VK_ANDROID_external_memory_android_hardware_buffer(3) Manual Page

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This extension enables an application to import Android AHardwareBuffer objects created outside of the Vulkan device into Vulkan memory objects, where they can be bound to images and buffers. It also allows exporting an AHardwareBuffer from a Vulkan memory object for symmetry with other operating systems. But since not all AHardwareBuffer usages and formats have Vulkan equivalents, exporting from Vulkan provides strictly less functionality than creating the AHardwareBuffer externally and importing it.

Some AHardwareBuffer images have implementation-defined external formats that may not correspond to Vulkan formats. Sampler Y′C_BC_R conversion can be used to sample from these images and convert them to a known color space.

If VK_KHR_format_feature_flags2 is supported:

If VK_KHR_format_feature_flags2 is supported:

1) Other external memory objects are represented as weakly-typed handles (e.g. Win32 HANDLE or POSIX file descriptor), and require a handle type parameter along with handles. AHardwareBuffer is strongly typed, so naming the handle type is redundant. Does symmetry justify adding handle type parameters/fields anyway?

RESOLVED: No. The handle type is already provided in places that treat external memory objects generically. In the places we would add it, the application code that would have to provide the handle type value is already dealing with AHardwareBuffer-specific commands/structures; the extra symmetry would not be enough to make that code generic.

2) The internal layout and therefore size of a AHardwareBuffer image may depend on native usage flags that do not have corresponding Vulkan counterparts. Do we provide this information to vkCreateImage somehow, or allow the allocation size reported by vkGetImageMemoryRequirements to be approximate?

RESOLVED: Allow the allocation size to be unspecified when allocating the memory. It has to work this way for exported image memory anyway, since AHardwareBuffer allocation happens in vkAllocateMemory, and internally is performed by a separate HAL, not the Vulkan implementation itself. There is a similar issue with vkGetImageSubresourceLayout: the layout is determined by the allocator HAL, so it is not known until the image is bound to memory.

3) Should the result of sampling an external-format image with the suggested Y′C_BC_R conversion parameters yield the same results as using a samplerExternalOES in OpenGL ES?

RESOLVED: This would be desirable, so that apps converting from OpenGL ES to Vulkan could get the same output given the same input. But since sampling and conversion from Y′C_BC_R images is so loosely defined in OpenGL ES, multiple implementations do it in a way that does not conform to Vulkan’s requirements. Modifying the OpenGL ES implementation would be difficult, and would change the output of existing unmodified applications. Changing the output only for applications that are being modified gives developers the chance to notice and mitigate any problems. Implementations are encouraged to minimize differences as much as possible without causing compatibility problems for existing OpenGL ES applications or violating Vulkan requirements.

4) Should an AHardwareBuffer with AHARDWAREBUFFER_USAGE_CPU_* usage be mappable in Vulkan? Should it be possible to export an AHardwareBuffers with such usage?

RESOLVED: Optional, and mapping in Vulkan is not the same as AHardwareBuffer_lock. The semantics of these are different: mapping in memory is persistent, just gives a raw view of the memory contents, and does not involve ownership. AHardwareBuffer_lock gives the host exclusive access to the buffer, is temporary, and allows for reformatting copy-in/copy-out. Implementations are not required to support host-visible memory types for imported Android hardware buffers or resources backed by them. If a host-visible memory type is supported and used, the memory can be mapped in Vulkan, but doing so follows Vulkan semantics: it is just a raw view of the data and does not imply ownership (this means implementations must not internally call AHardwareBuffer_lock to implement vkMapMemory, or assume the application has done so). Implementations are not required to support linear-tiled images backed by Android hardware buffers, even if the AHardwareBuffer has CPU usage. There is no reliable way to allocate memory in Vulkan that can be exported to a AHardwareBuffer with CPU usage.

5) Android may add new AHardwareBuffer formats and usage flags over time. Can reference to them be added to this extension, or do they need a new extension?

RESOLVED: This extension can document the interaction between the new AHB formats/usages and existing Vulkan features. No new Vulkan features or implementation requirements can be added. The extension version number will be incremented when this additional documentation is added, but the version number does not indicate that an implementation supports Vulkan memory or resources that map to the new AHardwareBuffer features: support for that must be queried with vkGetPhysicalDeviceImageFormatProperties2 or is implied by successfully allocating a AHardwareBuffer outside of Vulkan that uses the new feature and has a GPU usage flag.

In essence, these are new features added to a new Android API level, rather than new Vulkan features. The extension will only document how existing Vulkan features map to that new Android feature.

AHardwareBuffer, VkAndroidHardwareBufferFormatPropertiesANDROID, VkAndroidHardwareBufferPropertiesANDROID, VkAndroidHardwareBufferUsageANDROID, VkExternalFormatANDROID, VkImportAndroidHardwareBufferInfoANDROID, VkMemoryGetAndroidHardwareBufferInfoANDROID, vkGetAndroidHardwareBufferPropertiesANDROID, vkGetMemoryAndroidHardwareBufferANDROID

For more information, see the Vulkan Specification

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