glDrawElementsBaseVertex is not available in GL 3.1

[dpilawa]

Version/Branch of Dear ImGui:

Version: 1.72b
Branch: master

Back-end/Renderer/Compiler/OS

Back-ends: imgui_impl_glfw.cpp + imgui_impl_opengl3.cpp
Compiler: Clang 8.0.0
Operating System: win10

My Issue/Question:

This piece in imgui_impl_opengl3.cpp is not sufficient to decide on the use of glDrawElementsBaseVertex which is available from GL 3.2 onwards (and not in GL 3.1).

// Desktop GL has glDrawElementsBaseVertex() which GL ES and WebGL don't have. #if defined(IMGUI_IMPL_OPENGL_ES2) || defined(IMGUI_IMPL_OPENGL_ES3) #define IMGUI_IMPL_OPENGL_HAS_DRAW_WITH_BASE_VERTEX 0 #else #define IMGUI_IMPL_OPENGL_HAS_DRAW_WITH_BASE_VERTEX 1 #endif

[ocornut]

You are correct. We would need a PR that :

• Rename IMGUI_IMPL_OPENGL_HAS_DRAW_WITH_BASE_VERTEX to IMGUI_IMPL_OPENGL_MAY_HAVE_DRAW_WITH_BASE_VERTEX.

• In ImGui_ImplOpenGL3_Init() query the context version number using glGetIntegerv(GL_MAJOR_VERSION, ... etc, store in a single value (e.g. g_GlVersion = major * 1000 + minor), make that query outside of the ifdef for good measure.

• Set ImGuiBackendFlags_RendererHasVtxOffset accordingly or not.

• In the rendering function select between both functions depending on the version.

[ocornut]

@dpilawa Do you feel like attempting to make this PR?
Thanks!

[dpilawa]

@ocornut Yes, will do.

[dpilawa]

@ocornut I will need to find another solution and remove reference to glDrawElementsBaseVertex by compiler directives in contexts below 3.2. If I select between both functions in rendering function based on global variable, I still end up with code which won't compile on a pure 3.1 system (glDrawElementsBaseVertex is undeclared there and
IMGUI_IMPL_OPENGL_MAY_HAVE_DRAW_WITH_BASE_VERTEX precompiler condition is too weak).

If you have a quick idea, drop me a line, please.

[ocornut]

I still end up with code which won't compile on a pure 3.1 system

Which GL loader are you using?

[dpilawa]

@ocornut I am using GLAD with compatibility profile.

[ocornut]

OK so I guess we could

• introduce optional #define to enforce overally GL version and compile that out
• take advantage of standard #defined emitted in glad to automatically select a default at compile time? Can we find any of those?

[dpilawa]

Right, let's use these which GLAD emits: #define GL_VERSION_3_x 1. On 3.1 GL_VERSION_3_2 will not be defined. I will work on PR later this week.

[ocornut]

Fixed via #2866