使用GL_TRIANGLE_STRIP或索引gl_三角形来绘制动态数量的四边形是否更有效?

Question

我正在用c++开发一款简单的基于精灵的2D游戏，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。因为这是一款2D游戏，所以我只需要绘制四边形，但因为精灵的数量是动态的，所以我不能指望四边形的数量是恒定的。因此，我需要通过我的VBO每帧重新缓冲所有顶点数据(因为可能有更多或更少的四边形比上一帧，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，并允许用户通过使用向上和向下箭头键在对角线行中添加和删除四边形。现在我画的四边形是简单的，没有纹理的白色正方形。以下是我正在使用的代码(在OS X 10.6.8和Ubuntu 12.04与OpenGL 2.1下编译和正常工作):

#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, 0, 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, SCREEN_HEIGHT, 0, -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;
}

目前我使用GL_TRIANGLE_STRIPS与glMultiDrawArrays()来渲染我的四边形。这是有效的，并且在性能方面似乎相当不错，但我想知道是否使用gl_三角形与IBO结合以避免重复顶点将是一个更有效的方式来渲染?我做了一些研究，有些人认为索引gl_triangle通常优于GL_TRIANGLE_STRIPS，但他们似乎也假设quad的数量保持不变，因此VBO和IBO的大小不必每帧都重新缓冲。这是我对索引gl_三角形最大的犹豫:如果我确实实现了索引gl_三角形，除了每帧重新缓冲整个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quad的动态数量。

所以基本上，我的问题是:考虑到我必须重新缓冲我所有的顶点数据到GPU每帧由于动态数量的四边形，它是更有效的切换到索引gl_三角形来绘制四边形，或者我应该坚持我目前的GL_TRIANGLE_STRIP实现?

Answer 1

您可能会使用未索引的GL_QUADS/GL_TRIANGLES和glDrawArrays()调用。

---

SDL_Surface *screen; 
...
screen = SDL_SetVideoMode(SCREEN_WIDTH, SCREEN_HEIGHT, 0, SDL_OPENGL);
...
SDL_FreeSurface(screen);

不要那样做:

返回的表面由SDL_Quit释放，不能被调用者释放。此规则还包括连续调用SDL_SetVideoMode(即调整大小或分辨率更改)，因为现有表面将自动释放。

---

EDIT:简单顶点数组演示:

// 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, sizeof( SpriteWrangler::Vertex ), &verts[0].r );
    glDrawArrays( GL_TRIANGLES, 0, 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, timer, 0 );
    glutMainLoop();
    return 0;
}

你可以用顶点数组获得不错的性能。

理想情况下，大多数/所有的dt s应该是<= 16毫秒。