C++ 跟进：异步屏幕外查询性能

我最近问了一个问题：

我听说，但还不能确认，渲染到窗口比渲染到帧缓冲区更昂贵。首先，有人能对此发表评论吗？我能否以比窗口更快的速度将多个场景绘制到帧缓冲区？是否有其他选择，例如PBuffer或PBO

我已经开始使用帧缓冲区，但我无法让查询正常工作。以下是我目前设置的一些psuedo代码：

glfwWindowHint(GLFW_VISIBLE, GL_FALSE);
window = glfwCreateWindow(1, 1, "OpenGL", NULL, NULL);

glfwMakeContextCurrent(window);

glGenFramebuffers(1, &fbo);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo);

glGenRenderbuffers(1, &rbo);
glBindRenderbuffer(GL_RENDERBUFFER, rbo);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24, size, size);
glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, rbo);

glEnable(GL_DEPTH_TEST);
glGenQueries(numberOfQueries, queries);

for (scene in scenesToRender)
{
    glClear(GL_DEPTH_BUFFER_BIT);


    glDepthFunc(GL_LESS);
    drawShadingObjects(scene);

    glBeginQuery(GL_SAMPLES_PASSED, queries[index]);

    glDepthFunc(GL_LEQUAL);
    drawShadedObject(scene);

    glEndQuery(GL_SAMPLES_PASSED);

}

collectQueryResults();

deleteBuffers();

到目前为止，所有查询都运行，但所有查询都返回“0”。当绘制到帧缓冲区时查询是否与绘制到窗口缓冲区时查询有所不同

我的两个问题是：

我能否以比窗口更快的速度将多个场景绘制到帧缓冲区？是否有其他选择，例如PBuffer或PBO

当绘制到帧缓冲区时查询是否与绘制到窗口缓冲区时查询有所不同

---

试着这样做：

#include <GL/glew.h>
#include <GLFW/glfw3.h>

#include <iostream>
#include <vector>
using namespace std;

const unsigned int sz = 1024;
void drawScene( unsigned int multiplier )
{
    glViewport( 0, 0, sz, sz );
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glRotatef( (float)glfwGetTime() * 50.f * multiplier, 0.f, 0.f, 1.f);
    glBegin(GL_TRIANGLES);
    glColor3f(1.f, 0.f, 0.f);
    glVertex3f(-0.6f, -0.4f, 0.f);
    glColor3f(0.f, 1.f, 0.f);
    glVertex3f(0.6f, -0.4f, 0.f);
    glColor3f(0.f, 0.f, 1.f);
    glVertex3f(0.f, 0.6f, 0.f);
    glEnd();
}

bool available( const vector< GLuint >& queries )
{
    for( size_t i = 0; i < queries.size(); ++i )
    {
        GLuint available = 0;
        glGetQueryObjectuiv( queries[i], GL_QUERY_RESULT_AVAILABLE, &available );
        if( GL_FALSE == available )
            return false;
    }
    return true;
}

int main()
{
    glfwInit();
    GLFWwindow* window = glfwCreateWindow( 400, 400, "Simple example", NULL, NULL );
    glfwMakeContextCurrent( window );
    glewInit();

    if( !glewIsSupported( "GL_VERSION_2_1" ) )
        return -1;
    if( !glewIsSupported( "GL_EXT_framebuffer_object" ) )
        return -1;

    GLuint fbo = 0;
    glGenFramebuffersEXT( 1, &fbo );
    glBindFramebufferEXT( GL_DRAW_FRAMEBUFFER_EXT, fbo );

    GLuint rbo0 = 0;
    glGenRenderbuffersEXT( 1, &rbo0 );
    glBindRenderbufferEXT( GL_RENDERBUFFER_EXT, rbo0 );
    glRenderbufferStorageEXT( GL_RENDERBUFFER_EXT, GL_RGBA, sz, sz );
    glFramebufferRenderbufferEXT( GL_DRAW_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_RENDERBUFFER_EXT, rbo0 );

    GLuint rbo1 = 0;
    glGenRenderbuffersEXT( 1, &rbo1 );
    glBindRenderbufferEXT( GL_RENDERBUFFER_EXT, rbo1 );
    glRenderbufferStorageEXT( GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT24, sz, sz );
    glFramebufferRenderbufferEXT( GL_DRAW_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, rbo1 );

    GLenum status = glCheckFramebufferStatusEXT( GL_FRAMEBUFFER_EXT );
    if( status != GL_FRAMEBUFFER_COMPLETE_EXT )
        return -1;

    glBindFramebufferEXT( GL_FRAMEBUFFER_EXT, 0 );

    vector< GLuint > queries( 10 );
    glGenQueries( queries.size(), &queries[0] );

    glBindFramebufferEXT( GL_FRAMEBUFFER_EXT, fbo );

    for( size_t i  = 0; i < queries.size(); ++i )
    {
        glBeginQuery( GL_SAMPLES_PASSED, queries[i] );
        drawScene( i + 1 );
        glEndQuery( GL_SAMPLES_PASSED );
    }

    glBindFramebufferEXT( GL_FRAMEBUFFER_EXT, 0 );

    // wait for queries to become available
    unsigned int cnt = 0;
    while( !available( queries ) )
    {
        cnt++;
    }

    // all queries available, display query results
    cout << "cnt: " << cnt << endl;
    for( size_t i = 0; i < queries.size(); ++i )
    {
        GLuint samples = 0;
        glGetQueryObjectuiv( queries[i], GL_QUERY_RESULT, &samples );
        cout << i << ": " << samples << endl;
    }
    cout << endl;

    glfwDestroyWindow( window );
    glfwTerminate();
    return 0;
}

---

那么您有一个查询还是多个查询？您的

glgenquerys（）

和

glBeginQuery（）

调用似乎不一致。它应该是多个查询。我会解决的。这个想法是，在绘制其他场景时，可能会发生一些异步查询。您的代码在我的机器上运行良好，我已经实现了与我的代码（主要包括FBO的颜色附件）的显著差异。无论出于何种原因，我似乎仍在渲染到默认帧缓冲区（即1x1窗口）。你知道这可能是什么原因吗？我在绘图之前绑定到“fbo”。使用

glGetIntegerv（GL\u draw\u FRAMEBUFFER\u binding，&int）

后，它似乎正在渲染到我定义的帧缓冲区，但出于任何原因，大小仅为1x1。帧缓冲区继承了窗口的大小，即使renderbuffer存储设置为512x512。@NealKruis：绑定FBO后是否设置视口？绑定FBO后是否设置视图矩阵。现在我想知道窗口是否可能设置了一个剪切矩形，这会影响我的渲染。@NealKruis：不是视图矩阵，而是视口，via。请参阅中的“解释FBO与窗口的宽度/高度不同时会发生什么情况？”问题。

cnt: 1884
0: 157288
1: 157288
2: 157289
3: 157288
4: 157287
5: 157286
6: 157292
7: 157286
8: 157289
9: 157288