Add support for transparent backbuffers to GLFW contexts (OpenGL3 and Vulkan) (reopens #2766)

backends/imgui_impl_glfw.cpp

@@ -147,6 +147,7 @@
 #define GLFW_HAS_GAMEPAD_API            (GLFW_VERSION_COMBINED >= 3300) // 3.3+ glfwGetGamepadState() new api
 #define GLFW_HAS_GETKEYNAME             (GLFW_VERSION_COMBINED >= 3200) // 3.2+ glfwGetKeyName()
 #define GLFW_HAS_GETERROR               (GLFW_VERSION_COMBINED >= 3300) // 3.3+ glfwGetError()
+#define GLFW_HAS_TRANSPARENT_FRAMEBUFFERS (GLFW_VERSION_COMBINED >= 3300)
 
 // GLFW data
 enum GlfwClientApi
@@ -1089,6 +1090,9 @@ static void ImGui_ImplGlfw_CreateWindow(ImGuiViewport* viewport)
     glfwWindowHint(GLFW_DECORATED, (viewport->Flags & ImGuiViewportFlags_NoDecoration) ? false : true);
 #if GLFW_HAS_WINDOW_TOPMOST
     glfwWindowHint(GLFW_FLOATING, (viewport->Flags & ImGuiViewportFlags_TopMost) ? true : false);
+#endif
+#if GLFW_HAS_TRANSPARENT_FRAMEBUFFERS
+    glfwWindowHint(GLFW_TRANSPARENT_FRAMEBUFFER, true);
 #endif
     GLFWwindow* share_window = (bd->ClientApi == GlfwClientApi_OpenGL) ? bd->Window : nullptr;
     vd->Window = glfwCreateWindow((int)viewport->Size.x, (int)viewport->Size.y, "No Title Yet", nullptr, share_window);

backends/imgui_impl_opengl2.cpp

@@ -135,6 +135,9 @@ static void ImGui_ImplOpenGL2_SetupRenderState(ImDrawData* draw_data, int fb_wid
     // Setup render state: alpha-blending enabled, no face culling, no depth testing, scissor enabled, vertex/texcoord/color pointers, polygon fill.
     glEnable(GL_BLEND);
     glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
+    // technically supported by openGL2, but not exposed via GL.h, may need a loader?
+    // See https://registry.khronos.org/OpenGL-Refpages/gl2.1/xhtml/glBlendFuncSeparate.xml
+    // https://registry.khronos.org/OpenGL/specs/gl/glspec20.pdf
     //glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA); // In order to composite our output buffer we need to preserve alpha
     glDisable(GL_CULL_FACE);
     glDisable(GL_DEPTH_TEST);

backends/imgui_impl_opengl3.cpp

@@ -970,7 +970,7 @@ static void ImGui_ImplOpenGL3_RenderWindow(ImGuiViewport* viewport, void*)
 {
     if (!(viewport->Flags & ImGuiViewportFlags_NoRendererClear))
     {
-        ImVec4 clear_color = ImVec4(0.0f, 0.0f, 0.0f, 1.0f);
+        ImVec4 clear_color = ImGui::GetStyle().Colors[ImGuiCol_WindowBg];
         glClearColor(clear_color.x, clear_color.y, clear_color.z, clear_color.w);
         glClear(GL_COLOR_BUFFER_BIT);
     }

backends/imgui_impl_vulkan.cpp

@@ -1461,6 +1461,8 @@ void ImGui_ImplVulkanH_CreateWindowSwapChain(VkPhysicalDevice physical_device, V
         VkSurfaceCapabilitiesKHR cap;
         err = vkGetPhysicalDeviceSurfaceCapabilitiesKHR(physical_device, wd->Surface, &cap);
         check_vk_result(err);
+        if (cap.supportedCompositeAlpha & VK_COMPOSITE_ALPHA_POST_MULTIPLIED_BIT_KHR)
+            info.compositeAlpha = VK_COMPOSITE_ALPHA_POST_MULTIPLIED_BIT_KHR;
         if (info.minImageCount < cap.minImageCount)
             info.minImageCount = cap.minImageCount;
         else if (cap.maxImageCount != 0 && info.minImageCount > cap.maxImageCount)
@@ -1770,7 +1772,7 @@ static void ImGui_ImplVulkan_RenderWindow(ImGuiViewport* viewport, void*)
             check_vk_result(err);
         }
         {
-            ImVec4 clear_color = ImVec4(0.0f, 0.0f, 0.0f, 1.0f);
+            ImVec4 clear_color = ImGui::GetStyle().Colors[ImGuiCol_WindowBg];
             memcpy(&wd->ClearValue.color.float32[0], &clear_color, 4 * sizeof(float));
         }
 #ifdef IMGUI_IMPL_VULKAN_HAS_DYNAMIC_RENDERING

examples/example_glfw_opengl3/main.cpp

@@ -63,6 +63,7 @@ int main(int, char**)
     //glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);  // 3.2+ only
     //glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);            // 3.0+ only
 #endif
+    glfwWindowHint(GLFW_TRANSPARENT_FRAMEBUFFER, GL_TRUE); // Enable transparent backbuffers
 
     // Create window with graphics context
     GLFWwindow* window = glfwCreateWindow(1280, 720, "Dear ImGui GLFW+OpenGL3 example", nullptr, nullptr);
@@ -74,11 +75,13 @@ int main(int, char**)
     // Setup Dear ImGui context
     IMGUI_CHECKVERSION();
     ImGui::CreateContext();
+
     ImGuiIO& io = ImGui::GetIO(); (void)io;
     io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;     // Enable Keyboard Controls
     io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad;      // Enable Gamepad Controls
     io.ConfigFlags |= ImGuiConfigFlags_DockingEnable;         // Enable Docking
     io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable;       // Enable Multi-Viewport / Platform Windows
+    io.ConfigFlags |= ImGuiConfigFlags_TransparentBackbuffers; // Enable transparent backbuffer
     //io.ConfigViewportsNoAutoMerge = true;
     //io.ConfigViewportsNoTaskBarIcon = true;
 
@@ -156,7 +159,7 @@ int main(int, char**)
 
         // 2. Show a simple window that we create ourselves. We use a Begin/End pair to create a named window.
         {
-            static float f = 0.0f;
+            static float f = 1.0f;
             static int counter = 0;
 
             ImGui::Begin("Hello, world!");                          // Create a window called "Hello, world!" and append into it.
@@ -165,8 +168,10 @@ int main(int, char**)
             ImGui::Checkbox("Demo Window", &show_demo_window);      // Edit bools storing our window open/close state
             ImGui::Checkbox("Another Window", &show_another_window);
 
-            ImGui::SliderFloat("float", &f, 0.0f, 1.0f);            // Edit 1 float using a slider from 0.0f to 1.0f
-            ImGui::ColorEdit3("clear color", (float*)&clear_color); // Edit 3 floats representing a color
+            ImGui::SliderFloat("Window Opacity", &ImGui::GetStyle().Colors[ImGuiCol_WindowBg].w, 0.0f, 1.0f); // Edit background window opacity (alpha channel)
+            ImGui::ColorEdit4("clear color", (float*)&clear_color); // Edit 4 floats representing an RGBA color
+            clear_color.w = ImGui::GetStyle().Colors[ImGuiCol_WindowBg].w;
+
 
             if (ImGui::Button("Button"))                            // Buttons return true when clicked (most widgets return true when edited/activated)
                 counter++;

examples/example_glfw_vulkan/main.cpp

@@ -398,6 +398,7 @@ int main(int, char**)
 
     // Create window with Vulkan context
     glfwWindowHint(GLFW_CLIENT_API, GLFW_NO_API);
+    glfwWindowHint(GLFW_TRANSPARENT_FRAMEBUFFER, GLFW_TRUE);
     GLFWwindow* window = glfwCreateWindow(1280, 720, "Dear ImGui GLFW+Vulkan example", nullptr, nullptr);
     if (!glfwVulkanSupported())
     {
@@ -427,10 +428,11 @@ int main(int, char**)
     IMGUI_CHECKVERSION();
     ImGui::CreateContext();
     ImGuiIO& io = ImGui::GetIO(); (void)io;
-    io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;     // Enable Keyboard Controls
-    io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad;      // Enable Gamepad Controls
-    io.ConfigFlags |= ImGuiConfigFlags_DockingEnable;         // Enable Docking
-    io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable;       // Enable Multi-Viewport / Platform Windows
+    io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;      // Enable Keyboard Controls
+    io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad;       // Enable Gamepad Controls
+    io.ConfigFlags |= ImGuiConfigFlags_DockingEnable;          // Enable Docking
+    io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable;        // Enable Multi-Viewport / Platform Windows
+    io.ConfigFlags |= ImGuiConfigFlags_TransparentBackbuffers; // Enable transparent backbuffer support
     //io.ConfigViewportsNoAutoMerge = true;
     //io.ConfigViewportsNoTaskBarIcon = true;
 
@@ -532,8 +534,9 @@ int main(int, char**)
             ImGui::Checkbox("Demo Window", &show_demo_window);      // Edit bools storing our window open/close state
             ImGui::Checkbox("Another Window", &show_another_window);
 
-            ImGui::SliderFloat("float", &f, 0.0f, 1.0f);            // Edit 1 float using a slider from 0.0f to 1.0f
-            ImGui::ColorEdit3("clear color", (float*)&clear_color); // Edit 3 floats representing a color
+            ImGui::SliderFloat("Window Opacity", &ImGui::GetStyle().Colors[ImGuiCol_WindowBg].w, 0.0f, 1.0f); // Edit background window opacity (alpha channel)
+            ImGui::ColorEdit4("clear color", (float*)&clear_color); // Edit 4 floats representing an RGBA color
+            clear_color.w = ImGui::GetStyle().Colors[ImGuiCol_WindowBg].w;
 
             if (ImGui::Button("Button"))                            // Buttons return true when clicked (most widgets return true when edited/activated)
                 counter++;