imgui/examples/imgui_impl_opengl2.h

// ImGui Renderer for: OpenGL2 (legacy OpenGL, fixed pipeline)
// This needs to be used along with a Platform Binding (e.g. GLFW, SDL, Win32, custom..)

// Implemented features:
//  [X] User texture binding. Use 'GLUint' OpenGL texture identifier as void*/ImTextureID. Read the FAQ about ImTextureID in imgui.cpp.
//  [X] Multi-viewport rendering (when ImGuiConfigFlags_EnableViewports is enabled).

// **DO NOT USE THIS CODE IF YOUR CODE/ENGINE IS USING MODERN OPENGL (SHADERS, VBO, VAO, etc.)**
// **Prefer using the code in imgui_impl_opengl3.cpp**
// This code is mostly provided as a reference to learn how ImGui integration works, because it is shorter to read.
// If your code is using GL3+ context or any semi modern OpenGL calls, using this is likely to make everything more
// complicated, will require your code to reset every single OpenGL attributes to their initial state, and might
// confuse your GPU driver. 
// The GL2 code is unable to reset attributes or even call e.g. "glUseProgram(0)" because they don't exist in that API.

IMGUI_API bool  ImGui_ImplOpenGL2_Init();
IMGUI_API void  ImGui_ImplOpenGL2_Shutdown();
IMGUI_API void  ImGui_ImplOpenGL2_NewFrame();
IMGUI_API void  ImGui_ImplOpenGL2_RenderDrawData(ImDrawData* draw_data);

// Called by Init/NewFrame/Shutdown
IMGUI_API bool  ImGui_ImplOpenGL2_CreateFontsTexture();
IMGUI_API void  ImGui_ImplOpenGL2_DestroyFontsTexture();
IMGUI_API bool  ImGui_ImplOpenGL2_CreateDeviceObjects();
IMGUI_API void  ImGui_ImplOpenGL2_DestroyDeviceObjects();