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CornyKorn21

OpenGL Windows OpenGL Initialization Questions

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Hi,
I was wondering if anyone could answer a few questions I have about properly setting up an OpenGL 4.1 context on Windows. I scrapped together two quick code samples on my computer one using GLFW/GLEW and another creating my own window and using GLEW.

I ran both code samples through gDEBugger and I'm a little confused by the results. Based on the way I set up my window in both of the samples below I expected to only see a "Front Buffer" and "Back Buffer" when I viewed the context in gDEBugger. Instead I am seeing a depth, back, front, and four auxiliary buffers. Do I have any control over the creation of those buffers?

The other question I have is if I'm properly setting up my OpenGL context for Windows. The one issue I seem to be having is that if I specify WGL_CONTEXT_DEBUG_BIT_ARB in the sample where I create my own window it doesn't properly create a context.
 
Posted this over at the OpenGL forums as well.
Thanks for any help.
 
GLFW/GLEW:

#define GLEW_STATIC
#define WIN32_LEAN_AND_MEAN
 
#include <Windows.h>
#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include <GLM/glm.hpp>
#include "Debugging/glDebugging.h"
 
int main( int argc, char* argv[] )
{
        if( !glfwInit() )
                return EXIT_FAILURE;
 
        // define glfw window hints
#if defined( _DEBUG )
        glfwWindowHint( GLFW_OPENGL_DEBUG_CONTEXT, GL_TRUE );
#endif
        glfwWindowHint( GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE );
        glfwWindowHint( GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE );
        glfwWindowHint( GLFW_CONTEXT_VERSION_MAJOR, 4 );
        glfwWindowHint( GLFW_CONTEXT_VERSION_MINOR, 1 );
        glfwWindowHint( GLFW_OPENGL_DEBUG_CONTEXT, 1 );
        glfwWindowHint( GLFW_DEPTH_BITS, 0 );
        glfwWindowHint( GLFW_STENCIL_BITS, 0 );
        glfwWindowHint( GLFW_ACCUM_RED_BITS, 0 );
        glfwWindowHint( GLFW_ACCUM_GREEN_BITS, 0 );
        glfwWindowHint( GLFW_ACCUM_BLUE_BITS, 0 );
        glfwWindowHint( GLFW_ACCUM_ALPHA_BITS, 0 );
        glfwWindowHint( GLFW_AUX_BUFFERS, 0 );
 
        // open a window with GLFW
        GLFWwindow* pWindow = glfwCreateWindow( 640, 360, "LearnGL", nullptr, nullptr );
        if( pWindow == nullptr )
        {
                glfwTerminate();
                return EXIT_FAILURE;
        }
 
        glfwMakeContextCurrent( pWindow );
 
        // initialize GLEW
        glewExperimental = GL_TRUE;
        if( glewInit() != GLEW_OK )
        {
                glfwTerminate();
                return EXIT_FAILURE;
        }
 
        if( !GLEW_VERSION_4_1 )
        {
                glfwTerminate();
                return EXIT_FAILURE;
        }
 
#if defined( _DEBUG )
        GL::InitializeDebugging();
#endif
 
        GLuint frameBuffer;
        GLuint colorBufferTexture;
 
        glGenTextures( 1, &colorBufferTexture );
 
        // Set up texture
        glActiveTexture( GL_TEXTURE0 );
        glBindTexture( GL_TEXTURE_2D, colorBufferTexture );
        glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, 0 );
        glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, 0 );
        glTexImage2D( GL_TEXTURE_2D, GLint( 0 ), GL_RGBA8, GLsizei( 640 ), GLsizei( 360 ), 0,
                GL_RGBA, GL_UNSIGNED_BYTE, nullptr );
        glBindTexture( GL_TEXTURE_2D, 0 );
 
        glPixelStorei( GL_UNPACK_ALIGNMENT, 4 );
 
        // Set up frame buffer
        glGenFramebuffers( 1, &frameBuffer );
        glBindFramebuffer( GL_FRAMEBUFFER, frameBuffer );
        glFramebufferTexture( GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, colorBufferTexture, 0 );
        glBindFramebuffer( GL_FRAMEBUFFER, 0 );
 
        while( !glfwWindowShouldClose( pWindow ) )
        {
                // Viewport setup
                int width = 0;
                int height = 0;
 
                glfwGetWindowSize( pWindow, &width, &height );
                glViewport( 0, 0, width, height );
 
                // Clear render targets
                glBindFramebuffer( GL_FRAMEBUFFER, frameBuffer );
                glClearBufferfv( GL_COLOR, 0, &glm::vec4( 1.0f, 0.0f, 5.0f, 1.0f )[0] );
 
                glBindFramebuffer( GL_FRAMEBUFFER, 0 );
                glClearBufferfv( GL_COLOR, 0, &glm::vec4( 0.1f, 0.1f, 0.1f, 1.0f )[0] );
                glClearBufferfi( GL_DEPTH_STENCIL, 0, 1.0f, 0 );
 
                glfwSwapBuffers( pWindow );
                glfwPollEvents();
        }
 
        return EXIT_SUCCESS;
}

Win32/GLEW

[font='Helvetica Neue', Arial, Verdana, sans-serif]#define GLEW_STATIC[/font]
#define WIN32_LEAN_AND_MEAN
 
#include <Windows.h>
#include <GL/glew.h>
#include <GL/wglew.h>
#include <GLM/glm.hpp>
#include "Debugging/glDebugging.h"
 
LRESULT CALLBACK WndProc( HWND, UINT, WPARAM, LPARAM );
 
struct RunInfo
{
        bool bRunning;
};
 
int main( int argc, char* argv[] )
{
        STARTUPINFO startInfo;
        HINSTANCE hInstance = ( HINSTANCE )GetModuleHandle( NULL );
       
        GetStartupInfo( &startInfo );
 
        WNDCLASSEX wcex;
 
        wcex.cbSize                     = sizeof( WNDCLASSEX );
    wcex.style          = CS_HREDRAW | CS_VREDRAW | CS_OWNDC;
    wcex.lpfnWndProc    = WndProc;
    wcex.cbClsExtra     = 0;
    wcex.cbWndExtra     = 0;
    wcex.hInstance      = hInstance;
    wcex.hIcon          = LoadIcon( hInstance, MAKEINTRESOURCE( IDI_APPLICATION ) );
    wcex.hCursor        = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground  = ( HBRUSH )( COLOR_WINDOW + 1 );
    wcex.lpszMenuName   = NULL;
    wcex.lpszClassName  = L"NoGLFW";
    wcex.hIconSm        = LoadIcon( hInstance, MAKEINTRESOURCE( IDI_APPLICATION ) );
 
        if( !RegisterClassEx( &wcex ) )
                return -1;
 
        RECT rc = { 0, 0, 640, 360 };
        AdjustWindowRectEx( &rc, WS_OVERLAPPEDWINDOW, FALSE, WS_EX_APPWINDOW | WS_EX_WINDOWEDGE );
 
        HWND hWnd = CreateWindowEx(
                WS_EX_APPWINDOW | WS_EX_WINDOWEDGE,
                L"NoGLFW",
                L"NoGLFW",
                WS_OVERLAPPEDWINDOW,
                0,
                0,
                rc.right - rc.left,
                rc.bottom - rc.top,
                NULL,
                NULL,
                hInstance,
                NULL );
        if( !hWnd )
                return -1;
 
        PIXELFORMATDESCRIPTOR pfd =
        {
                sizeof( PIXELFORMATDESCRIPTOR ),                                                        // Struct size
                1,                                                                                                                      // Version
                PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER,     // Support OpenGL/Double Buffering
                PFD_TYPE_RGBA,                                                                                          // Request RGBA format
                32,                                                                                                                     // Output color depth
                0, 0, 0, 0, 0, 0,                                                                                       // Color bits, ignored
                0,                                                                                                                      // No alpha buffer
                0,                                                                                                                      // Shift bit, ignored
                0,                                                                                                                      // No accumulation buffer
                0, 0, 0, 0,                                                                                                     // Accumulation bits, ignored
                0,                                                                                                                      // No z-buffer
                0,                                                                                                                      // No stencil buffer
                0,                                                                                                                      // No auxillary buffer
                PFD_MAIN_PLANE,                                                                                         // Main drawing layer
                0,                                                                                                                      // Reserved
                0, 0, 0                                                                                                         // Layer masks, ignored
        };
 
        HDC hDc = GetDC( hWnd );
        if( !hDc )
                return -1;
 
        unsigned int pixelFormat = ChoosePixelFormat( hDc, &pfd );
        if( !pixelFormat )
                return -1;
 
        if( !SetPixelFormat( hDc, pixelFormat, &pfd ) )
                return -1;
 
        HGLRC tempContext = wglCreateContext( hDc );
        wglMakeCurrent( hDc, tempContext );
 
        glewExperimental = GL_TRUE;
        GLenum err = glewInit();
 
        int attribs[] =
        {
                WGL_CONTEXT_MAJOR_VERSION_ARB, 4,
                WGL_CONTEXT_MINOR_VERSION_ARB, 1,
                WGL_CONTEXT_FLAGS_ARB, WGL_CONTEXT_FORWARD_COMPATIBLE_BIT_ARB,
                WGL_CONTEXT_PROFILE_MASK_ARB, WGL_CONTEXT_CORE_PROFILE_BIT_ARB,
                WGL_CONTEXT_FLAGS_ARB, 0,
                0
        };
 
        HGLRC hRc = tempContext;
        if( wglewIsSupported( "WGL_ARB_create_context" ) == 1 )
        {
                hRc = wglCreateContextAttribsARB( hDc, 0, attribs );
                wglMakeCurrent( NULL, NULL );
                wglDeleteContext( tempContext );
                wglMakeCurrent( hDc, hRc );
        }
 
        //Checking GL version
    const GLubyte* GLVersionString = glGetString( GL_VERSION );
 
    //Or better yet, use the GL3 way to get the version number
    int OpenGLVersion[2];
    glGetIntegerv( GL_MAJOR_VERSION, &OpenGLVersion[0] );
    glGetIntegerv( GL_MINOR_VERSION, &OpenGLVersion[1] );
 
        RunInfo runInfo;
        runInfo.bRunning = true;
 
        SetWindowLongPtr( hWnd, GWLP_USERDATA, ( LONG_PTR )&runInfo );
 
        ShowWindow( hWnd, SW_SHOWDEFAULT );
        UpdateWindow( hWnd );
 
        MSG msg = { 0 };
        while( runInfo.bRunning )
        {
                if( PeekMessage( &msg, hWnd, 0, 0, PM_REMOVE ) )
                {
                        TranslateMessage( &msg );
                        DispatchMessage( &msg );
                }
 
                glViewport( 0, 0, 640, 360 );
                glClearBufferfv( GL_COLOR, 0, &glm::vec4( 0.1f, 0.1f, 0.1f, 1.0f )[0] );
 
                SwapBuffers( hDc );
        }
 
        return 0;
}
 
LRESULT CALLBACK WndProc( HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam )
{
        RunInfo* pRunInfo = ( RunInfo* )GetWindowLongPtr( hWnd, GWLP_USERDATA );
 
        switch( message )
        {
                case WM_DESTROY:
                        {
                                pRunInfo->bRunning = false;
                                PostQuitMessage( 0 );
                        }
                        break;
        }
 
        return DefWindowProc( hWnd, message, wParam, lParam );
}

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To get a pixel format with no depth buffer, you have to specify the PFD_DEPTH_DONTCARE flag.

 

In any event, ChoosePixelFormat returns the first supported pixel format closest to your requirements. It may be that, for your card, the only formats supporting the requirements you specified also have 4 auxiliary buffers.

 

So to answer your first question - yes and no. You have control over what you ask for, but the drivers will decide what to give you. ;-)

 

I have no insight into your second question.

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I guess my next question would be is what is the purpose of the auxiliary buffers?

 

For example if I were to set up a GBuffer for a deferred pipeline should I take advantage of the fact the auxiliary buffers exist and use those as my GBuffer surfaces instead of manually creating my own?

 

My guess is that I'm not guaranteed that they are going to exist. So I would want to allocate my own framebuffer/texture sets anyway. But then at that point it seems like I'm just throwing away usable GPU memory.

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See http://www.glprogramming.com/red/chapter10.html

 

 

OpenGL doesn't specify any particular uses for these buffers, so you can define and use them however you please.

 

I don't think it's specified anywhere, but I do think it's safe to assume that your driver isn't going allocate memory for these buffers until the first time you actually use them.

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Afaik ChoosePixelFormat is just a convenience function that could return a worse format than you need and that you dont need to use.

There are also other ways to request information about pixel formats and choose one yourself. You could for example try to loop over all formats and extract info from DescribePixelFormat and then decide on one of those yourself (maybe from 1 up until you get an error?).

Still that would be more work than most people would do and you are investing much time into replicating something librarys like glfw or sfml could do in a crossplatform way already and still get something worse.

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Still that would be more work than most people would do and you are investing much time into replicating something librarys like glfw or sfml could do in a crossplatform way already and still get something worse.

It's part of my job to investigate.

 

Didn't keep a log of everything I changed but the debug context is now creating properly in my Win32 example. Haven't been able to remove the auxiliary buffers but at least on the Win32 example I was able to eliminate the depth/stencil so I have the option to manually create it later for use in an FBO. Not sure what I have set up wrong with GLFW that it still creates the depth/stencil.

Edited by CornyKorn

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