moltenvk example code

static void demo_draw(struct demo *demo) {
    VkResult U_ASSERT_ONLY err;

    // Ensure no more than FRAME_LAG renderings are outstanding
    vkWaitForFences(demo->device, 1, &demo->fences[demo->frame_index], VK_TRUE, UINT64_MAX);
    vkResetFences(demo->device, 1, &demo->fences[demo->frame_index]);

    do {
        // Get the index of the next available swapchain image:
        err =
            demo->fpAcquireNextImageKHR(demo->device, demo->swapchain, UINT64_MAX,
                                        demo->image_acquired_semaphores[demo->frame_index], VK_NULL_HANDLE, &demo->current_buffer);

        if (err == VK_ERROR_OUT_OF_DATE_KHR) {
            // demo->swapchain is out of date (e.g. the window was resized) and
            // must be recreated:
            demo_resize(demo);
        } else if (err == VK_SUBOPTIMAL_KHR) {
            // demo->swapchain is not as optimal as it could be, but the platform's
            // presentation engine will still present the image correctly.
            break;
        } else {
            assert(!err);
        }
    } while (err != VK_SUCCESS);

    demo_update_data_buffer(demo);

    if (demo->VK_GOOGLE_display_timing_enabled) {
        // Look at what happened to previous presents, and make appropriate
        // adjustments in timing:
        DemoUpdateTargetIPD(demo);

        // Note: a real application would position its geometry to that it's in
        // the correct locatoin for when the next image is presented.  It might
        // also wait, so that there's less latency between any input and when
        // the next image is rendered/presented.  This demo program is so
        // simple that it doesn't do either of those.
    }

    // Wait for the image acquired semaphore to be signaled to ensure
    // that the image won't be rendered to until the presentation
    // engine has fully released ownership to the application, and it is
    // okay to render to the image.
    VkPipelineStageFlags pipe_stage_flags;
    VkSubmitInfo submit_info;
    submit_info.sType = VK_STRUCTURE_TYPE_SUBMIT_INFO;
    submit_info.pNext = NULL;
    submit_info.pWaitDstStageMask = &pipe_stage_flags;
    pipe_stage_flags = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
    submit_info.waitSemaphoreCount = 1;
    submit_info.pWaitSemaphores = &demo->image_acquired_semaphores[demo->frame_index];
    submit_info.commandBufferCount = 1;
    submit_info.pCommandBuffers = &demo->swapchain_image_resources[demo->current_buffer].cmd;
    submit_info.signalSemaphoreCount = 1;
    submit_info.pSignalSemaphores = &demo->draw_complete_semaphores[demo->frame_index];
    err = vkQueueSubmit(demo->graphics_queue, 1, &submit_info, demo->fences[demo->frame_index]);
    assert(!err);

    if (demo->separate_present_queue) {
        // If we are using separate queues, change image ownership to the
        // present queue before presenting, waiting for the draw complete
        // semaphore and signalling the ownership released semaphore when finished
        VkFence nullFence = VK_NULL_HANDLE;
        pipe_stage_flags = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
        submit_info.waitSemaphoreCount = 1;
        submit_info.pWaitSemaphores = &demo->draw_complete_semaphores[demo->frame_index];
        submit_info.commandBufferCount = 1;
        submit_info.pCommandBuffers = &demo->swapchain_image_resources[demo->current_buffer].graphics_to_present_cmd;
        submit_info.signalSemaphoreCount = 1;
        submit_info.pSignalSemaphores = &demo->image_ownership_semaphores[demo->frame_index];
        err = vkQueueSubmit(demo->present_queue, 1, &submit_info, nullFence);
        assert(!err);
    }

    // If we are using separate queues we have to wait for image ownership,
    // otherwise wait for draw complete
    VkPresentInfoKHR present = {
        .sType = VK_STRUCTURE_TYPE_PRESENT_INFO_KHR,
        .pNext = NULL,
        .waitSemaphoreCount = 1,
        .pWaitSemaphores = (demo->separate_present_queue) ? &demo->image_ownership_semaphores[demo->frame_index]
                                                          : &demo->draw_complete_semaphores[demo->frame_index],
        .swapchainCount = 1,
        .pSwapchains = &demo->swapchain,
        .pImageIndices = &demo->current_buffer,
    };

    VkRectLayerKHR rect;
    VkPresentRegionKHR region;
    VkPresentRegionsKHR regions;
    if (demo->VK_KHR_incremental_present_enabled) {
        // If using VK_KHR_incremental_present, we provide a hint of the region
        // that contains changed content relative to the previously-presented
        // image.  The implementation can use this hint in order to save
        // work/power (by only copying the region in the hint).  The
        // implementation is free to ignore the hint though, and so we must
        // ensure that the entire image has the correctly-drawn content.
        uint32_t eighthOfWidth = demo->width / 8;
        uint32_t eighthOfHeight = demo->height / 8;

        rect.offset.x = eighthOfWidth;
        rect.offset.y = eighthOfHeight;
        rect.extent.width = eighthOfWidth * 6;
        rect.extent.height = eighthOfHeight * 6;
        rect.layer = 0;

        region.rectangleCount = 1;
        region.pRectangles = ▭

        regions.sType = VK_STRUCTURE_TYPE_PRESENT_REGIONS_KHR;
        regions.pNext = present.pNext;
        regions.swapchainCount = present.swapchainCount;
        regions.pRegions = &region;
        present.pNext = &regions;
    }

    if (demo->VK_GOOGLE_display_timing_enabled) {
        VkPresentTimeGOOGLE ptime;
        if (demo->prev_desired_present_time == 0) {
            // This must be the first present for this swapchain.
            //
            // We don't know where we are relative to the presentation engine's
            // display's refresh cycle.  We also don't know how long rendering
            // takes.  Let's make a grossly-simplified assumption that the
            // desiredPresentTime should be half way between now and
            // now+target_IPD.  We will adjust over time.
            uint64_t curtime = getTimeInNanoseconds();
            if (curtime == 0) {
                // Since we didn't find out the current time, don't give a
                // desiredPresentTime:
                ptime.desiredPresentTime = 0;
            } else {
                ptime.desiredPresentTime = curtime + (demo->target_IPD >> 1);
            }
        } else {
            ptime.desiredPresentTime = (demo->prev_desired_present_time + demo->target_IPD);
        }
        ptime.presentID = demo->next_present_id++;
        demo->prev_desired_present_time = ptime.desiredPresentTime;

        VkPresentTimesInfoGOOGLE present_time = {
            .sType = VK_STRUCTURE_TYPE_PRESENT_TIMES_INFO_GOOGLE,
            .pNext = present.pNext,
            .swapchainCount = present.swapchainCount,
            .pTimes = &ptime,
        };
        if (demo->VK_GOOGLE_display_timing_enabled) {
            present.pNext = &present_time;
        }
    }

    err = demo->fpQueuePresentKHR(demo->present_queue, &present);
    demo->frame_index += 1;
    demo->frame_index %= FRAME_LAG;

    if (err == VK_ERROR_OUT_OF_DATE_KHR) {
        // demo->swapchain is out of date (e.g. the window was resized) and
        // must be recreated:
        demo_resize(demo);
    } else if (err == VK_SUBOPTIMAL_KHR) {
        // demo->swapchain is not as optimal as it could be, but the platform's
        // presentation engine will still present the image correctly.
    } else {
        assert(!err);
    }
}