Bug#582801: libsdl1.2debian-all: SDL_GL_SWAP_CONTROL can't work (video tearing)

[Sylvain Beucler]

Package: libsdl1.2debian-all
Version: 1.2.14-6
Severity: important

SDL supports SDL_GL_SWAP_CONTROL since 1.2.10:

  SDL_GL_SetAttribute(SDL_GL_SWAP_CONTROL, 1);

This setting allows the graphics card to update the screen memory in
sync with the physical's screen vertical refresh rate (typically
60Hz).

However, there is a bug, because it does not work under GNU/Linux.  I
set this bug to important, because this is the _only_ way to avoid
tearing and jumpy scrolling under X11, and get a truly smooth
animation.

The bug is due to (AFAICT) an implementation mistake: it tries to load
'glXSwapIntervalMESA' or 'glXSwapIntervalSGI' dynamically ('dlsym')
from '/usr/lib/libGL.so.1', while it should load them through
'glXGetProcAddressARB', e.g.:

  myfunc = glXGetProcAddressARB("glXSwapIntervalMESA").

To exhibit this behavior:

- the following will fail:

  SDL_Init(SDL_INIT_EVERYTHING);
  if (SDL_GL_SetAttribute(SDL_GL_SWAP_CONTROL, 1) < 0) // since SDL v1.2.10
    fprintf(stderr, "SDL_GL_SWAP_CONTROL: %s\n", SDL_GetError());
  SDL_Surface* screen = SDL_SetVideoMode(SCREEN_WIDTH, SCREEN_HEIGHT, 0, SDL_OPENGL|SDL_DOUBLEBUF);
  int swap_control = -1;
  if (SDL_GL_GetAttribute(SDL_GL_SWAP_CONTROL, &swap_control) < 0)
    fprintf(stderr, "SDL_GL_SWAP_CONTROL: %s\n", SDL_GetError());
  else
    printf("SDL_GL_SWAP_CONTROL = %d\n", swap_control);

  => "SDL_GL_SWAP_CONTROL: OpenGL attribute is unsupported on this system"
  => 200Hz FPS and scrolling that "shakes"

- the following will work around the bug:

  GLint (*my_glXSwapIntervalMESA) ( unsigned interval );
  my_glXSwapIntervalMESA = (GLint (*)(unsigned)) glXGetProcAddressARB("glXSwapIntervalMESA");
  my_glXSwapIntervalMESA(1);

  => 60Hz FPS and smooth scrolling


I'm going to attach a scrolling demo so one can test the problem.
I'll also send a mail to the SDL mailing list to get their input.


-- System Information:
Debian Release: squeeze/sid
  APT prefers testing
  APT policy: (500, 'testing'), (300, 'unstable')
Architecture: i386 (i686)

Kernel: Linux 2.6.32-3-686 (SMP w/1 CPU core)
Locale: LANG=fr_FR.UTF-8, LC_CTYPE=fr_FR.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash

Versions of packages libsdl1.2debian-all depends on:
ii  libaa1                    1.4p5-38       ascii art library
ii  libartsc0                 1.5.9-3+b1     aRts sound system C support librar
ii  libasound2                1.0.22-2       shared library for ALSA applicatio
ii  libc6                     2.10.2-6       Embedded GNU C Library: Shared lib
ii  libcaca0                  0.99.beta17-1  colour ASCII art library
ii  libdirectfb-1.2-9         1.2.10.0-3     direct frame buffer graphics - sha
ii  libesd0                   0.2.41-7       Enlightened Sound Daemon - Shared 
ii  libncurses5               5.7+20100313-2 shared libraries for terminal hand
ii  libpulse0                 0.9.21-1       PulseAudio client libraries
ii  libslang2                 2.2.2-4        The S-Lang programming library - r
ii  libsvga1                  1:1.4.3-29     console SVGA display libraries
ii  libx11-6                  2:1.3.3-3      X11 client-side library

libsdl1.2debian-all recommends no packages.

libsdl1.2debian-all suggests no packages.

-- no debconf information