rendering_vulkan.cpp: add notes / custom_background & clear_color

src/hello_imgui/internal/backend_impls/rendering_vulkan.cpp

@@ -56,14 +56,28 @@ namespace HelloImGui
             // ImGui_ImplGlfw_NewFrame();
         };
 
-        callbacks->Impl_Frame_3D_ClearColor  = [] (ImVec4) {};
+        callbacks->Impl_Frame_3D_ClearColor  = [] (ImVec4)
+        {
+            // Note: at this moment, Impl_Frame_3D_ClearColor is empty, because the clearing is done in Impl_RenderDrawData_To_3D
+            //
+            // If you want to skip the clearing because you provided a CustomBackground() callback
+            // some adaptations are probably needed.
+            // For example:
+            //    - Impl_NewFrame_3D could set loadOp to VK_ATTACHMENT_LOAD_OP_LOAD to preserve the existing content
+            //    - Impl_Frame_3D_ClearColor(), if called, would then set it to VK_ATTACHMENT_LOAD_OP_CLEAR
+            //      (Impl_Frame_3D_ClearColor is not called if CustomBackground() is provided)
+            //
+            // However, this requires that the render pass in Vulkan is configured to support dynamic changes.
+            // (probably by setting ImGui_ImplVulkan_InitInfo init_info.UseDynamicRendering = true; inside
+            //  PrepareGlfwForVulkan() and PrepareSdlForVulkan() (see rendering_vulkan_glfw.cpp and rendering_vulkan_sdl.cpp)
+        };
 
         callbacks->Impl_RenderDrawData_To_3D = []
         {
             auto & gVkGlobals = HelloImGui::GetVulkanGlobals();
+
             ImGui_ImplVulkanH_Window* wd = &gVkGlobals.ImGuiMainWindowData;
             ImVec4 clear_color = HelloImGui::GetRunnerParams()->imGuiWindowParams.backgroundColor;
-
             ImDrawData* main_draw_data = ImGui::GetDrawData();
             const bool main_is_minimized = (main_draw_data->DisplaySize.x <= 0.0f || main_draw_data->DisplaySize.y <= 0.0f);
             wd->ClearValue.color.float32[0] = clear_color.x * clear_color.w;