Viewport: Removed back-end renderer code that implied we need to clear with WindowBg color. Added NoRendererClear flag. (#1542, #1042)

examples/imgui_impl_dx10.cpp

@@ -587,10 +587,10 @@ static void ImGui_ImplDX10_ResizeViewport(ImGuiViewport* viewport, ImVec2 size)
 static void ImGui_ImplDX10_RenderViewport(ImGuiViewport* viewport)
 {
     ImGuiPlatformDataDx10* data = (ImGuiPlatformDataDx10*)viewport->RendererUserData;
-    ImVec4 clear_color = ImGui::GetStyle().Colors[ImGuiCol_WindowBg]; // FIXME-PLATFORM
-    clear_color.w = 1.0f;
+    ImVec4 clear_color = ImVec4(0.0f, 0.0f, 0.0f, 1.0f);
     g_pd3dDevice->OMSetRenderTargets(1, &data->RTView, NULL);
-    g_pd3dDevice->ClearRenderTargetView(data->RTView, (float*)&clear_color);
+    if (!(viewport->Flags & ImGuiViewportFlags_NoRendererClear))
+        g_pd3dDevice->ClearRenderTargetView(data->RTView, (float*)&clear_color);
     ImGui_ImplDX10_RenderDrawData(&viewport->DrawData);
 }
 

examples/imgui_impl_dx11.cpp

@@ -595,10 +595,10 @@ static void ImGui_ImplDX11_ResizeViewport(ImGuiViewport* viewport, ImVec2 size)
 static void ImGui_ImplDX11_RenderViewport(ImGuiViewport* viewport)
 {
     ImGuiPlatformDataDx11* data = (ImGuiPlatformDataDx11*)viewport->RendererUserData;
-    ImVec4 clear_color = ImGui::GetStyle().Colors[ImGuiCol_WindowBg]; // FIXME-PLATFORM
-    clear_color.w = 1.0f;
+    ImVec4 clear_color = ImVec4(0.0f, 0.0f, 0.0f, 1.0f);
     g_pd3dDeviceContext->OMSetRenderTargets(1, &data->RTView, NULL);
-    g_pd3dDeviceContext->ClearRenderTargetView(data->RTView, (float*)&clear_color);
+    if (!(viewport->Flags & ImGuiViewportFlags_NoRendererClear))
+        g_pd3dDeviceContext->ClearRenderTargetView(data->RTView, (float*)&clear_color);
     ImGui_ImplDX11_RenderDrawData(&viewport->DrawData);
 }
 

examples/imgui_impl_dx12.cpp

@@ -734,10 +734,10 @@ static void ImGui_ImplDX12_RenderViewport(ImGuiViewport* viewport)
     IM_ASSERT(0);
     (void)data;
     /*
-    ImVec4 clear_color = ImGui::GetStyle().Colors[ImGuiCol_WindowBg]; // FIXME-PLATFORM
-    clear_color.w = 1.0f;
+    ImVec4 clear_color = ImVec4(0.0f, 0.0f, 0.0f, 1.0f);
     g_pd3dDeviceContext->OMSetRenderTargets(1, &data->RTView, NULL);
-    g_pd3dDeviceContext->ClearRenderTargetView(data->RTView, (float*)&clear_color);
+    if (!(viewport->Flags & ImGuiViewportFlags_NoRendererClear))
+        g_pd3dDeviceContext->ClearRenderTargetView(data->RTView, (float*)&clear_color);
     */
     ImGui_ImplDX12_RenderDrawData(&viewport->DrawData);
 }

examples/imgui_impl_opengl3.cpp

@@ -331,9 +331,12 @@ void    ImGui_ImplOpenGL3_DestroyDeviceObjects()
 
 static void ImGui_ImplOpenGL3_RenderViewport(ImGuiViewport* viewport)
 {
-    ImVec4 clear_color = ImGui::GetStyle().Colors[ImGuiCol_WindowBg];
-    glClearColor(clear_color.x, clear_color.y, clear_color.z, 1.0f);    // FIXME-PLATFORM
-    glClear(GL_COLOR_BUFFER_BIT);
+    if (!(viewport->Flags & ImGuiViewportFlags_NoRendererClear))
+    {
+        ImVec4 clear_color = ImVec4(0.0f, 0.0f, 0.0f, 1.0f);
+        glClearColor(clear_color.x, clear_color.y, clear_color.z, clear_color.w);
+        glClear(GL_COLOR_BUFFER_BIT);
+    }
     ImGui_ImplOpenGL3_RenderDrawData(&viewport->DrawData);
 }
 

examples/imgui_impl_vulkan.cpp

@@ -757,6 +757,7 @@ ImGui_ImplVulkan_WindowData::ImGui_ImplVulkan_WindowData()
     memset(&SurfaceFormat, 0, sizeof(SurfaceFormat));
     PresentMode = VK_PRESENT_MODE_MAX_ENUM_KHR;
     RenderPass = VK_NULL_HANDLE;
+    ClearEnable = true;
     memset(&ClearValue, 0, sizeof(ClearValue));
     BackBufferCount = 0;
     memset(&BackBuffer, 0, sizeof(BackBuffer));
@@ -959,7 +960,7 @@ void ImGui_ImplVulkanH_CreateWindowDataSwapChainAndFramebuffer(VkPhysicalDevice
         VkAttachmentDescription attachment = {};
         attachment.format = wd->SurfaceFormat.format;
         attachment.samples = VK_SAMPLE_COUNT_1_BIT;
-        attachment.loadOp = VK_ATTACHMENT_LOAD_OP_CLEAR;
+        attachment.loadOp = wd->ClearEnable ? VK_ATTACHMENT_LOAD_OP_CLEAR : VK_ATTACHMENT_LOAD_OP_DONT_CARE;
         attachment.storeOp = VK_ATTACHMENT_STORE_OP_STORE;
         attachment.stencilLoadOp = VK_ATTACHMENT_LOAD_OP_DONT_CARE;
         attachment.stencilStoreOp = VK_ATTACHMENT_STORE_OP_DONT_CARE;
@@ -1091,6 +1092,7 @@ static void ImGui_ImplVulkan_CreateViewport(ImGuiViewport* viewport)
     wd->PresentMode = ImGui_ImplVulkanH_SelectPresentMode(g_PhysicalDevice, wd->Surface, &present_mode, 1);
 
     // Create SwapChain, RenderPass, Framebuffer, etc.
+    wd->ClearEnable = (viewport->Flags & ImGuiViewportFlags_NoRendererClear) ? false : true;
     ImGui_ImplVulkanH_CreateWindowDataCommandBuffers(g_PhysicalDevice, g_Device, g_QueueFamily, wd, g_Allocator);
     ImGui_ImplVulkanH_CreateWindowDataSwapChainAndFramebuffer(g_PhysicalDevice, g_Device, wd, g_Allocator, (int)viewport->Size.x, (int)viewport->Size.y);
 }
@@ -1110,6 +1112,7 @@ static void ImGui_ImplVulkan_ResizeViewport(ImGuiViewport* viewport, ImVec2 size
     ImGuiPlatformDataVulkan* data = (ImGuiPlatformDataVulkan*)viewport->RendererUserData;
     if (data == NULL) // This is NULL for the main viewport (which is left to the user/app to handle)
         return;
+    data->WindowData.ClearEnable = (viewport->Flags & ImGuiViewportFlags_NoRendererClear) ? false : true;
     ImGui_ImplVulkanH_CreateWindowDataSwapChainAndFramebuffer(g_PhysicalDevice, g_Device, &data->WindowData, g_Allocator, (int)size.x, (int)size.y);
 }
 
@@ -1119,9 +1122,6 @@ static void ImGui_ImplVulkan_RenderViewport(ImGuiViewport* viewport)
     ImGui_ImplVulkan_WindowData* wd = &data->WindowData;
     VkResult err;
 
-    ImVec4 clear_color = ImGui::GetStyle().Colors[ImGuiCol_WindowBg]; // FIXME-PLATFORM
-    clear_color.w = 1.0f;
-
     {
         ImGui_ImplVulkan_FrameData* fd = &wd->Frames[wd->FrameIndex];
         for (;;)
@@ -1145,19 +1145,21 @@ static void ImGui_ImplVulkan_RenderViewport(ImGuiViewport* viewport)
             check_vk_result(err);
         }
         {
+            ImVec4 clear_color = ImVec4(0.0f, 0.0f, 0.0f, 1.0f);
+            memcpy(&wd->ClearValue.color.float32[0], &clear_color, 4 * sizeof(float));
+
             VkRenderPassBeginInfo info = {};
             info.sType = VK_STRUCTURE_TYPE_RENDER_PASS_BEGIN_INFO;
             info.renderPass = wd->RenderPass;
             info.framebuffer = wd->Framebuffer[fd->BackbufferIndex];
             info.renderArea.extent.width = wd->Width;
             info.renderArea.extent.height = wd->Height;
-            info.clearValueCount = 1;
-            info.pClearValues = &wd->ClearValue;
+            info.clearValueCount = (viewport->Flags & ImGuiViewportFlags_NoRendererClear) ? 0 : 1;
+            info.pClearValues = (viewport->Flags & ImGuiViewportFlags_NoRendererClear) ? NULL : &wd->ClearValue;
             vkCmdBeginRenderPass(fd->CommandBuffer, &info, VK_SUBPASS_CONTENTS_INLINE);
         }
     }
 
-    memcpy(&wd->ClearValue.color.float32[0], &clear_color, 4 * sizeof(float));
     ImGui_ImplVulkan_RenderDrawData(wd->Frames[wd->FrameIndex].CommandBuffer, &viewport->DrawData);
 
     {

examples/imgui_impl_vulkan.h

@@ -77,6 +77,7 @@ struct ImGui_ImplVulkan_WindowData
     VkSurfaceFormatKHR  SurfaceFormat;
     VkPresentModeKHR    PresentMode;
     VkRenderPass        RenderPass;
+    bool                ClearEnable;
     VkClearValue        ClearValue;
     uint32_t            BackBufferCount;
     VkImage             BackBuffer[16];

imgui.cpp

@@ -6053,6 +6053,10 @@ static void ImGui::UpdateWindowViewport(ImGuiWindow* window, bool window_pos_set
 
     if (window->Flags & ImGuiWindowFlags_FullViewport)
     {
+        // We currently have window fully covering a viewport and we disable WindowBg alpha, so clearing is not necessary
+        window->Viewport->Flags |= ImGuiViewportFlags_NoRendererClear;
+
+        // Position
         SetWindowPos(window, window->Viewport->Pos, ImGuiCond_Always);
         if (window->Viewport->PlatformRequestResize)
             SetWindowSize(window, window->Viewport->Size, ImGuiCond_Always);
@@ -6606,6 +6610,8 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
             ImU32 bg_col = GetColorU32(GetWindowBgColorIdxFromFlags(flags));
             if (g.NextWindowData.BgAlphaCond != 0)
                 bg_col = (bg_col & ~IM_COL32_A_MASK) | (IM_F32_TO_INT8_SAT(g.NextWindowData.BgAlphaVal) << IM_COL32_A_SHIFT);
+            if (window->Flags & ImGuiWindowFlags_FullViewport)
+                bg_col = (bg_col | IM_COL32_A_MASK);
             window->DrawList->AddRectFilled(window->Pos+ImVec2(0,window->TitleBarHeight()), window->Pos+window->Size, bg_col, window_rounding, (flags & ImGuiWindowFlags_NoTitleBar) ? ImDrawCornerFlags_All : ImDrawCornerFlags_Bot);
 
             // Title bar

imgui_internal.h

@@ -510,9 +510,10 @@ enum ImGuiViewportFlags_
 {
     ImGuiViewportFlags_MainViewport         = 1 << 0,
     ImGuiViewportFlags_HostOtherWindows     = 1 << 1,
-    ImGuiViewportFlags_NoDecoration         = 1 << 2,   // Platform Window: Disable platform title bar, borders, etc.
-    ImGuiViewportFlags_NoFocusOnAppearing   = 1 << 3,   // Platform Window: Don't take focus when created.
-    ImGuiViewportFlags_NoInputs             = 1 << 4    // Platform Window: Make mouse pass through so we can drag this window while peaking behind it.
+    ImGuiViewportFlags_NoRendererClear      = 1 << 2,   // Platform Window: Renderer doesn't need to clear the framebuffer ahead.
+    ImGuiViewportFlags_NoDecoration         = 1 << 3,   // Platform Window: Disable platform title bar, borders, etc.
+    ImGuiViewportFlags_NoFocusOnAppearing   = 1 << 4,   // Platform Window: Don't take focus when created.
+    ImGuiViewportFlags_NoInputs             = 1 << 5    // Platform Window: Make mouse pass through so we can drag this window while peaking behind it.
 };
 
 struct ImGuiViewport