CODEWITHSUNDEEP
vulkan
lvella
lvella

Reputation: 13443

How to synchronize vkCmdCopyBufferToImage()?

I need to render an image and copy it back to host. I issue the vkCmdCopyImageToBuffer() from the render pass result to a host readable buffer right after the vkCmdEndRenderPass(), it seems to work, but I am worried the copy starts before the rendering is finished (or the image is transitioned).

Do I need to perform some kind of synchronization, or it is implicitly guaranteed the image will be transitioned to the needed VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL, which happens at the end of the render pass, before the copy is initiated? In the specs, where is this sequence defined?

Upvotes: 2

Views: 1075

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473447

The layout of an image after the execution of a renderpass is explicitly specified by you, when you constructed that renderpass. This is specified by VkAttachmentDescription::finalLayout for the attached image.

As for synchronization, that again is specified by you at renderpass creation time. Subpasses can have external dependencies, where they depend on something that happens before the renderpass, or where they cause something after the renderpass to depend on them.

That being said, if you do not specify a subpass dependency for which the destination subpass is VK_SUBPASS_EXTERNAL, then an implicit dependency is created (one of the few times Vulkan does synchronization implicitly). This implicit dependency synchronizes color, depth, and input attachments from any command with the bottom of the pipe. However, because it does not specify any destination access forms in its mask, this is not useful and you should provide an explicit external dependency.

Also, using bottom of the pipe as the destination stage is pretty much never useful.

Upvotes: 3

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