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Perico El de los Palotes

OpenGL
WGL_EXT_swap_control extension present? to activate it?

5 posts in this topic

Hi,
in my multiplatform OpenGL application, i require WGL_EXT_swap_control extension capability. 
I have it running properly on Linux and MacOSX. 

In WinXP and Win7, glGetString(GL_EXTENSIONS) returns null (i still dont kwno why), so,
- how to know if WGL_EXT_swap_control extension is present?
- how to enable it?

regards
dani

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First, WGL extensions belongs to the WGL API, which is the Windows layer for OpenGL. Not saying that other platforms don't have an equivalent extension, but WGL_EXT_swap_control itself is necessarily Windows only.
 

Second, if glGetString returns a null pointer, then that's because you haven't created and activated a rendering context yet.

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Thank you, of course you're right.

 

- After having my context created, glGetString(GL_EXTENSIONS) returns: GL_EXT_blend_minmax GL_EXT_blend_subtract GL_EXT_blend_color GL_EXT_abgr GL_EXT_texture3D GL_EXT_clip_volume_hint GL_EXT_compiled_vertex_array GL_SGIS_texture_edge_clamp GL_SGIS_generate_mipmap GL_EXT_draw_range_elements GL_SGIS_texture_lod GL_EXT_rescale_normal GL_EXT_packed_pixels GL_EXT_separate_specular_color GL_ARB_multitexture GL_EXT_texture_env_combine GL_EXT_bgra GL_EXT_blend_func_separate GL_EXT_secondary_color GL_EXT_fog_coord GL_EXT_texture_env_add GL_ARB_texture_cube_map GL_ARB_transpose_matrix GL_ARB_texture_env_add GL_IBM_texture_mirrored_repeat GL_EXT_multi_draw_arrays GL_NV_blend_square GL_ARB_texture_compression GL_3DFX_texture_compression_FXT1 GL_EXT_texture_filter_anisotropic GL_ARB_texture_border_clamp GL_ARB_point_parameters GL_ARB_texture_env_combine GL_ARB_texture_env_dot3 GL_ARB_texture_env_crossbar GL_EXT_texture_compression_s3tc GL_ARB_shadow GL_ARB_window_pos GL_EXT_shadow_funcs GL_EXT_stencil_wrap GL_ARB_vertex_program GL_EXT_texture_rectangle GL_ARB_fragment_program GL_EXT_stencil_two_side GL_ATI_separate_stencil GL_ARB_vertex_buffer_object GL_EXT_texture_lod_bias GL_ARB_fragment_shader GL_ARB_shader_objects GL_ARB_shading_language_100 GL_ARB_texture_non_power_of_two GL_ARB_vertex_shader GL_NV_texgen_reflection GL_ARB_point_sprite GL_EXT_blend_equation_separate GL_ARB_depth_texture GL_ARB_texture_rectangle GL_ARB_draw_buffers GL_ARB_pixel_buffer_object GL_WIN_swap_hint GL_EXT_framebuffer_object 

--> WGL_EXT_swap_control is not found. Current OS is WinXP.

 

- I didn't want to go on and on, and i made a mistake: i require WGL_EXT_swap_control under Windows, and GLX_SGI_video_sync under Linux, because what i want to do at the end is activating synchronization for swap buffering and vertical refresh. In WinXP it doesn't work for me.

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If this is an older NVIDIA card (which it looks like from the available extensions) then you should note the text in the NVIDIA control panel advising that for GL applications you should use the control panel for selecting vsync.

 

Despite this, I have seen some more recent NVIDIA cards where using WGL_EXT_swap_control does work, but there are a few gotchas with it.

 

First one is that the driver may be automatically chopping your extensions string.  It does this for compatibility with some older games (e.g. in the Quake series) where the extensions string was copied to a fixed size buffer, causing the game to crash if the extensions string was too long.  The driver may be mis-identifying your game as one that needs this attention and you can verify if this is the case by comparing your extensions string with e.g. that from the GL extensions viewer.  Unfortunately NVIDIA have removed the capability to control this behaviour from more recent drivers, so you're stuck with it.

 

Secondly, you may be able to access the WGL extensions by calling wglGetExtensionsStringEXT.  Check that for the presence of WGL_EXT_swap_control.

 

Thirdy, it's always an option to bypass the extensions string and just call wglGetProcAddress on wglSwapIntervalEXT - if it returns non-NULL for you then you should be able to make wglSwapIntervalEXT calls even despite it not being present in the extensions string (whether or not they work is another matter).

 

It's worth noting that this is absolutely nothing to do with the OS - this is all behaviour dictated by the driver.  Yes, the driver is OS-specific, but the wacky stuff is in the driver, not the OS.

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If this is an older NVIDIA card (which it looks like from the available extensions) then you should note the text in the NVIDIA control panel advising that for GL applications you should use the control panel for selecting vsync.

 

Despite this, I have seen some more recent NVIDIA cards where using WGL_EXT_swap_control does work, but there are a few gotchas with it.

 

First one is that the driver may be automatically chopping your extensions string.  It does this for compatibility with some older games (e.g. in the Quake series) where the extensions string was copied to a fixed size buffer, causing the game to crash if the extensions string was too long.  The driver may be mis-identifying your game as one that needs this attention and you can verify if this is the case by comparing your extensions string with e.g. that from the GL extensions viewer.  Unfortunately NVIDIA have removed the capability to control this behaviour from more recent drivers, so you're stuck with it.

 

Secondly, you may be able to access the WGL extensions by calling wglGetExtensionsStringEXT.  Check that for the presence of WGL_EXT_swap_control.

 

Thirdy, it's always an option to bypass the extensions string and just call wglGetProcAddress on wglSwapIntervalEXT - if it returns non-NULL for you then you should be able to make wglSwapIntervalEXT calls even despite it not being present in the extensions string (whether or not they work is another matter).

 

It's worth noting that this is absolutely nothing to do with the OS - this is all behaviour dictated by the driver.  Yes, the driver is OS-specific, but the wacky stuff is in the driver, not the OS.

 

WGL_EXT_swap_control works with nvidia GPUs as long as the user has set v-sync to "use the application settings"(or something equivalent, varies a bit between driver versions) in the control panel, if it is set to something else (adaptive, adaptive(half refresh rate) , Off, On, etc),  WGL_EXT_swap_control is ignored. (It is possible that nvidia hides the extension if the user has forced a setting for it (I know i would do that if i was nvidia atleast to show applications that it is pointless to display that option to the end user)

Edited by SimonForsman
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