class ImagePipe2
Defined at line 1222 of file fidling/gen/sdk/fidl/fuchsia.images/fuchsia.images/hlcpp/fuchsia/images/cpp/fidl.h
ImagePipe is a mechanism for streaming shared images between a producer
and a consumer which may be running in different processes.
Conceptually, the image pipe maintains a table of image resources supplied
by the producer into which graphical content may be stored as well as a
presentation queue containing a sequence of images which the producer has
asked the consumer to present.
The presentation queue is initially empty.
Each entry in the presentation queue consists of an image together with a
pair of optional synchronization fences:
- Acquire fence: signaled by the producer when the image is ready to be consumed
- Release fence: signaled by the consumer when the image is free to be freed or
modified by the producer
The producer performs the following sequence of steps to present content:
- Allocate and add some number of BufferCollections to the image pipe to allow
consumer to set constraints.
- Allocate and add some number of images (often 2 or 3) to the image pipe
to establish a pool using `AddImage()`.
- Obtain the next available image from the pool.
- Ask the consumer to enqueue an image for presentation and provide fences
using `PresentImage()`.
- Start rendering the image.
- Signal the image's acquire fence when rendering is complete.
- Loop to present more image, listen for signals on release fences to recycle
images back into the pool.
The consumer performs the following sequence of steps for each image which
is enqueued in the presentation queue:
- Await signals on the image's acquire fence.
- If the fence wait cannot be satisfied or if some other error is detected,
close the image pipe.
Otherwise, begin presenting the image's content.
- Retire the previously presented image (if any) from the presentation queue
and signal its release fence when no longer needed.
- Continue presenting the same image until the next one is ready. Loop.
If the producer wants to close the image pipe, it should:
- Close its side of the connection.
- Wait on all release fences for buffers that it has submitted with
`PresentImage()`.
- Proceed with resource cleanup.
When the consumer detects the image pipe has closed, it should:
- Stop using/presenting any images from the pipe.
- Unmap all memory objects associated with the images in the pipe.
- Close all BufferCollection resources.
- Signal all release fences for presented and queued buffers.
- Close all handles to fences.
- Close its side of the connection.
When either party detects that a fence has been abandoned (remotely closed
without being signaled) it should assume that the associated image is in
an indeterminate state. This will typically happen when the other party
(or one of its delegates) has crashed. The safest course of action is to
close the image pipe, release all resources which were shared with the
other party, and re-establish the connection to recover.
Public Methods
void ~ImagePipe2 ()void AddBufferCollection2 (uint32_t buffer_collection_id, ::fidl::InterfaceHandle< ::fuchsia::sysmem2::BufferCollectionToken> buffer_collection_token)Adds a BufferCollection resource to the image pipe.
The producer is expected to set constraints on this resource for images added
via `AddImage()`. The consumer can set its constraints on
`buffer_collection_token` before or after. Note that the buffers won't be
allocated until all BufferCollectionToken instances are used to set
constraints, on both the producer and consumer side. See collection.fidl for
details.
The following errors will cause the connection to be closed:
- `buffer_collection_id` is already registered
void AddBufferCollection (uint32_t buffer_collection_id, ::fidl::InterfaceHandle< ::fuchsia::sysmem::BufferCollectionToken> buffer_collection_token)void AddImage (uint32_t image_id, uint32_t buffer_collection_id, uint32_t buffer_collection_index, ::fuchsia::sysmem::ImageFormat_2 image_format)Adds an image resource to image pipe.
`buffer_collection_id` refers to the BufferCollectionToken instance that is
registered via `AddBufferCollection()`. The underlying memory objects allocated
are used to address to the image data. `buffer_collection_index` refers to the
index of the memory object allocated in BufferCollection.
`image_format` specifiies image properties. `coded_width` and `coded_height` are
used to set image dimensions.
It is valid to create multiple images backed by the same memory object; they
may even overlap. Consumers must detect this and handle it accordingly.
The following errors will cause the connection to be closed:
- `image_id` is already registered
- `buffer_collection_id` refers to an unregistered BufferCollection.
- `buffer_collection_index` points to a resource index out of the initialized
BufferCollection bounds
- No resource is allocated in the registered BufferCollection.
void RemoveBufferCollection (uint32_t buffer_collection_id)Removes a BufferCollection resource from the pipe.
The `buffer_collection_id` resource is detached as well as all Images that are
associated with that BufferCollection. Leads to the same results as calling
`RemoveImage()` on all Images for `buffer_collection_id`.
The producer must wait for all release fences associated with the Images to
be signaled before freeing or modifying the underlying memory object since
the image may still be in use in the presentation queue.
The following errors will cause the connection to be closed:
- `buffer_collection_id` does not reference a currently registered BufferCollection
void RemoveImage (uint32_t image_id)Removes an image resource from the pipe.
The `image_id` is detached from the image resource and is free to be
reused to add a new image resource.
Removing an image from the image pipe does not affect the presentation
queue or the currently presented image.
The producer must wait for all release fences associated with the image to
be signaled before freeing or modifying the underlying memory object since
the image may still be in use in the presentation queue.
The following errors will cause the connection to be closed:
- `image_id` does not reference a currently registered image resource
void PresentImage (uint32_t image_id, uint64_t presentation_time, ::std::vector< ::zx::event> acquire_fences, ::std::vector< ::zx::event> release_fences, PresentImageCallback callback)Enqueues the specified image for presentation by the consumer.
The `acquire_fences` are a set of fences which must all be signaled by
the producer before the consumer presents the image.
The `release_fences` are a set of fences which inform the producer that
it's safe to free or modify the `image_id` image, and it's safe to
re-use the fences in `acquire_fences`. The consumer must signal all the
fences in `release_fences` after `image_id` is no longer being
presented. The producer may reuse resources after any of the
`release_fences` is signaled.
This design allows a producer to distribute image processing across
multiple threads / processes without unnecessary coordination delay.
Each thread / process signals its own fence in `acquire_fences` when
it's done rendering its piece of `image_id`, and waits on its own fence
in `release_fences` to render new content in `image_id`.
`presentation_time` specifies the time on or after which the
client would like the enqueued operations should take visible effect
(light up pixels on the screen), expressed in nanoseconds in the
`CLOCK_MONOTONIC` timebase. Desired presentation times must be
monotonically non-decreasing.
`presentation_info` returns timing information about the submitted frame
and future frames (see presentation_info.fidl).
The producer may decide not to signal `acquire_fences` for an image.
In that case, if a later image is enqueued and later image's
`presentation_time` is reached, the consumer presents the later image when
later image's `acquire_fences` are signaled. The consumer also signals
earlier image's `release_fences` and removes it from the presentation queue.
This sequence works as a cancellation mechanism.
The following errors will cause the connection to be closed:
- `image_id` does not reference a currently registered image resource