OpenGL -> wglCreateContext, wglMakeCurrent in WndProc's WM_CREATE makes rendering fail - windows

Question

OS: Windows 7, 64 bit
Visual Studio 2010, debug, 32 bit
I'm trying out a simple Windows program to start working with openGL: all the program should do is clear the color buffer with glClear(GL_COLOR_BUFFER_BIT).
In the tutorials I found on the web, I find that people are creating and setting the openGL context during the creation of the window (and thus during the treatment of the WM_CREATE message). Somehow, I find that this doesn't work for my code and I don't seem to be able to figure it out. So if anyone could point me in the right direction.
Please find hereafter the code how I expect that it should work based on tutorials:
#include <windows.h>
#include <GL\glew.h>
#include <GL\wglew.h>
#include "Application.h"
HWND ghMainWnd = 0;
bool InitWindowsApp(HINSTANCE instanceHandle, int show);
int Run();
LRESULT CALLBACK
WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
HDC hDC;
HGLRC hGLRC;
HPALETTE hPalette;
void SetupPixelFormat()
{
PIXELFORMATDESCRIPTOR pfd = {};
pfd.nSize = sizeof(PIXELFORMATDESCRIPTOR);
pfd.nVersion = 1;
pfd.dwFlags = PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;
pfd.iPixelType = PFD_TYPE_RGBA;
pfd.cColorBits = 24;
pfd.cAlphaBits = 8;
pfd.cDepthBits = 32;
pfd.iLayerType = PFD_MAIN_PLANE;
int pixelFormat = ChoosePixelFormat(hDC, &pfd);
if(pixelFormat == 0)
{
MessageBox(ghMainWnd, L"ChoosePixelFormat() failed", L"Error", MB_ICONERROR | MB_OK);
exit(1);
}
if(!SetPixelFormat(hDC, pixelFormat, &pfd))
{
MessageBox(ghMainWnd, L"SetPixelFormat() failed", L"Error", MB_ICONERROR | MB_OK);
exit(1);
}
}
int WINAPI
WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR pCmdLine, int nShowCmd)
{
if(!InitWindowsApp(hInstance, nShowCmd))
return 0;
return Run();
}
bool InitWindowsApp(HINSTANCE instanceHandle, int show)
{
WNDCLASS wc;
wc.style = CS_OWNDC | CS_HREDRAW | CS_VREDRAW;
wc.lpfnWndProc = WndProc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hInstance = instanceHandle;
wc.hIcon = LoadIcon(0, IDI_APPLICATION);
wc.hCursor = LoadCursor(0, IDC_ARROW);
wc.hbrBackground = (HBRUSH) GetStockObject(BLACK_BRUSH);
wc.lpszMenuName = 0;
wc.lpszClassName = L"BasicWndClass";
if(!RegisterClass(&wc))
{
MessageBox(0, L"RegisterClass failed", 0, 0);
return false;
}
ghMainWnd = CreateWindow(
L"BasicWndClass",
L"BasicWindowsApp",
WS_OVERLAPPEDWINDOW,
0,
0,
600,
600,
0,
0,
instanceHandle,
0);
if(ghMainWnd == 0)
{
MessageBox(0, L"CreateWindow failed", 0, 0);
return false;
}
ShowWindow(ghMainWnd, show);
UpdateWindow(ghMainWnd);
return true;
}
int Run()
{
MSG msg = {0};
Application *pApp = new Application();
pApp->Init();
while(msg.message != WM_QUIT)
{
if(PeekMessage(&msg, 0, 0, 0, PM_REMOVE))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
else
{
//Do Other stuff
pApp->Draw();
SwapBuffers(hDC);
}
}
if(pApp)
{
delete pApp;
pApp = nullptr;
}
return (int)msg.wParam;
}
LRESULT CALLBACK
WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch(msg)
{
case WM_CREATE:
hDC = GetDC(ghMainWnd);
SetupPixelFormat();
hGLRC = wglCreateContext(hDC);
if(!hGLRC)
{
MessageBox(ghMainWnd, L"wglCreateContext() failed", L"Error", MB_ICONERROR | MB_OK);
exit(1);
}
if(!wglMakeCurrent(hDC, hGLRC))
{
MessageBox(ghMainWnd, L"wglMakeCurrent() failed", L"Error", MB_ICONERROR | MB_OK);
exit(1);
}
glViewport(0, 0, 600, 600);
glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
return 0;
case WM_LBUTTONDOWN:
return 0;
case WM_KEYDOWN:
if(wParam == VK_ESCAPE)
DestroyWindow(ghMainWnd);
return 0;
case WM_DESTROY:
if(hGLRC)
{
wglMakeCurrent(nullptr, nullptr);
wglDeleteContext(hGLRC);
}
ReleaseDC(ghMainWnd, hDC);
PostQuitMessage(0);
return 0;
}
return DefWindowProc(hWnd, msg, wParam, lParam);
}
Below, I found a "hack" to make it work: I basically move the opengl context creation into the InitWindowsApp() function... But why does the code above doesn't work?
#include <windows.h>
#include <GL\glew.h>
#include <GL\wglew.h>
#include "Application.h"
HWND ghMainWnd = 0;
bool InitWindowsApp(HINSTANCE instanceHandle, int show);
int Run();
LRESULT CALLBACK
WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
HDC hDC;
HGLRC hGLRC;
HPALETTE hPalette;
void SetupPixelFormat()
{
PIXELFORMATDESCRIPTOR pfd = {};
pfd.nSize = sizeof(PIXELFORMATDESCRIPTOR);
pfd.nVersion = 1;
pfd.dwFlags = PFD_DRAW_TO_WINDOW | PFD_SUPPORT_OPENGL | PFD_DOUBLEBUFFER;
pfd.iPixelType = PFD_TYPE_RGBA;
pfd.cColorBits = 24;
pfd.cAlphaBits = 8;
pfd.cDepthBits = 32;
pfd.iLayerType = PFD_MAIN_PLANE;
int pixelFormat = ChoosePixelFormat(hDC, &pfd);
if(pixelFormat == 0)
{
MessageBox(ghMainWnd, L"ChoosePixelFormat() failed", L"Error", MB_ICONERROR | MB_OK);
exit(1);
}
if(!SetPixelFormat(hDC, pixelFormat, &pfd))
{
MessageBox(ghMainWnd, L"SetPixelFormat() failed", L"Error", MB_ICONERROR | MB_OK);
exit(1);
}
}
int WINAPI
WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR pCmdLine, int nShowCmd)
{
if(!InitWindowsApp(hInstance, nShowCmd))
return 0;
return Run();
}
bool InitWindowsApp(HINSTANCE instanceHandle, int show)
{
WNDCLASS wc;
wc.style = CS_OWNDC | CS_HREDRAW | CS_VREDRAW;
wc.lpfnWndProc = WndProc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hInstance = instanceHandle;
wc.hIcon = LoadIcon(0, IDI_APPLICATION);
wc.hCursor = LoadCursor(0, IDC_ARROW);
wc.hbrBackground = (HBRUSH) GetStockObject(BLACK_BRUSH);
wc.lpszMenuName = 0;
wc.lpszClassName = L"BasicWndClass";
if(!RegisterClass(&wc))
{
MessageBox(0, L"RegisterClass failed", 0, 0);
return false;
}
ghMainWnd = CreateWindow(
L"BasicWndClass",
L"BasicWindowsApp",
WS_OVERLAPPEDWINDOW,
0,
0,
600,
600,
0,
0,
instanceHandle,
0);
if(ghMainWnd == 0)
{
MessageBox(0, L"CreateWindow failed", 0, 0);
return false;
}
ShowWindow(ghMainWnd, show);
UpdateWindow(ghMainWnd);
hDC = GetDC(ghMainWnd);
SetupPixelFormat();
hGLRC = wglCreateContext(hDC);
if(!hGLRC)
{
MessageBox(ghMainWnd, L"wglCreateContext() failed", L"Error", MB_ICONERROR | MB_OK);
exit(1);
}
if(!wglMakeCurrent(hDC, hGLRC))
{
MessageBox(ghMainWnd, L"wglMakeCurrent() failed", L"Error", MB_ICONERROR | MB_OK);
exit(1);
}
glViewport(0, 0, 600, 600);
glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
return true;
}
int Run()
{
MSG msg = {0};
Application *pApp = new Application();
pApp->Init();
while(msg.message != WM_QUIT)
{
if(PeekMessage(&msg, 0, 0, 0, PM_REMOVE))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
else
{
//Do Other stuff
pApp->Draw();
SwapBuffers(hDC);
}
}
if(pApp)
{
delete pApp;
pApp = nullptr;
}
return (int)msg.wParam;
}
LRESULT CALLBACK
WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch(msg)
{
case WM_CREATE:
return 0;
case WM_LBUTTONDOWN:
return 0;
case WM_KEYDOWN:
if(wParam == VK_ESCAPE)
DestroyWindow(ghMainWnd);
return 0;
case WM_DESTROY:
if(hGLRC)
{
wglMakeCurrent(nullptr, nullptr);
wglDeleteContext(hGLRC);
}
ReleaseDC(ghMainWnd, hDC);
PostQuitMessage(0);
return 0;
}
return DefWindowProc(hWnd, msg, wParam, lParam);
}

Answer 1

When WM_CREATE arrives, your ghMainWnd is still NULL (the message is sent before the CreateWindow call returns). Instead, use the hWnd parameter of the message in your GetDC call.