Call ImGui::NewFrame() early in the main loop

src/engine/engine.cc

@@ -130,6 +130,8 @@ void Engine::Update(float delta_time) {
   seconds_accumulated_ += delta_time;
   ++tick_;
 
+  imgui_backend_.NewFrame(delta_time);
+
   for (auto d : animators_)
     d->Update(delta_time);
 
@@ -140,6 +142,9 @@ void Engine::Update(float delta_time) {
     fps_ = renderer_->GetAndResetFPS();
     fps_seconds_ = 0;
   }
+
+  if (stats_visible_)
+    ShowStats();
 }
 
 void Engine::Draw(float frame_frac) {
@@ -150,29 +155,12 @@ void Engine::Draw(float frame_frac) {
       [](auto& a, auto& b) { return a->GetZOrder() < b->GetZOrder(); });
 
   renderer_->PrepareForDrawing();
-
   for (auto d : drawables_) {
     if (d->IsVisible())
       d->Draw(frame_frac);
   }
-
-  if (stats_visible_) {
-    ImVec2 center = ImGui::GetMainViewport()->GetCenter();
-    ImGui::SetNextWindowPos(center, ImGuiCond_Appearing, ImVec2(0.5f, 0.5f));
-    ImGuiWindowFlags window_flags =
-        ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoTitleBar |
-        ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoCollapse |
-        ImGuiWindowFlags_NoNav | ImGuiWindowFlags_AlwaysAutoResize;
-    ImGui::Begin("Stats", nullptr, window_flags);
-    ImGui::Text("%s", renderer_->GetDebugName());
-    ImGui::Text("%d fps", fps_);
-    ImGui::End();
-  }
-
-  imgui_backend_.Render();
-
+  imgui_backend_.Draw();
   renderer_->Present();
-  imgui_backend_.NewFrame();
 }
 
 void Engine::AddDrawable(Drawable* drawable) {
@@ -696,4 +684,17 @@ void Engine::WaitForAsyncWork() {
   }
 }
 
+void Engine::ShowStats() {
+  ImVec2 center = ImGui::GetMainViewport()->GetCenter();
+  ImGui::SetNextWindowPos(center, ImGuiCond_Appearing, ImVec2(0.5f, 0.5f));
+  ImGuiWindowFlags window_flags =
+      ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoTitleBar |
+      ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoCollapse |
+      ImGuiWindowFlags_NoNav | ImGuiWindowFlags_AlwaysAutoResize;
+  ImGui::Begin("Stats", nullptr, window_flags);
+  ImGui::Text("%s", renderer_->GetDebugName());
+  ImGui::Text("%d fps", fps_);
+  ImGui::End();
+}
+
 }  // namespace eng

src/engine/engine.h

@@ -242,6 +242,8 @@ class Engine : public PlatformObserver {
 
   void WaitForAsyncWork();
 
+  void ShowStats();
+
   Engine(const Engine&) = delete;
   Engine& operator=(const Engine&) = delete;
 };

src/engine/imgui_backend.cc

@@ -52,7 +52,6 @@ void ImguiBackend::Initialize() {
   Engine::Get().SetImageSource(
       "imgui_atlas",
       []() -> std::unique_ptr<Image> {
-        // Build texture atlas
         unsigned char* pixels;
         int width, height;
         ImGui::GetIO().Fonts->GetTexDataAsRGBA32(&pixels, &width, &height);
@@ -67,7 +66,7 @@ void ImguiBackend::Initialize() {
   ImGui::GetIO().Fonts->SetTexID(
       (ImTextureID)(intptr_t)Engine::Get().AcquireTexture("imgui_atlas"));
 
-  NewFrame();
+  NewFrame(0);
 }
 
 void ImguiBackend::Shutdown() {
@@ -117,28 +116,31 @@ std::unique_ptr<InputEvent> ImguiBackend::OnInputEvent(
   return event;
 }
 
-void ImguiBackend::NewFrame() {
+void ImguiBackend::NewFrame(float delta_time) {
+  if (is_new_frame_)
+    ImGui::EndFrame();
+  is_new_frame_ = true;
+
   ImGuiIO& io = ImGui::GetIO();
   io.DisplaySize = ImVec2((float)renderer_->GetScreenWidth(),
                           (float)renderer_->GetScreenHeight());
-  io.DeltaTime = timer_.Delta();
+  io.DeltaTime = delta_time;
   ImGui::NewFrame();
 }
 
-void ImguiBackend::Render() {
+void ImguiBackend::Draw() {
+  is_new_frame_ = false;
   ImGui::Render();
-
   ImDrawData* draw_data = ImGui::GetDrawData();
   if (draw_data->CmdListsCount <= 0)
     return;
 
-  float L = draw_data->DisplayPos.x;
-  float R = draw_data->DisplayPos.x + draw_data->DisplaySize.x;
-  float T = draw_data->DisplayPos.y;
-  float B = draw_data->DisplayPos.y + draw_data->DisplaySize.y;
   base::Matrix4f ortho_projection;
-  ortho_projection.CreateOrthoProjection(L, R, B, T);
-
+  ortho_projection.CreateOrthoProjection(
+      draw_data->DisplayPos.x,
+      draw_data->DisplayPos.x + draw_data->DisplaySize.x,
+      draw_data->DisplayPos.y + draw_data->DisplaySize.y,
+      draw_data->DisplayPos.y);
   shader_->Activate();
   shader_->SetUniform("projection", ortho_projection);
   shader_->UploadUniforms();

src/engine/imgui_backend.h

@@ -3,8 +3,6 @@
 
 #include <memory>
 
-#include "base/timer.h"
-
 namespace eng {
 
 class InputEvent;
@@ -23,13 +21,13 @@ class ImguiBackend {
 
   std::unique_ptr<InputEvent> OnInputEvent(std::unique_ptr<InputEvent> event);
 
-  void NewFrame();
-  void Render();
+  void NewFrame(float delta_time);
+  void Draw();
 
  private:
   std::unique_ptr<Shader> shader_;
   Renderer* renderer_ = nullptr;
-  base::DeltaTimer timer_;
+  bool is_new_frame_ = false;
 };
 
 }  // namespace eng

src/third_party/imgui/LICENSE.txt

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2014-2023 Omar Cornut
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.