单个子通道的子通道依赖项。如何？

Question

所以我有一个渲染通道，它带有一个直接绘制到帧缓冲区的子通道。规范并没有强迫我使用依赖项——如果我省略它们，实现会隐式插入它们（虽然我不明白为什么它srcStageMask = VK_PIPELINE_STAGE_TOP_OF_PIPE_BIT用于第一个子通道——这个阶段意味着最开始，即不要等待任何东西）。

但与 Vulkan 一样 - 最好是明确的。这是混淆 - 多个来源以不同的方式使用子通道。

1. Sdk 的立方体示例根本不使用它们。

2. Vulkan-tutorial 只使用一个：

   dependency.srcSubpass = VK_SUBPASS_EXTERNAL;
   dependency.dstSubpass = 0;
   dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
   dependency.srcAccessMask = 0;
   dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
   dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_READ_BIT | VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;
   

   为什么srcAccessMask这里是零？

3. 没有秘密的 API 使用两个：

   dependency.srcSubpass = VK_SUBPASS_EXTERNAL;
   dependency.dstSubpass = 0;
   dependency.srcStageMask = VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT;
   dependency.srcAccessMask = VK_ACCESS_MEMORY_READ_BIT;
   dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
   dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;
   

   和

   dependency.srcSubpass = 0;
   dependency.dstSubpass = VK_SUBPASS_EXTERNAL;
   dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
   dependency.srcAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;
   dependency.dstStageMask = VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT;
   dependency.dstAccessMask = VK_ACCESS_MEMORY_READ_BIT;
   

   这不是很清楚，为什么srcStageMask是 VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT第一subpass -是不是这个阶段应该用于执行依赖性的，但在这里，我们需要存储的依赖？为什么同样的问题dstStageMask 是VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT在第二subpass？

4. Khronos 同步示例使用一种：

   dependency.srcSubpass = VK_SUBPASS_EXTERNAL;
   dependency.dstSubpass = 0;
   dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
   dependency.srcAccessMask = 0;
   dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT;
   dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;
   

   为什么srcAccessMask是0？

5. 这是我对两个依赖项的尝试：

       dependency.srcSubpass = VK_SUBPASS_EXTERNAL;
       dependency.dstSubpass = 0;
       dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT; // need to wait until
   presentation is finished reading the image
       dependency.srcAccessMask = VK_ACCESS_COLOR_ATTACHMENT_READ_BIT;
       dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT; // we are writing to
   the image in this stage
       dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;
   
   and
   
       dependency.srcSubpass = 0;
       dependency.dstSubpass = VK_SUBPASS_EXTERNAL;
       dependency.srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT; // we are writing to
   the image in this stage
       dependency.srcAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT;
       dependency.dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT; // presentation reads
   image in this stage (is it?)
       dependency.dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_READ_BIT;
   

这一切都非常令人困惑。如您所见，多个主管来源有不同的愿景。使用哪一种？如何理解这些依赖？

Answer 1

哦，亲爱的，这是一个大多数人不太了解的话题。许多错误信息被假设和传播。规范示例的正确位置是github repo 中的维基页面。

可以在此处找到有关交换链图像获取/呈现的相关部分。

1. 这是在许多同步细节被正确确定之前的一个老例子。

2. 目的是同步信号量使用以等待 vkAcquireNextImage 的结果与渲染。这是读后写的危害，信号量将包括内存可见性同步。所以 srcAccessMask 是没有必要的。

3. 同样，屏障旨在与信号量同步。将命令缓冲区提交到队列时，您可以设置哪些阶段等待哪些信号量。在这种情况下，他们使用管道阶段的底部而不是VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT

4. 与 2 相同的答案，唯一的区别是 dstAccessMask

5. 我上面链接的维基页面使用以下内容：

   /* Only need a dependency coming in to ensure that the first
      layout transition happens at the right time.
      Second external dependency is implied by having a different
      finalLayout and subpass layout. */
   VkSubpassDependency dependency = {
       .srcSubpass = VK_SUBPASS_EXTERNAL,
       .dstSubpass = 0,
       // .srcStageMask needs to be a part of pWaitDstStageMask in the WSI semaphore.
       .srcStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT,
       .dstStageMask = VK_PIPELINE_STAGE_COLOR_ATTACHMENT_OUTPUT_BIT,
       .srcAccessMask = 0,
       .dstAccessMask = VK_ACCESS_COLOR_ATTACHMENT_WRITE_BIT,
       .dependencyFlags = 0};
   
   /* Normally, we would need an external dependency at the end as well since we are changing layout in finalLayout,
      but since we are signalling a semaphore, we can rely on Vulkan's default behavior,
      which injects an external dependency here with
      dstStageMask = VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT,
      dstAccessMask = 0. */
   

   您混淆了第二个障碍的子通道索引。虽然正如评论中提到的，在渲染过程结束时已经有一个隐式障碍，它反过来与您用来同步当前的信号量或栅栏同步。所以不需要显式依赖。