This document describes the design for buffers.
GstBuffer is the object that is passed from an upstream element to a
downstream element and contains memory and metadata information.
- It must be fast
- allocation, free, low fragmentation
- Must be able to attach multiple memory blocks to the buffer
- Must be able to attach arbitrary metadata to buffers
- efficient handling of subbuffer, copy, span, trim
GstMemory extends from
GstMiniObject and therefore uses its lifecycle
management (See miniobject).
GstBuffer is writable as returned by
metadata can be added/removed and the metadata can be changed
GstMemoryblocks can be added/removed
The individual memory blocks have their own locking and READONLY flags that might influence their writability.
Buffers can be made writable with
will copy the buffer with the metadata and will ref the memory in the
buffer. This means that the memory is not automatically copied when
GstBuffer contains an array of pointers to
When the buffer is writable,
gst_buffer_insert_memory() can be used
to add a new
GstMemory object to the buffer. When the array of memory is
full, memory will be merged to make room for the new memory object.
gst_buffer_n_memory() is used to get the amount of memory blocks on
gst_buffer_peek_memory(), memory can be retrieved from the
memory array. The desired access pattern for the memory block should be
specified so that appropriate checks can be made and, in case of
GST_MAP_WRITE, a writable copy can be constructed when needed.
can be used to remove memory from the
Subbuffers are made by copying only a region of the memory blocks and copying all of the metadata.
Spanning will merge together the data of 2 buffers into a new buffer
Accessing the data of the buffer can happen by retrieving the individual
GstMemory objects in the
GstBuffer or by using the
_unmap() functions will always return the memory of all
blocks as one large contiguous region. Using these functions might be more
convenient than accessing the individual memory blocks at the expense of
being more expensive because it might perform memcpy operations.
For buffers with only one
GstMemory object (the most common case),
_unmap() have no performance penalty at all.
Read access with 1 memory block: The memory block is accessed and mapped for read access. The memory block is unmapped after usage
write access with 1 memory block: The buffer should be writable or this operation will fail. The memory block is accessed. If the memory block is readonly, a copy is made and the original memory block is replaced with this copy. Then the memory block is mapped in write mode and unmapped after usage.
Read access with multiple memory blocks: The memory blocks are combined into one large memory block. If the buffer is writable, the memory blocks are replaced with this new combined block. If the buffer is not writable, the memory is returned as is. The memory block is then mapped in read mode. When the memory is unmapped after usage and the buffer has multiple memory blocks, this means that the map operation was not able to store the combined buffer and it thus returned memory that should be freed. Otherwise, the memory is unmapped.
Write access with multiple memory blocks: The buffer should be writable or the operation fails. The memory blocks are combined into one large memory block and the existing blocks are replaced with this new block. The memory is then mapped in write mode and unmapped after usage.
Generating RTP packets from h264 video
We receive as input a
GstBuffer with an encoded h264 image and we need
to create RTP packets containing this h264 data as the payload. We
typically need to fragment the h264 data into multiple packets, each
with their own RTP and payload specific header.
+-------+-------+---------------------------+--------+ input H264 buffer: | NALU1 | NALU2 | ..... | NALUx | +-------+-------+---------------------------+--------+ | V array of +-+ +-------+ +-+ +-------+ +-+ +-------+ output buffers: | | | NALU1 | | | | NALU2 | .... | | | NALUx | +-+ +-------+ +-+ +-------+ +-+ +-------+ : : : : \-----------/ \-----------/ buffer 1 buffer 2
The output buffer array consists of x buffers consisting of an RTP
payload header and a subbuffer of the original input H264 buffer. Since
the rtp headers and the h264 data don’t need to be contiguous in memory,
they are added to the buffer as separate
GstMemory blocks and we can
avoid to memcpy the h264 data into contiguous memory.
A typical udpsink will then use something like sendmsg to send the memory regions on the network inside one UDP packet. This will further avoid having to memcpy data into contiguous memory.
Using bufferlists, the complete array of output buffers can be pushed in one operation to the peer element.
The results of the search are