C++ Vulkan swapchain image_index vs current_frame

Question

I am new to vulkan and following the vulkan-tutorial . In the chapter about swapchain and multiple frames in flight ( frames_in_flight ) there is something I dont understand.

The variable imageIndex gets set by the function vkAcquireNextImageKHR

uint32_t imageIndex;
vkAcquireNextImageKHR(device, swapChain, UINT64_MAX, imageAvailableSemaphores[currentFrame], VK_NULL_HANDLE, &imageIndex);

and the variable currentFrame gets incremented each frame

currentFrame = (currentFrame + 1) % MAX_FRAMES_IN_FLIGHT;

The imageIndex variable just gets used for the pImageIndices field of the VkPresentInfoKHR struct and for the indexing into the std::vector<VkFramebuffer> .

All other vectors eg the VkFence or VkCommandBuffer are indexed with the currentFrame variable.

What exactly is the defferece between imageIndex and currentFrame and why do I need to keep track of woth?

Answer 1

The difference is:

vkAcquireNextImageKHR::imageIndex is "random". It can return any number in any order.

The currentFrame changes strictly in round-robin fashion. Additionally the max-count may differ from swapchain image count.

You would use vkAcquireNextImageKHR::imageIndex for things tied to a specific swapchain image. And you would use currentFrame for things tied to the sequence of frames (eg odd frames vs even frames).