Follow up: Asynchronous off-screen query performance - c++

Question

I recently asked this question:
How to perform asynchronous off-screen queries?
What I've heard, but haven't been able to confirm yet, is that rendering to the window is more expensive than rendering to a framebuffer. First of all, can anyone comment on this? Can I draw multiple scenes to framebuffers faster than I can to the window? Are there other options, e.g., pbuffers or PBOs?
I have started playing around with framebuffers, but I have not been able to get the query to work. Here's some psuedo code for what I have set up so far:
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();
So far everything runs, but all of the queries return "0". Is there something about querying when drawing to a framebuffer that is different than when drawing to the window buffer?
Again, my two questions are:
Can I draw multiple scenes to framebuffers faster than I can to the window? Are there other options, e.g., pbuffers or PBOs?
Is there something about querying when drawing to a framebuffer that is different than when drawing to the window buffer?

Answer 1

Try something like this:
#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;
}
Representative output on my system:
cnt: 1884
0: 157288
1: 157288
2: 157289
3: 157288
4: 157287
5: 157286
6: 157292
7: 157286
8: 157289
9: 157288