io.DeltaTime and all that

[voidware]

OK, so this is what i've done which might help others.

Also if you're getting this assert fail

IM_ASSERT((g.IO.DeltaTime > 0.0f || g.FrameCount == 0)              && "Need a positive DeltaTime!");`

This is with SDL + emscripten + firefox.

Turned out, after a lot of time wasted, that my problem was privacy.resistFingerprinting true. This has (at least) two side effects;

1. makes the canvas go weird and fonts come out blobby.
2. screws up the browser performance counter to 100ms (yes 10fps!!)

(2) caused the assert fail, as you can imagine.

In my journey of discovery, i reworked some imgui code which might be a good idea in general;

imgui_impl_sdl.cpp

bool ImGui_ImplSDL2_Tick()
{
    ImGuiIO& io = ImGui::GetIO();

    /* note if you have resist fingerprinting enabled in the browser
     * this will cause returns of FALSE
     */

    static Uint64 frequency = SDL_GetPerformanceFrequency();
    Uint64 current_time = SDL_GetPerformanceCounter();
    float dt = (float)((double)(current_time - g_Time) / frequency);
    if (dt <= 0) return false;
    io.DeltaTime = dt;
    g_Time = current_time;
    return true;
}

void ImGui_ImplSDL2_NewFrame(SDL_Window* window)
{
    ImGuiIO& io = ImGui::GetIO();
    IM_ASSERT(io.Fonts->IsBuilt() && "Font atlas not built! It is generally built by the renderer backend. Missing call to renderer _NewFrame() function? e.g. ImGui_ImplOpenGL3_NewFrame().");

    // Setup display size (every frame to accommodate for window resizing)
    int w, h;
    int display_w, display_h;
    SDL_GetWindowSize(window, &w, &h);
    if (SDL_GetWindowFlags(window) & SDL_WINDOW_MINIMIZED)
        w = h = 0;
    SDL_GL_GetDrawableSize(window, &display_w, &display_h);
    io.DisplaySize = ImVec2((float)w, (float)h);
    if (w > 0 && h > 0)
        io.DisplayFramebufferScale = ImVec2((float)display_w / w, (float)display_h / h);
		
		/*

    // Setup time step (we don't use SDL_GetTicks() because it is using millisecond resolution)
    static Uint64 frequency = SDL_GetPerformanceFrequency();
    Uint64 current_time = SDL_GetPerformanceCounter();
    io.DeltaTime = g_Time > 0 ? (float)((double)(current_time - g_Time) / frequency) : (float)(1.0f / 60.0f);
    g_Time = current_time;
	*/
    ImGui_ImplSDL2_UpdateMousePosAndButtons();
    ImGui_ImplSDL2_UpdateMouseCursor();

    // Update game controllers (if enabled and available)
    ImGui_ImplSDL2_UpdateGamepads();
}

So, split out the "tick" bit. THen i change my main loop to;

   for (;;)
    {
        SDL_Event event;
        while (SDL_PollEvent(&event))
        {
            ImGui_ImplSDL2_ProcessEvent(&event);
            // Capture events here, based on io.WantCaptureMouse and io.WantCaptureKeyboard
        }

        bool v = ImGui_ImplSDL2_Tick();
        if (v) break;

        SDL_Delay(0);
    }

So this will cause a yielded semi-busy wait for the next tick time. And then it will always be positive!

So now if i re-enable resist fingerprinting, it doesnt crash, but i get an amusing 10fps experience.

Obviously, you cant fix stupid, but there are a lot of browser addons that might enable this "security" feature.

[voidware]

Just to add;

You can not call Tick at all and it still works, even though time is never advanced!! How weird is that.

[rokups]

Or we could include this information next to assert. While this change does prevent a crash - application is still unusable, so why bother..