when using ImGui with Glut it does not show any objects

Question

I'm coding a rendering engine in C++ with OpenGL and GLUT and trying to integrate ImGUI into my engine, but I have a problem. it either renders the gui and only the background (no objects), or it only renders the objects and background (no GUI). This code:

glutDisplayFunc(renderScene);
glutIdleFunc(renderScene);
glutReshapeFunc(sizeChange);
glutSpecialUpFunc(releaseKey);
glutSpecialFunc(pressKey);
IMGUI_CHECKVERSION();
ImGui::CreateContext();
ImGuiIO& io = ImGui::GetIO(); (void)io;
ImGui::StyleColorsLight();
ImGui_ImplGLUT_Init();
ImGui_ImplGLUT_InstallFuncs();
ImGui_ImplOpenGL2_Init();

creates this:

and this code:

IMGUI_CHECKVERSION();
ImGui::CreateContext();
ImGuiIO& io = ImGui::GetIO(); (void)io;
ImGui::StyleColorsLight();
ImGui_ImplGLUT_Init();
ImGui_ImplGLUT_InstallFuncs();
ImGui_ImplOpenGL2_Init();

glutDisplayFunc(renderScene);
glutIdleFunc(renderScene);
glutReshapeFunc(sizeChange);
glutSpecialUpFunc(releaseKey);
glutSpecialFunc(pressKey);

creates this:

score 1 · Accepted Answer · answered Aug 16 '22 at 05:42

The problem is that GLUT callback handlers in both of your examples are set both manually (glut...Func) and by ImGui via ImGui_ImplGLUT_InstallFuncs. The latter sets default ImGui handlers for many GLUT callbacks (see the source), in particular glutReshapeFunc is used to set current window resize callback to ImGui_ImplGLUT_ReshapeFunc (source), which sets internal ImGui display size according to given parameters.

When in the second example glutReshapeFunc(sizeChange) gets called, ImGui handler gets unset, so it doesn't get called during window resize (which, in particular, happens also before the first display according to the GLUT documentation), which leaves internal ImGui state not set properly. In the first example, the situation is reversed - your reshape handler sizeChange gets unset and replaced by ImGui one, so initialization and rendering of your cube doesn't go as intended because sizeChange never gets called.

To resolve this, you can omit ImGui_ImplGLUT_InstallFuncs and use your GLUT handlers while, apart from custom logic, calling default ImGui handlers from them if they are present and their functionality is needed (you can see what default handlers exist by inspecting the sources linked above). E.g. you can change sizeChange like this:

void sizeChange(int w, int h)
{
    // Your logic
    // ...
    ImGui_ImplGLUT_ReshapeFunc(w, h);
}

If you want key presses and releases to be registered by ImGui as well as your own code, you may change releaseKey and pressKey in a similar way by adding, respectively, ImGui_ImplGLUT_SpecialUpFunc and ImGui_ImplGLUT_SpecialFunc calls to them.

when using ImGui with Glut it does not show any objects

1 Answers1