Image presentation now depends on the completeness of command buffer submission through semaphores.

To maintain maximum frame rate we render to the last acquired swapchain image but present the last but one drawn image. This behavior is optional through conditional compilation macros.

examples/vulkan_example/main.cpp

@@ -13,7 +13,9 @@
 
 #define IMGUI_MAX_POSSIBLE_BACK_BUFFERS 16
 #define IMGUI_UNLIMITED_FRAME_RATE
+//#ifdef _DEBUG
 //#define IMGUI_VULKAN_DEBUG_REPORT
+//#endif
 
 static VkAllocationCallbacks*   g_Allocator = NULL;
 static VkInstance               g_Instance = VK_NULL_HANDLE;
@@ -34,7 +36,7 @@ static VkPipelineCache          g_PipelineCache = VK_NULL_HANDLE;
 static VkDescriptorPool         g_DescriptorPool = VK_NULL_HANDLE;
 
 static int                      fb_width, fb_height;
-static uint32_t                 g_BackBufferIndex = 0;
+static uint32_t                 g_BackbufferIndices[IMGUI_VK_QUEUED_FRAMES];    // keep track of recently rendered swapchain frame indices
 static uint32_t                 g_BackBufferCount = 0;
 static VkImage                  g_BackBuffer[IMGUI_MAX_POSSIBLE_BACK_BUFFERS] = {};
 static VkImageView              g_BackBufferView[IMGUI_MAX_POSSIBLE_BACK_BUFFERS] = {};
@@ -44,7 +46,8 @@ static uint32_t                 g_FrameIndex = 0;
 static VkCommandPool            g_CommandPool[IMGUI_VK_QUEUED_FRAMES];
 static VkCommandBuffer          g_CommandBuffer[IMGUI_VK_QUEUED_FRAMES];
 static VkFence                  g_Fence[IMGUI_VK_QUEUED_FRAMES];
-static VkSemaphore              g_Semaphore[IMGUI_VK_QUEUED_FRAMES];
+static VkSemaphore              g_PresentCompleteSemaphore[IMGUI_VK_QUEUED_FRAMES];
+static VkSemaphore              g_RenderCompleteSemaphore[IMGUI_VK_QUEUED_FRAMES];
 
 static VkClearValue             g_ClearValue = {};
 
@@ -452,7 +455,9 @@ static void setup_vulkan(GLFWwindow* window)
         {
             VkSemaphoreCreateInfo info = {};
             info.sType = VK_STRUCTURE_TYPE_SEMAPHORE_CREATE_INFO;
-            err = vkCreateSemaphore(g_Device, &info, g_Allocator, &g_Semaphore[i]);
+            err = vkCreateSemaphore(g_Device, &info, g_Allocator, &g_PresentCompleteSemaphore[i]);
+            check_vk_result(err);
+            err = vkCreateSemaphore(g_Device, &info, g_Allocator, &g_RenderCompleteSemaphore[i]);
             check_vk_result(err);
         }
     }
@@ -492,7 +497,8 @@ static void cleanup_vulkan()
         vkDestroyFence(g_Device, g_Fence[i], g_Allocator);
         vkFreeCommandBuffers(g_Device, g_CommandPool[i], 1, &g_CommandBuffer[i]);
         vkDestroyCommandPool(g_Device, g_CommandPool[i], g_Allocator);
-        vkDestroySemaphore(g_Device, g_Semaphore[i], g_Allocator);
+        vkDestroySemaphore(g_Device, g_PresentCompleteSemaphore[i], g_Allocator);
+        vkDestroySemaphore(g_Device, g_RenderCompleteSemaphore[i], g_Allocator);
     }
     for (uint32_t i = 0; i < g_BackBufferCount; i++)
     {
@@ -525,7 +531,7 @@ static void frame_begin()
         check_vk_result(err);
     }
     {
-        err = vkAcquireNextImageKHR(g_Device, g_Swapchain, UINT64_MAX, g_Semaphore[g_FrameIndex], VK_NULL_HANDLE, &g_BackBufferIndex);
+        err = vkAcquireNextImageKHR(g_Device, g_Swapchain, UINT64_MAX, g_PresentCompleteSemaphore[g_FrameIndex], VK_NULL_HANDLE, &g_BackbufferIndices[g_FrameIndex]);
         check_vk_result(err);
     }
     {
@@ -541,7 +547,7 @@ static void frame_begin()
         VkRenderPassBeginInfo info = {};
         info.sType = VK_STRUCTURE_TYPE_RENDER_PASS_BEGIN_INFO;
         info.renderPass = g_RenderPass;
-        info.framebuffer = g_Framebuffer[g_BackBufferIndex];
+        info.framebuffer = g_Framebuffer[g_BackbufferIndices[g_FrameIndex]];
         info.renderArea.extent.width = fb_width;
         info.renderArea.extent.height = fb_height;
         info.clearValueCount = 1;
@@ -559,10 +565,12 @@ static void frame_end()
         VkSubmitInfo info = {};
         info.sType = VK_STRUCTURE_TYPE_SUBMIT_INFO;
         info.waitSemaphoreCount = 1;
-        info.pWaitSemaphores = &g_Semaphore[g_FrameIndex];
+        info.pWaitSemaphores = &g_PresentCompleteSemaphore[g_FrameIndex];
         info.pWaitDstStageMask = &wait_stage;
         info.commandBufferCount = 1;
         info.pCommandBuffers = &g_CommandBuffer[g_FrameIndex];
+        info.signalSemaphoreCount = 1;
+        info.pSignalSemaphores = &g_RenderCompleteSemaphore[g_FrameIndex];
 
         err = vkEndCommandBuffer(g_CommandBuffer[g_FrameIndex]);
         check_vk_result(err);
@@ -571,17 +579,31 @@ static void frame_end()
         err = vkQueueSubmit(g_Queue, 1, &info, g_Fence[g_FrameIndex]);
         check_vk_result(err);
     }
+}
+
+static void frame_present()
 {
+    VkResult err;
+// If IMGUI_UNLIMITED_FRAME_RATE is defined we present the latest but one frame
+// Othrewise we present the latest rendered frame
+#ifdef IMGUI_UNLIMITED_FRAME_RATE
+    uint32_t PresentIndex = (g_FrameIndex + IMGUI_VK_QUEUED_FRAMES - 1) % IMGUI_VK_QUEUED_FRAMES;
+#else
+    uint32_t PresentIndex = g_FrameIndex;
+#endif // IMGUI_UNLIMITED_FRAME_RATE
+
     VkSwapchainKHR swapchains[1] = {g_Swapchain};
-        uint32_t indices[1] = {g_BackBufferIndex};
+    uint32_t indices[1] = {g_BackbufferIndices[PresentIndex]};
     VkPresentInfoKHR info = {};
     info.sType = VK_STRUCTURE_TYPE_PRESENT_INFO_KHR;
+    info.waitSemaphoreCount = 1;
+    info.pWaitSemaphores = &g_RenderCompleteSemaphore[PresentIndex];
     info.swapchainCount = 1;
     info.pSwapchains = swapchains;
     info.pImageIndices = indices;
     err = vkQueuePresentKHR(g_Queue, &info);
     check_vk_result(err);
-    }
+
     g_FrameIndex = (g_FrameIndex + 1) % IMGUI_VK_QUEUED_FRAMES;
 }
 
@@ -660,6 +682,18 @@ int main(int, char**)
     bool show_another_window = false;
     ImVec4 clear_color = ImColor(114, 144, 154);
 
+    // When IMGUI_UNLIMITED_FRAME_RATE is defined we render into latest image acquired from the swapchain
+    // but we display the image which was rendered before
+    // hence we must render once and increase the g_FrameIndex without presenting, which we do befor entering the render loop 
+    // this is also the reason why frame_end() is split into frame_end() and frame_present(), the latter one not being called here
+#ifdef IMGUI_UNLIMITED_FRAME_RATE
+    ImGui_ImplGlfwVulkan_NewFrame();
+    frame_begin();
+    ImGui_ImplGlfwVulkan_Render(g_CommandBuffer[g_FrameIndex]);
+    frame_end();
+    g_FrameIndex = (g_FrameIndex + 1) % IMGUI_VK_QUEUED_FRAMES;
+#endif // IMGUI_UNLIMITED_FRAME_RATE
+
     // Main loop
     while (!glfwWindowShouldClose(window))
     {
@@ -702,6 +736,7 @@ int main(int, char**)
         frame_begin();
         ImGui_ImplGlfwVulkan_Render(g_CommandBuffer[g_FrameIndex]);
         frame_end();
+        frame_present();
     }
 
     // Cleanup