I'm trying to support an Optimus system, that allows me to run my framework on an Intel GPU or on a NVIDIA GPU. No problems since I tried to support instanced drawing with instanced arrays.
While NVIDIA supports OpenGL 4.5, the Intel card supports OpenGL 3.1 with few extensions. During this debugging session, I discovered that glVertexAttribDivisor was defined by GL_ARB_instanced_arrays, not by GL_ARB_draw_instanced as I always believed till now.
However I've noticed that glVertexAttribDivisor is currently implemented (and it actually works!), but the relative extension is not advertised. Investigating on this issue, I found few other functions currently implemented, without being declared supported via the extension mechanism.
Here is a log of the extension retrieval:
glGetString(Version) = 3.1.0 - Build 8.15.10.2622
glGetError() = NoError
glGetIntegerv(NumExtensions, 117)
glGetError() = NoError
glGetStringi(0x1F03, 0) = GL_EXT_blend_minmax
glGetError() = NoError
glGetStringi(0x1F03, 1) = GL_EXT_blend_subtract
glGetError() = NoError
glGetStringi(0x1F03, 2) = GL_EXT_blend_color
glGetError() = NoError
glGetStringi(0x1F03, 3) = GL_EXT_abgr
glGetError() = NoError
glGetStringi(0x1F03, 4) = GL_EXT_texture3D
glGetError() = NoError
glGetStringi(0x1F03, 5) = GL_EXT_clip_volume_hint
glGetError() = NoError
glGetStringi(0x1F03, 6) = GL_EXT_compiled_vertex_array
glGetError() = NoError
glGetStringi(0x1F03, 7) = GL_SGIS_texture_edge_clamp
glGetError() = NoError
glGetStringi(0x1F03, 8) = GL_SGIS_generate_mipmap
glGetError() = NoError
glGetStringi(0x1F03, 9) = GL_EXT_draw_range_elements
glGetError() = NoError
glGetStringi(0x1F03, 10) = GL_SGIS_texture_lod
glGetError() = NoError
glGetStringi(0x1F03, 11) = GL_EXT_rescale_normal
glGetError() = NoError
glGetStringi(0x1F03, 12) = GL_EXT_packed_pixels
glGetError() = NoError
glGetStringi(0x1F03, 13) = GL_EXT_texture_edge_clamp
glGetError() = NoError
glGetStringi(0x1F03, 14) = GL_EXT_separate_specular_color
glGetError() = NoError
glGetStringi(0x1F03, 15) = GL_ARB_multitexture
glGetError() = NoError
glGetStringi(0x1F03, 16) = GL_EXT_texture_env_combine
glGetError() = NoError
glGetStringi(0x1F03, 17) = GL_EXT_bgra
glGetError() = NoError
glGetStringi(0x1F03, 18) = GL_EXT_blend_func_separate
glGetError() = NoError
glGetStringi(0x1F03, 19) = GL_EXT_secondary_color
glGetError() = NoError
glGetStringi(0x1F03, 20) = GL_EXT_fog_coord
glGetError() = NoError
glGetStringi(0x1F03, 21) = GL_EXT_tIl thread 0xe14 è terminato con il codice 0 (0x0).
exture_env_add
glGetError() = NoError
glGetStringi(0x1F03, 22) = GL_ARB_texture_cube_map
glGetError() = NoError
glGetStringi(0x1F03, 23) = GL_ARB_transpose_matrix
glGetError() = NoError
glGetStringi(0x1F03, 24) = GL_ARB_texture_env_add
glGetError() = NoError
glGetStringi(0x1F03, 25) = GL_IBM_texture_mirrored_repeat
glGetError() = NoError
glGetStringi(0x1F03, 26) = GL_EXT_multi_draw_arrays
glGetError() = NoError
glGetStringi(0x1F03, 27) = GL_NV_blend_square
glGetError() = NoError
glGetStringi(0x1F03, 28) = GL_ARB_texture_compression
glGetError() = NoError
glGetStringi(0x1F03, 29) = GL_3DFX_texture_compression_FXT1
glGetError() = NoError
glGetStringi(0x1F03, 30) = GL_EXT_texture_filter_anisotropic
glGetError() = NoError
glGetStringi(0x1F03, 31) = GL_ARB_texture_border_clamp
glGetError() = NoError
glGetStringi(0x1F03, 32) = GL_ARB_point_parameters
glGetError() = NoError
glGetStringi(0x1F03, 33) = GL_ARB_texture_env_combine
glGetError() = NoError
glGetStringi(0x1F03, 34) = GL_ARB_texture_env_dot3
glGetError() = NoError
glGetStringi(0x1F03, 35) = GL_ARB_texture_env_crossbar
glGetError() = NoError
glGetStringi(0x1F03, 36) = GL_EXT_texture_compression_s3tc
glGetError() = NoError
glGetStringi(0x1F03, 37) = GL_ARB_shadow
glGetError() = NoError
glGetStringi(0x1F03, 38) = GL_ARB_window_pos
glGetError() = NoError
glGetStringi(0x1F03, 39) = GL_EXT_shadow_funcs
glGetError() = NoError
glGetStringi(0x1F03, 40) = GL_EXT_stencil_wrap
glGetError() = NoError
glGetStringi(0x1F03, 41) = GL_ARB_vertex_program
glGetError() = NoError
glGetStringi(0x1F03, 42) = GL_EXT_texture_rectangle
glGetError() = NoError
glGetStringi(0x1F03, 43) = GL_ARB_fragment_program
glGetError() = NoError
glGetStringi(0x1F03, 44) = GL_EXT_stencil_two_side
glGetError() = NoError
glGetStringi(0x1F03, 45) = GL_ATI_separate_stencil
glGetError() = NoError
glGetStringi(0x1F03, 46) = GL_ARB_vertex_buffer_object
glGetError() = NoError
glGetStringi(0x1F03, 47) = GL_EXT_texture_lod_bias
glGetError() = NoError
glGetStringi(0x1F03, 48) = GL_ARB_occlusion_query
glGetError() = NoError
glGetStringi(0x1F03, 49) = GL_ARB_fragment_shader
glGetError() = NoError
glGetStringi(0x1F03, 50) = GL_ARB_shader_objects
glGetError() = NoError
glGetStringi(0x1F03, 51) = GL_ARB_shading_language_100
glGetError() = NoError
glGetStringi(0x1F03, 52) = GL_ARB_texture_non_power_of_two
glGetError() = NoError
glGetStringi(0x1F03, 53) = GL_ARB_vertex_shader
glGetError() = NoError
glGetStringi(0x1F03, 54) = GL_NV_texgen_reflection
glGetError() = NoError
glGetStringi(0x1F03, 55) = GL_ARB_point_sprite
glGetError() = NoError
glGetStringi(0x1F03, 56) = GL_ARB_fragment_program_shadow
glGetError() = NoError
glGetStringi(0x1F03, 57) = GL_EXT_blend_equation_separate
glGetError() = NoError
glGetStringi(0x1F03, 58) = GL_ARB_depth_texture
glGetError() = NoError
glGetStringi(0x1F03, 59) = GL_ARB_texture_rectangle
glGetError() = NoError
glGetStringi(0x1F03, 60) = GL_ARB_draw_buffers
glGetError() = NoError
glGetStringi(0x1F03, 61) = GL_ARB_color_buffer_float
glGetError() = NoError
glGetStringi(0x1F03, 62) = GL_ARB_half_float_pixel
glGetError() = NoError
glGetStringi(0x1F03, 63) = GL_ARB_texture_float
glGetError() = NoError
glGetStringi(0x1F03, 64) = GL_ARB_pixel_buffer_object
glGetError() = NoError
glGetStringi(0x1F03, 65) = GL_EXT_framebuffer_object
glGetError() = NoError
glGetStringi(0x1F03, 66) = GL_ARB_draw_instanced
glGetError() = NoError
glGetStringi(0x1F03, 67) = GL_ARB_half_float_vertex
glGetError() = NoError
glGetStringi(0x1F03, 68) = GL_EXT_draw_buffers2
glGetError() = NoError
glGetStringi(0x1F03, 69) = GL_WIN_swap_hint
glGetError() = NoError
glGetStringi(0x1F03, 70) = GL_EXT_texture_sRGB
glGetError() = NoError
glGetStringi(0x1F03, 71) = GL_ARB_multisample
glGetError() = NoError
glGetStringi(0x1F03, 72) = GL_EXT_packed_float
glGetError() = NoError
glGetStringi(0x1F03, 73) = GL_EXT_texture_shared_exponent
glGetError() = NoError
glGetStringi(0x1F03, 74) = GL_ARB_texture_rg
glGetError() = NoError
glGetStringi(0x1F03, 75) = GL_ARB_texture_compression_rgtc
glGetError() = NoError
glGetStringi(0x1F03, 76) = GL_NV_conditional_render
glGetError() = NoError
glGetStringi(0x1F03, 77) = GL_EXT_texture_swizzle
glGetError() = NoError
glGetStringi(0x1F03, 78) = GL_ARB_sync
glGetError() = NoError
glGetStringi(0x1F03, 79) = GL_ARB_framebuffer_sRGB
glGetError() = NoError
glGetStringi(0x1F03, 80) = GL_EXT_packed_depth_stencil
glGetError() = NoError
glGetStringi(0x1F03, 81) = GL_ARB_depth_buffer_float
glGetError() = NoError
glGetStringi(0x1F03, 82) = GL_EXT_transform_feedback
glGetError() = NoError
glGetStringi(0x1F03, 83) = GL_EXT_framebuffer_blit
glGetError() = NoError
glGetStringi(0x1F03, 84) = GL_EXT_framebuffer_multisample
glGetError() = NoError
glGetStringi(0x1F03, 85) = GL_ARB_framebuffer_object
glGetError() = NoError
glGetStringi(0x1F03, 86) = GL_EXT_texture_array
glGetError() = NoError
glGetStringi(0x1F03, 87) = GL_EXT_texture_integer
glGetError() = NoError
glGetStringi(0x1F03, 88) = GL_ARB_map_buffer_range
glGetError() = NoError
glGetStringi(0x1F03, 89) = GL_EXT_texture_snorm
glGetError() = NoError
glGetStringi(0x1F03, 90) = GL_INTEL_performance_queries
glGetError() = NoError
glGetStringi(0x1F03, 91) = GL_ARB_copy_buffer
glGetError() = NoError
glGetStringi(0x1F03, 92) = GL_ARB_sampler_objects
glGetError() = NoError
glGetStringi(0x1F03, 93) = GL_NV_primitive_restart
glGetError() = NoError
glGetStringi(0x1F03, 94) = GL_ARB_seamless_cube_map
glGetError() = NoError
glGetStringi(0x1F03, 95) = GL_ARB_uniform_buffer_object
glGetError() = NoError
glGetStringi(0x1F03, 96) = GL_ARB_depth_clamp
glGetError() = NoError
glGetStringi(0x1F03, 97) = GL_ARB_vertex_array_bgra
glGetError() = NoError
glGetStringi(0x1F03, 98) = GL_ARB_draw_elements_base_vertex
glGetError() = NoError
glGetStringi(0x1F03, 99) = GL_ARB_fragment_coord_conventions
glGetError() = NoError
glGetStringi(0x1F03, 100) = GL_EXT_gpu_program_parameters
glGetError() = NoError
glGetStringi(0x1F03, 101) = GL_ARB_compatibility
glGetError() = NoError
glGetStringi(0x1F03, 102) = GL_ARB_vertex_array_object
glGetError() = NoError
glGetStringi(0x1F03, 103) = WGL_EXT_depth_float
glGetError() = NoError
glGetStringi(0x1F03, 104) = WGL_ARB_buffer_region
glGetError() = NoError
glGetStringi(0x1F03, 105) = WGL_ARB_extensions_string
glGetError() = NoError
glGetStringi(0x1F03, 106) = WGL_ARB_make_current_read
glGetError() = NoError
glGetStringi(0x1F03, 107) = WGL_ARB_pixel_format
glGetError() = NoError
glGetStringi(0x1F03, 108) = WGL_ARB_pbuffer
glGetError() = NoError
glGetStringi(0x1F03, 109) = WGL_EXT_extensions_string
glGetError() = NoError
glGetStringi(0x1F03, 110) = WGL_EXT_swap_control
glGetError() = NoError
glGetStringi(0x1F03, 111) = WGL_EXT_swap_control_tear
glGetError() = NoError
glGetStringi(0x1F03, 112) = WGL_ARB_multisample
glGetError() = NoError
glGetStringi(0x1F03, 113) = WGL_ARB_pixel_format_float
glGetError() = NoError
glGetStringi(0x1F03, 114) = WGL_ARB_framebuffer_sRGB
glGetError() = NoError
glGetStringi(0x1F03, 115) = WGL_ARB_create_context
glGetError() = NoError
glGetStringi(0x1F03, 116) = WGL_EXT_pixel_format_packed_float
glGetError() = NoError
And here there is a log of the function mismatches:
// The command FramebufferTextureFaceARB is defined, but not supported by any feature.
// The command CullParameterdvEXT is defined, but not supported by any feature.
// The command CullParameterfvEXT is defined, but not supported by any feature.
// The command TransformFeedbackVaryings_Unmanaged is defined, but not supported by any feature.
// The command FramebufferTexture is defined, but not supported by any feature.
// The command TexImage2DMultisample is defined, but not supported by any feature.
// The command TexImage3DMultisample is defined, but not supported by any feature.
// The command GetMultisamplefv is defined, but not supported by any feature.
// The command SampleMaski is defined, but not supported by any feature.
// The command VertexAttribDivisor is defined, but not supported by any feature.
// The command ProgramParameteri is defined, but not supported by any feature.
// The command BeginPerfQueryINTEL is defined, but not supported by any feature.
// The command CreatePerfQueryINTEL is defined, but not supported by any feature.
// The command DeletePerfQueryINTEL is defined, but not supported by any feature.
// The command EndPerfQueryINTEL is defined, but not supported by any feature.
// The command GetFirstPerfQueryIdINTEL is defined, but not supported by any feature.
// The command GetNextPerfQueryIdINTEL is defined, but not supported by any feature.
// The command GetPerfCounterInfoINTEL is defined, but not supported by any feature.
// The command GetPerfQueryDataINTEL is defined, but not supported by any feature.
// The command GetPerfQueryIdByNameINTEL is defined, but not supported by any feature.
// The command GetPerfQueryInfoINTEL is defined, but not supported by any feature.
My question is: is it possible the usage of the above functions, even if they are not supported by any extension?
My first thought was that this is a way of the driver implementor to avoid the usage of those functions.
However, I want to use glVertexAttribDivisor to avoid multiple shader definition...
Is it possible? Yes. Is it a good idea to use it? No.
If an implementation doesn't feel confident enough in an implementation of an extension to actually advertise it, then you should not feel confident enough that it will work to use it.