Still have tearing with double buffering

SDL Development
1

I’m using Linux, XFree 4.0.3, SDL 1.2.2 and OpenGL, and I’ve setup double
buffering, and yet I still am getting tearing. This is using an NVidia
GeForce2 DDR with the latest driver. (glxinfo shows I’ve got hardware
acceleration.) I set up double buffering with

SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER,1);

and then I set my video mode using

SDL_SetVideoMode(640, 480, 0, SDL_HWSURFACE | SDL_HWACCEL | SDL_OPENGL )

Using SDL_GL_GetAttribute shows that I did get double buffering. This
problem also rears its head if I set the video mode to use fullscreen.

Perhaps the problem lies herein: My monitor is being refreshed at 85 Hz.
My frames per second calculations show about 265Hz. How can this be
possible if SDL_GL_SwapBuffers() waits for the vertical blanking interval
before swapping? If it’s not waiting for the blanking interval, how can I
tell it to do so?

I can explain the tearing or the rest of my program in more detail if this
isn’t enough.

Any solution (including dropping SDL) will be very appreciated!

Thanks,
Andrew Straw

2

You could also limit the frames drawn per second with code like:

Uint32 lastticks=0;
Uint32 tick=0;
Uint32 delta_tick=0;

//Timer stuff
  tick = SDL_GetTicks();
  delta_tick = tick - lastticks;
  lastticks = tick;

  //limit to 40 revolutions per second (40*25ms = 1 second)
  if(delta_tick < 25)
    {
      SDL_Delay(25 - delta_tick);
      tick = SDL_GetTicks();
      delta_tick = delta_tick + tick - lastticks;
      lastticks = tick;
    }

I forget who’s code that came from, it was located somewhere in the mailing
list (possibly months ago) or on a webpage linked from the mailing list.

However this doesn’t solve the wait for vsync problem.

You may want to check if there is a configuration for the video driver that
will lock to vsync.

BTW this line:

SDL_SetVideoMode(640, 480, 0, SDL_HWSURFACE | SDL_HWACCEL | SDL_OPENGL )

Doesn’t need the SDL_HWSURFACE | SDL_HWACCEL flags (as they are for the 2D
api) if you are only using OpenGL. You only need the fullscreen flag for
fullscreen.

3

In the NVidia linux driver README file, it says to set the environment
variable __GL_SYNC_TO_VBLANK to a non-zero value.

It works! (But it doesn’t seem very portable.)

4

I’m using Linux, XFree 4.0.3, SDL 1.2.2 and OpenGL, and I’ve setup double
buffering, and yet I still am getting tearing. This is using an NVidia
GeForce2 DDR with the latest driver. (glxinfo shows I’ve got hardware
acceleration.) I set up double buffering with

SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER,1);

and then I set my video mode using

SDL_SetVideoMode(640, 480, 0, SDL_HWSURFACE | SDL_HWACCEL | SDL_OPENGL )

Using SDL_GL_GetAttribute shows that I did get double buffering. This
problem also rears its head if I set the video mode to use fullscreen.

Perhaps the problem lies herein: My monitor is being refreshed at 85 Hz.
My frames per second calculations show about 265Hz. How can this be
possible if SDL_GL_SwapBuffers() waits for the vertical blanking interval
before swapping? If it’s not waiting for the blanking interval, how can I
tell it to do so?

I can explain the tearing or the rest of my program in more detail if this
isn’t enough.

From the nvidia-driver readme:
__GL_SYNC_TO_VBLANK
Setting this to a non-zero value will force glXSwapBuffers
to sync to your monitor’s vertical refresh rate (perform
a swap only during the vertical blanking period).

I think you should try this option to force waiting for a vsync.
I have the same tearing with UT on my TNT.

HTH

Pius II.