Changing SDL/OpenGL Display Resolution Without Application Restart

Problem

In many modern Windows games, the user will be forced (either manually or automatically) to restart his/her game when changing display resolution. The reason for this is often technical, but primarily programmer lazyness. Often it is more convenient to let the user restart the game/application than to code a mechanism that is rarely invoked. But those of us old enough to have played non-hardware accelerated games will recognise this to be a regression in functionality.

For SDL-based OpenGL games and applications, changing the display resolution indeed proves somewhat problematic, but the solution is not as complex as some might have thought.

Changing the display resolution in SDL done through the function SDL_SetVideoMode() . This function creates a window and an OpenGL Rendering Context. Unfortunately, on Windows, the Rendering Context is tied to the window surface. Subsequent calls to SDL_SetVideoMode() will cause SDL to destroy the window and OpenGL context before re-creating it. If your game/application uses OpenGL to render the very graphics options menu, then the textures and shaders (as well as any VBOs, FBOs, display lists etc) used to render it will be invalid once the window re-appears. You'll be looking at a blank and/or corrupted screen, or worse, the application may crash.

One option is to simply reload all the textures and shaders after the switch, but this will take up some time, especially if the game has a complete level loaded with many textures, VBOs and shaders. If the player is trying different settings to see which give the best performance/quality, this waiting time will quickly become very annoying.

Thankfully, there is a solution that automagically solves this.

Solution

The solution is to create a second, temporary OpenGL context, to which we share the GL resources. On the Windows platform, there is the function wglShareLists() . Originally, this function was intended to allow CAD/CAM applications to render the same 'display lists' to different viewports/windows (with each their own GL context). As textures, geometry and shaders became 'objects' (resources) like display lists in later GL versions, they also inherited (by design) the same resource management behavior of display list sharing.

The following code snipped details the operation:

int g_display_width = 800;
int g_display_height = 600;
int g_colorbits = 32;
int g_depthbits = 16;
int g_multisample = 8;
bool g_vsync = true;
bool g_fullscreen = true;
bool g_resize = false;


// sets video mode
bool CApp::SetVideoMode()
{
 // framebuffer
 SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
 SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, g_depthbits);
 
 // v-sync
 if (g_vsync) {
  SDL_GL_SetAttribute(SDL_GL_SWAP_CONTROL, 1);
 } else {
  SDL_GL_SetAttribute(SDL_GL_SWAP_CONTROL, 0);
 }
 
 // anti-aliasing
 if (g_multisample > 1) {
  SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1);
  SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, g_multisample);
 } else {
  SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 0);
  SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, 0);
 }
 
 // window flags
 int flags = 0;
 if (g_fullscreen) flags |= SDL_FULLSCREEN;
 if (g_resize)     flags |= SDL_RESIZABLE;
 
 // set video mode
 SDL_Surface *surface = SDL_SetVideoMode(g_display_width, g_display_height,
                                         g_colorbits, SDL_OPENGL | flags );
 if (!surface) {
  PrintError(" Failed to set video mode: %s\n", SDL_GetError());
  return false;
 }
 
 // center window
 CenterWindow();
 
 // success
 return true;
}


// changes display resolution at any time
bool CApp::ChangeVideoMode()
{
 SDL_SysWMinfo info;
 
 // get window handle from SDL
 SDL_VERSION(&info.version);
 if (SDL_GetWMInfo(&info) == -1) {
  PrintError("SDL_GetWMInfo #1 failed\n");
  return false;
 }
 
 // get device context handle
 HDC tempDC = GetDC( info.window );
 
 // create temporary context
 HGLRC tempRC = wglCreateContext( tempDC );
 if (tempRC == NULL) {
  PrintError("wglCreateContext failed\n");
  return false;
 }
 
 // share resources to temporary context
 SetLastError(0);
 if (!wglShareLists(info.hglrc, tempRC)) {
  PrintError("wglShareLists #1 failed\n");
  return false;
 }
 
 // set video mode
 if (!SetVideoMode()) return false;
 
 // previously used structure may possibly be invalid, to be sure we get it again
 SDL_VERSION(&info.version);
 if (SDL_GetWMInfo(&info) == -1) {
  PrintError("SDL_GetWMInfo #2 failed\n");
  return false;
 }
 
 // share resources to new SDL-created context
 if (!wglShareLists(tempRC, info.hglrc)) {
  PrintError("wglShareLists #2 failed\n");
  return false;
 }
 
 // we no longer need our temporary context
 if (!wglDeleteContext(tempRC)) {
  PrintError("wglDeleteContext failed\n");
  return false;
 }
 
 // success
 return true;
}

SetVideoMode() should be called at application launch. It does the usual SDL initialization.

ChangeVideoMode() can later be called at any time. ChangeVideoMode will in turn calls SetVideoMode(), but before doing so creates a temporary GL context, and shares the application's current resources with it. The OpenGL drivers will then not destroy those resources once SDL destroys it's own GL context.

Once SDL has switched the display resolution and created a new window & GL context, we share the resources back, and we finally destroy our temporary context, which we no longer need.

Caveat

While wglShareLists() shares resources between contexts, it does not restore or copy any OpenGL states over. So for example, if you call glEnable(GL_NORMALIZE) at application launch, you will need to call it again to restore the state in a new context. If your application relies a lot on 'lingering' states (e.g. to minimize redundant state changes) then you will need to be careful.

Then there is a function named wglCopyContext() , which as it's name implies can copy the entire OpenGL context from one to another. Unfortunately, it turns out to be highly unreliable, considered deprecated and should be avoided.

Centering SDL Window

For completeness sake, here is also a snippet of Windows-only code that centers the SDL window, when not running the application in full-screen mode.

// centers window
void CApp::CenterWindow()
{
 if (!g_fullscreen) return;
 SDL_SysWMinfo info;
 SDL_VERSION(&info.version);
 if (SDL_GetWMInfo(&info) > 0) {
  HWND hwnd = info.window;
  int w = GetSystemMetrics(SM_CXSCREEN);
  int h = GetSystemMetrics(SM_CYSCREEN);
  RECT rc;
  GetWindowRect(hwnd, &rc);
  int x = (w - (rc.right - rc.left))/2;
  int y = (h - (rc.bottom - rc.top))/2;
  SetWindowPos(hwnd, NULL, x, y, 0, 0, SWP_NOSIZE | SWP_NOZORDER);
 }
}

MSDN unreliable

Along the way, I made the mistake to rely on the MSDN section that documents the functionality of wglShareLists() . For unknown reasons, a number of WGL related pages have been re-written to become garbled and ambigeous. So for carity: the first argument of wglShareLists is the source context, the second the destination context, not the other way around.

Also, the claim that the Windows API function GetLastError() returns an error code containing further info when wglShareLists fails, is incorrect, or at least unreliable. During my tests it would consistently return an error code that does not conform to any Microsoft error number specifications, so it is pretty much useless for debugging.

Then there is also the claim that wglShareLists() only works with two identical pixel formats. This could be interpreted to mean that, specific to our usage, if the display depth or other framebuffer properties other than the width and height dimensions are changed, the function will fail. I have not been able to determine yet if this assertion holds true in all situations, but it appears that wglShareLists() succeeds reliably even when if the pixel formats vary.

Linux

One final note: on Linux the SDL-owned OpenGL context is never destroyed in the first place (as it should), and as such no special measures need to be taken on Linux when switching display resolutions. It just works!

Email: martijn AT bytehazard DOT com
Page created: 2010-05-31
Last modified: 2013-06-03