Instead of starting my own thread I'm just going to chime in here and say that I'm experiencing the same issues. I have lots of Render Textures and the profiler reveals that wglMakeCurrent is taking a substantial amount of time. Afaict SFML uses one GL context per render texture and then has to switch between them. I was curious whether the setActive() call was the buffer being flushed as binary suggested, so I made two minimal examples.
(A) uses 20 manually created FBOs and (B) uses 20 sf::RenderTextures. The code for each is below. (A) runs at around 340 fps and (B) runs at 220 fps on my machine. MSVC's profiler shows (B) on the right and you can see wglMakeCurrent takes up a big chunk of the total run-time.
I talked about it a little on
twitter and I think that having one context per render texture is not a great approach when rendertexture count is high. Also, a discussion
here comments about a few strategies to optimise the use of many fbos. One strategy is to have a single FBO and then rebind different textures to them as you render. It's a very specific use case, but unfortunately seems a little tricky with the one context-per-render texture approach of sfml.
#include <GL/glew.h>
#include <SFML/Graphics.hpp>
// #include <SFML/OpenGL.hpp>
// #include "mm/common/profiler.h"
#include <array>
#include <iostream>
#include <Windows.h>
// as per: http://developer.download.nvidia.com/devzone/devcenter/gamegraphics/files/OptimusRenderingPolicies.pdf
extern "C" {
_declspec(dllexport) DWORD NvOptimusEnablement = 0x00000001;
}
int main1();
int main2();
void drawThing();
int main(){
// return main1();
return main2();
};
static const int W = 800;
static const int N = 20;
int main1()
{
sf::RenderWindow window(sf::VideoMode(W, W), "test");
window.setFramerateLimit(1000);
std::array<sf::RenderTexture, N> rts;
for (auto& rt: rts) rt.create(W, W);
sf::Clock clock;
int frameCount = 0;
while (window.isOpen())
{
// BROFILER_FRAME("App");
sf::Event event;
while (window.pollEvent(event))
{
if (event.type == sf::Event::Closed) window.close();
if (event.type == sf::Event::KeyPressed){
if (event.key.code == sf::Keyboard::Escape){
window.close();
}
}
}
// draw to texture
for (auto& rt : rts){
rt.clear(sf::Color(255,0,0));
rt.setActive(true);
//rt.pushGLStates();
rt.setView(sf::View(sf::FloatRect(0,W,W,W)));
glViewport(0, 0, W, W);
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glOrtho(0.0, W, 0.0, W, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
drawThing();
glPopMatrix();
glPopMatrix();
/*
sf::RectangleShape shape(sf::Vector2f(W / 2, W / 2));
shape.setOrigin(W / 4, W / 4);
shape.setPosition(W / 2, W / 2);
rt.draw(shape);
*/
rt.display();
//rt.popGLStates();
}
window.setActive(true);
window.pushGLStates();
// draw textures to window
window.clear();
for (auto& rt : rts){
auto spr = sf::Sprite(rt.getTexture());
window.draw(spr);
}
window.display();
window.popGLStates();
frameCount++;
if (frameCount > 100){
std::cout << 1./(clock.getElapsedTime().asSeconds() / 100) << std::endl;
frameCount = 0;
clock.restart();
}
}
return EXIT_SUCCESS;
}
std::pair<int,int> makeFBO(int w, int h){
GLuint tex, fb;
glGenTextures(1, &tex);
glBindTexture(GL_TEXTURE_2D, tex);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
//NULL means reserve texture memory, but texels are undefined
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, w, h, 0, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
//-------------------------
glGenFramebuffers(1, &fb);
glBindFramebuffer(GL_FRAMEBUFFER, fb);
//Attach 2D texture to this FBO
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, tex, 0);
//Does the GPU support current FBO configuration?
GLenum status;
status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
switch (status)
{
case GL_FRAMEBUFFER_COMPLETE:
std::cout << "good";
break;
default:
std::exit(EXIT_FAILURE);
}
return { fb, tex };
}
int main2()
{
sf::RenderWindow window(sf::VideoMode(W, W), "test");
window.setFramerateLimit(1000);
glewInit();
std::array<std::pair<int,int>, N> fbos;
for (int i = 0; i < N; i++){
fbos[i] = makeFBO(W, W);
}
sf::Clock clock;
int frameCount = 0;
while (window.isOpen())
{
// BROFILER_FRAME("App");
sf::Event event;
while (window.pollEvent(event))
{
if (event.type == sf::Event::Closed) window.close();
if (event.type == sf::Event::KeyPressed){
if (event.key.code == sf::Keyboard::Escape){
window.close();
}
}
}
// draw to fbos
for (auto& fbo: fbos){
window.pushGLStates();
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glOrtho(0.0, W, 0.0, W, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo.first);
glClearColor(1, 0, 0, 1);
glClear(GL_COLOR_BUFFER_BIT);
drawThing();
glPopMatrix();
glPopMatrix();
window.popGLStates();
}
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
// draw textures to window
window.clear();
window.pushGLStates();
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glOrtho(0.0, W, 0.0, W, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
for (auto& fb: fbos){
glColor3f(1, 1, 1);
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, fb.second);
glBegin(GL_QUADS);
glTexCoord2f(0, 0); glVertex3f(0, 0, 0);
glTexCoord2f(0, 1); glVertex3f(0, W, 0);
glTexCoord2f(1, 1); glVertex3f(W, W, 0);
glTexCoord2f(1, 0); glVertex3f(W, 0, 0);
glEnd();
// auto spr = sf::Sprite(rt.getTexture());
// window.draw(spr);
}
glPopMatrix();
glPopMatrix();
window.popGLStates();
window.display();
frameCount++;
if (frameCount > 100){
std::cout << 1. / (clock.getElapsedTime().asSeconds() / 100) << std::endl;
frameCount = 0;
clock.restart();
}
}
return EXIT_SUCCESS;
}
void drawThing(){
int W = 500;
// do drawing
glBegin(GL_QUADS);
glColor3f(1, 1, 1);
glVertex2f(0, 0);
glVertex2f(W, 0);
glVertex2f(W, W/2);
glVertex2f(0, W / 2);
glColor3f(1, 0, 1);
glVertex2f(0, 0);
glVertex2f(W/2, 0);
glVertex2f(W/2, W / 4);
glVertex2f(0, W / 4);
glEnd();
}
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Saw this thread from chatting to Hiura on twitter. Plenty more screens etc at the 
