Special treatment of UNDEFINED layout for external images to preserve contents #3713
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- Preserve the initial contents of UNDEFINED images if they are external - Don't consider a pipeline barrier from UNDEFINED a resource discard for external images Change-Id: Ife30e7c542a66668d681d492363a25745238119e
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External images have to be created with VK_IMAGE_LAYOUT_UNDEFINED: "If the pNext chain includes a VkExternalMemoryImageCreateInfo or VkExternalMemoryImageCreateInfoNV structure whose handleTypes member is not 0, initialLayout must be VK_IMAGE_LAYOUT_UNDEFINED" (https://registry.khronos.org/vulkan/specs/latest/html/vkspec.html#VUID-VkImageCreateInfo-pNext-01443)
So even if image has contents that importing application would like to be preserved, it will have a transition from UNDEFINED to new layout.
This section https://registry.khronos.org/vulkan/specs/latest/html/vkspec.html#resources-external-sharing adds that "Therefore, the layout specified as part of this transition will be the true initial layout of the image. The undefined layout specified when creating it is a placeholder to simplify valid usage requirements."