertex coordinates
    GLint x;
    GLint y;
};

//Constants
const int SCREEN_WIDTH = 1024;
const int SCREEN_HEIGHT = 768;
const int FPS = 60; //our framerate
//Globals
SDL_Surface *screen;                    //the screen
std::vector<Vertex> vertices;           //the actual vertices for the quads
std::vector<GLint> startingElements;    //the index where the 4 vertices of each quad begin in the 'vertices' vector
std::vector<GLint> counts;              //the number of vertices for each quad
GLuint VBO = 0;                         //the handle to the vertex buffer


void createVertex(GLint x,GLint y)
{
    Vertex vertex;
    vertex.x = x;
    vertex.y = y;
    vertices.push_back(vertex);
}

//creates a quad at position x,y,with a width of w and a height of h (in pixels)
void createQuad(GLint x,GLint y,GLint w,GLint h)
{
    //Since we're drawing the quads using GL_TRIANGLE_STRIP,the vertex drawing
    //order is from top to bottom,left to right,like so:
    //
    //    1-----3
    //    |     |
    //    |     |
    //    2-----4

    createVertex(x,y);     //top-left vertex
    createVertex(x,y+h);   //bottom-left vertex
    createVertex(x+w,y);   //top-right vertex
    createVertex(x+w,y+h); //bottom-right vertex

    counts.push_back(4);    //each quad will always have exactly 4 vertices
    startingElements.push_back(startingElements.size()*4);

    std::cout << "Number of Quads: " << counts.size() << std::endl; //print out the current number of quads
}

//removes the most recently created quad
void removeQuad()
{
    if (counts.size() > 0)  //we don't want to remove a quad if there aren't any to remove
    {
        for (int i=0; i<4; i++)
        {
            vertices.pop_back();
        }

        startingElements.pop_back();
        counts.pop_back();

        std::cout << "Number of Quads: " << counts.size() << std::endl;
    }
    else
    {
        std::cout << "Sorry,you can't remove a quad if there are no quads to remove!" << std::endl;
    }
}


void init()
{
    //initialize SDL
    SDL_Init(SDL_INIT_VIDEO | SDL_INIT_TIMER);

    screen = SDL_SetVideoMode(SCREEN_WIDTH,SCREEN_HEIGHT,SDL_OPENGL);

#if defined(__APPLE__)
    //Enable vsync so that we don't get tearing when rendering
    GLint swapInterval = 1;
    CGLSetParameter(CGLGetCurrentContext(),kCGLCPSwapInterval,&swapInterval);
#endif

    //Disable depth testing,lighting,and dithering,since we're going to be doing 2D rendering only
    glDisable(GL_DEPTH_TEST);
    glDisable(GL_LIGHTING);
    glDisable(GL_DITHER);
    glPushAttrib(GL_DEPTH_BUFFER_BIT | GL_LIGHTING_BIT);

    //Set the projection matrix
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0,SCREEN_WIDTH,-1.0,1.0);

    //Set the modelview matrix
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    //Create VBO
    glGenBuffers(1,&VBO);
    glBindBuffer(GL_ARRAY_BUFFER,VBO);
}


void gameLoop()
{   
    int frameDuration = 1000/FPS;   //the set duration (in milliseconds) of a single frame      
    int currentTicks;       
    int pastTicks = SDL_GetTicks();
    bool done = false;  
    SDL_Event event;

    while(!done)
    {   
        //handle user input
        while(SDL_PollEvent(&event))
        {
            switch(event.type)
            {
                case SDL_KEYDOWN:
                    switch (event.key.keysym.sym)
                    {
                        case SDLK_UP:   //create a new quad every time the up arrow key is pressed
                            createQuad(64*counts.size(),64*counts.size(),64,64);
                            break;
                        case SDLK_DOWN: //remove the most recently created quad every time the down arrow key is pressed
                            removeQuad();
                            break;
                        default:
                            break;
                    }
                    break;
                case SDL_QUIT:
                    done = true;
                    break;
                default:
                    break;
            }           
        }


        //Clear the color buffer
        glClear(GL_COLOR_BUFFER_BIT);

        glBindBuffer(GL_ARRAY_BUFFER,VBO);
        //replace the current contents of the VBO with a completely new set of data (possibly including either more or fewer quads)
        glBufferData(GL_ARRAY_BUFFER,vertices.size()*sizeof(Vertex),&vertices.front(),GL_DYNAMIC_DRAW);

        glEnableClientState(GL_VERTEX_ARRAY);

            //Set vertex data
            glVertexPointer(2,GL_INT,sizeof(Vertex),0);
            //Draw the quads
            glMultiDrawArrays(GL_TRIANGLE_STRIP,&startingElements.front(),&counts.front(),counts.size());

        glDisableClientState(GL_VERTEX_ARRAY);

        glBindBuffer(GL_ARRAY_BUFFER,0);


        //Check to see if we need to delay the duration of the current frame to match the set framerate
        currentTicks = SDL_GetTicks();
        int currentDuration = (currentTicks - pastTicks);   //the duration of the frame so far
        if (currentDuration < frameDuration)
        {
            SDL_Delay(frameDuration - currentDuration);
        }
        pastTicks = SDL_GetTicks();

        // flip the buffers
        SDL_GL_SwapBuffers();
    }
}


void cleanUp()
{   
    glDeleteBuffers(1,&VBO);

    SDL_FreeSurface(screen);
    SDL_Quit();
}


int main(int argc,char *argv[])
{
    std::cout << "To create a quad,press the up arrow. To remove the most recently created quad,press the down arrow." << std::endl;

    init();
    gameLoop();
    cleanUp();

    return 0;
}

目前我正在使用GL_TRIANGLE_STRIPS和glMultiDrawArrays()来渲染我的四边形.这有效,而且在性能方面似乎相当不错,但我不得不怀疑将GL_TRIANGLES与IBO结合使用以避免重复顶点是一种更有效的渲染方式吗？我做了一些研究,有人建议索引GL_TRIANGLES通常优于GL_TRIANGLE_STRIPS,但他们似乎也假设四边形的数量保持不变,因此每帧不需要重新调整VBO和IBO的大小.这是我对索引GL_TRIANGLES的最大犹豫：如果我确实实现了索引GL_TRIANGLES,除了每帧重新调整整个VBO之外,我还必须每帧重新缓冲整个索引缓冲区,这也是因为动态的四边形数量.

所以基本上,我的问题是：鉴于由于四边形的动态数量,我必须将每个顶点数据重新缓冲到GPU的每一帧,切换到索引GL_TRIANGLES来绘制四边形会更高效,还是应该我坚持我目前的GL_TRIANGLE_STRIP实施？

SDL_Surface *screen; 
...
screen = SDL_SetVideoMode(SCREEN_WIDTH,SDL_OPENGL);
...
SDL_FreeSurface(screen);

// g++ main.cpp -lglut -lGL
#include <GL/glut.h>
#include <vector>
using namespace std;

// OpenGL Mathematics (GLM): http://glm.g-truc.net/
#include <glm/glm.hpp>
#include <glm/gtc/random.hpp>
using namespace glm;

struct SpriteWrangler
{
    SpriteWrangler( unsigned int aSpriteCount )
    {
        verts.resize( aSpriteCount * 6 );
        states.resize( aSpriteCount );

        for( size_t i = 0; i < states.size(); ++i )
        {
            states[i].pos = linearRand( vec2( -400,-400 ),vec2( 400,400 ) );
            states[i].vel = linearRand( vec2( -30,-30 ),vec2( 30,30 ) );

            Vertex vert;
            vert.r = (unsigned char)linearRand( 64.0f,255.0f );
            vert.g = (unsigned char)linearRand( 64.0f,255.0f );
            vert.b = (unsigned char)linearRand( 64.0f,255.0f );
            vert.a = 255;
            verts[i*6 + 0] = verts[i*6 + 1] = verts[i*6 + 2] =
            verts[i*6 + 3] = verts[i*6 + 4] = verts[i*6 + 5] = vert;
        }
    }

    void wrap( const float minVal,float& val,const float maxVal )
    {
        if( val < minVal )
            val = maxVal - fmod( maxVal - val,maxVal - minVal );
        else
            val = minVal + fmod( val - minVal,maxVal - minVal );
    }

    void Update( float dt )
    {
        for( size_t i = 0; i < states.size(); ++i )
        {
            states[i].pos += states[i].vel * dt;
            wrap( -400.0f,states[i].pos.x,400.0f );
            wrap( -400.0f,states[i].pos.y,400.0f );

            float size = 20.0f;
            verts[i*6 + 0].pos = states[i].pos + vec2( -size,-size );
            verts[i*6 + 1].pos = states[i].pos + vec2(  size,-size );
            verts[i*6 + 2].pos = states[i].pos + vec2(  size,size );
            verts[i*6 + 3].pos = states[i].pos + vec2(  size,size );
            verts[i*6 + 4].pos = states[i].pos + vec2( -size,size );
            verts[i*6 + 5].pos = states[i].pos + vec2( -size,-size );
        }
    }

    struct Vertex
    {
        vec2 pos;
        unsigned char r,g,b,a;
    };

    struct State
    {
        vec2 pos;
        vec2 vel;       // units per second
    };

    vector< Vertex > verts;
    vector< State > states;
};

void display()
{
    // timekeeping
    static int prvTime = glutGet(GLUT_ELAPSED_TIME);
    const int curTime = glutGet(GLUT_ELAPSED_TIME);
    const float dt = ( curTime - prvTime ) / 1000.0f;
    prvTime = curTime;

    // sprite updates
    static SpriteWrangler wrangler( 2000 );
    wrangler.Update( dt );
    vector< SpriteWrangler::Vertex >& verts = wrangler.verts;

    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

    // set up projection and camera
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    double w = glutGet( GLUT_WINDOW_WIDTH );
    double h = glutGet( GLUT_WINDOW_HEIGHT );
    double ar = w / h;
    glOrtho( -400 * ar,400 * ar,-400,400,-1,1);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glEnableClientState( GL_VERTEX_ARRAY );
    glEnableClientState( GL_COLOR_ARRAY );

    glVertexPointer( 2,GL_FLOAT,sizeof( SpriteWrangler::Vertex ),&verts[0].pos.x );
    glColorPointer( 4,GL_UNSIGNED_BYTE,&verts[0].r );
    glDrawArrays( GL_TRIANGLES,verts.size() );

    glDisableClientState( GL_VERTEX_ARRAY );
    glDisableClientState( GL_COLOR_ARRAY );

    glutSwapBuffers();
}

// run display() every 16ms or so
void timer( int extra )
{
    glutTimerFunc( 16,timer,0 );
    glutPostRedisplay();
}

int main(int argc,char **argv)
{
    glutInit( &argc,argv );
    glutInitWindowSize( 600,600 );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE );
    glutCreateWindow( "Sprites" );

    glutDisplayFunc( display );
    glutTimerFunc( 0,0 );
    glutMainLoop();
    return 0;
}