Method Render

imgui_tiny_app/imgui/imgui.cpp:6342–6412

Prepare the data for rendering so you can call GetDrawData() (As with anything within the ImGui:: namespace this doesn't touch your GPU or graphics API at all: it is the role of the ImGui_ImplXXXX_RenderDrawData() function provided by the renderer backend)

// (As with anything within the ImGui:: namespace this doesn't touch your GPU or graphics API at all:
// it is the role of the ImGui_ImplXXXX_RenderDrawData() function provided by the renderer backend)
void ImGui::Render()
{
    ImGuiContext& g = *GImGui;
    IM_ASSERT(g.Initialized);

    if (g.FrameCountEnded != g.FrameCount)
        EndFrame();
    if (g.FrameCountRendered == g.FrameCount)
        return;
    g.FrameCountRendered = g.FrameCount;

    g.IO.MetricsRenderWindows = 0;
    CallContextHooks(&g, ImGuiContextHookType_RenderPre);

    // Add background ImDrawList (for each active viewport)
    for (ImGuiViewportP* viewport : g.Viewports)
    {
        InitViewportDrawData(viewport);
        if (viewport->BgFgDrawLists[0] != NULL && viewport->BgFgDrawListsLastTimeActive[0] == (float)g.Time)
            AddDrawListToDrawDataEx(&viewport->DrawDataP, viewport->DrawDataBuilder.Layers[0], GetBackgroundDrawList(viewport));
    }

    // Draw modal/window whitening backgrounds
    RenderDimmedBackgrounds();

    // Add ImDrawList to render
    ImGuiWindow* windows_to_render_top_most[2];
    windows_to_render_top_most[0] = (g.NavWindowingTarget && !(g.NavWindowingTarget->Flags & ImGuiWindowFlags_NoBringToFrontOnFocus)) ? g.NavWindowingTarget->RootWindowDockTree : NULL;
    windows_to_render_top_most[1] = (g.NavWindowingTarget ? g.NavWindowingListWindow : NULL);
    for (ImGuiWindow* window : g.Windows)
    {
        IM_MSVC_WARNING_SUPPRESS(6011); // Static Analysis false positive "warning C6011: Dereferencing NULL pointer 'window'"
        if (IsWindowActiveAndVisible(window) && (window->Flags & ImGuiWindowFlags_ChildWindow) == 0 && window != windows_to_render_top_most[0] && window != windows_to_render_top_most[1])
            AddRootWindowToDrawData(window);
    }
    for (int n = 0; n < IM_COUNTOF(windows_to_render_top_most); n++)
        if (windows_to_render_top_most[n] && IsWindowActiveAndVisible(windows_to_render_top_most[n])) // NavWindowingTarget is always temporarily displayed as the top-most window
            AddRootWindowToDrawData(windows_to_render_top_most[n]);

    // Draw software mouse cursor if requested by io.MouseDrawCursor flag
    if (g.IO.MouseDrawCursor && g.MouseCursor != ImGuiMouseCursor_None)
        RenderMouseCursor(g.IO.MousePos, g.Style.MouseCursorScale, g.MouseCursor, IM_COL32_WHITE, IM_COL32_BLACK, IM_COL32(0, 0, 0, 48));

    // Setup ImDrawData structures for end-user
    g.IO.MetricsRenderVertices = g.IO.MetricsRenderIndices = 0;
    for (ImGuiViewportP* viewport : g.Viewports)
    {
        FlattenDrawDataIntoSingleLayer(&viewport->DrawDataBuilder);

        // Add foreground ImDrawList (for each active viewport)
        if (viewport->BgFgDrawLists[1] != NULL && viewport->BgFgDrawListsLastTimeActive[1] == (float)g.Time)
            AddDrawListToDrawDataEx(&viewport->DrawDataP, viewport->DrawDataBuilder.Layers[0], GetForegroundDrawList(viewport));

        // We call _PopUnusedDrawCmd() last thing, as RenderDimmedBackgrounds() rely on a valid command being there (especially in docking branch).
        ImDrawData* draw_data = &viewport->DrawDataP;
        IM_ASSERT(draw_data->CmdLists.Size == draw_data->CmdListsCount);
        for (ImDrawList* draw_list : draw_data->CmdLists)
            draw_list->_PopUnusedDrawCmd();