我正在用 C++ 开发一个简单的基于 sprite 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，所以我永远不能依赖于恒定数量的四边形。因此，我需要通过我的 VBO 每帧重新缓冲所有顶点数据（因为可能有比上一帧更多或更少的四边形，因此缓冲区的大小可能不同）。
到目前为止，我的原型程序创建了一个窗口，并允许用户使用向上和向下箭头键在对角线行中添加和删除四边形。现在我正在绘制的四边形是简单的、无纹理的白色方块。这是我正在使用的代码（在 OS X 10.6.8 和带有 OpenGL 2.1 的 Ubuntu 12.04 下编译并正常工作）：
#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;
}
目前我正在使用 GL_TRIANGLE_STRIPS 和 glMultiDrawArrays() 来渲染我的四边形。这很有效，而且在性能方面似乎相当不错，但我想知道是否将 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？我做了一些研究，有些人认为索引 GL_TRIANGLES 通常优于 GL_TRIANGLE_STRIPS，但他们似乎也假设四边形的数量将保持不变，因此 VBO 和 IBO 的大小不必每帧都重新缓冲. 这是我对索引 GL_TRIANGLES 最大的犹豫：如果我确实实现了索引 GL_TRIANGLES，除了每帧重新缓冲整个 VBO 之外，我还必须重新缓冲每个帧的整个索引缓冲区，这也是因为四边形的动态数量。
所以基本上，我的问题是：鉴于由于四边形的动态数量，我必须在每帧将所有顶点数据重新缓冲到 GPU，切换到索引 GL_TRIANGLES 来绘制四边形会更有效吗？坚持我目前的 GL_TRIANGLE_STRIP 实施？