This class is for elements that receive buffers in an undesired size.
While for example raw video contains one image per buffer, the same is not
true for a lot of other formats, especially those that come directly from
a file. So if you have undefined buffer sizes and require a specific size,
this object is for you.
For example, a sink pad's chain function that needs to pass data to a library
in 512-byte chunks could be implemented like this:
static GstFlowReturn
sink_pad_chain (GstPad *pad, GstObject *parent, GstBuffer *buffer)
{
MyElement *this;
GstAdapter *adapter;
GstFlowReturn ret = GST_FLOW_OK;
this = MY_ELEMENT (parent);
adapter = this->adapter;
// put buffer into adapter
gst_adapter_push (adapter, buffer);
// while we can read out 512 bytes, process themwhile (gst_adapter_available (adapter) >= 512 && ret == GST_FLOW_OK) {
const guint8 *data = gst_adapter_map (adapter, 512);
// use flowreturn as an error value
ret = my_library_foo (data);
gst_adapter_unmap (adapter);
gst_adapter_flush (adapter, 512);
}
return ret;
}
For another example, a simple element inside GStreamer that uses #GstAdapter
is the libvisual element.
An element using #GstAdapter in its sink pad chain function should ensure that
when the FLUSH_STOP event is received, that any queued data is cleared using
gstbase.adapter.Adapter.clear. Data should also be cleared or processed on EOS and
when changing state from gst.types.State.Paused to gst.types.State.Ready.
Also check the GST_BUFFER_FLAG_DISCONT flag on the buffer. Some elements might
need to clear the adapter after a discontinuity.
The adapter will keep track of the timestamps of the buffers
that were pushed. The last seen timestamp before the current position
can be queried with gstbase.adapter.Adapter.prevPts. This function can
optionally return the number of bytes between the start of the buffer that
carried the timestamp and the current adapter position. The distance is
useful when dealing with, for example, raw audio samples because it allows
you to calculate the timestamp of the current adapter position by using the
last seen timestamp and the amount of bytes since. Additionally, the
gstbase.adapter.Adapter.prevPtsAtOffset can be used to determine the last
seen timestamp at a particular offset in the adapter.
The adapter will also keep track of the offset of the buffers
(#GST_BUFFER_OFFSET) that were pushed. The last seen offset before the
current position can be queried with gstbase.adapter.Adapter.prevOffset. This function
can optionally return the number of bytes between the start of the buffer
that carried the offset and the current adapter position.
A last thing to note is that while #GstAdapter is pretty optimized,
merging buffers still might be an operation that requires a malloc() and
memcpy() operation, and these operations are not the fastest. Because of
this, some functions like gstbase.adapter.Adapter.availableFast are provided to help
speed up such cases should you want to. To avoid repeated memory allocations,
gstbase.adapter.Adapter.copy can be used to copy data into a (statically allocated)
user provided buffer.
#GstAdapter is not MT safe. All operations on an adapter must be serialized by
the caller. This is not normally a problem, however, as the normal use case
of #GstAdapter is inside one pad's chain function, in which case access is
serialized via the pad's STREAM_LOCK.
Note that gstbase.adapter.Adapter.push takes ownership of the buffer passed. Use
gst_buffer_ref() before pushing it into the adapter if you still want to
access the buffer later. The adapter will never modify the data in the
buffer pushed in it.
This class is for elements that receive buffers in an undesired size. While for example raw video contains one image per buffer, the same is not true for a lot of other formats, especially those that come directly from a file. So if you have undefined buffer sizes and require a specific size, this object is for you.
An adapter is created with gstbase.adapter.Adapter.new_. It can be freed again with gobject.object.ObjectG.unref.
The theory of operation is like this: All buffers received are put into the adapter using gstbase.adapter.Adapter.push and the data is then read back in chunks of the desired size using gstbase.adapter.Adapter.map/gstbase.adapter.Adapter.unmap and/or gstbase.adapter.Adapter.copy. After the data has been processed, it is freed using gstbase.adapter.Adapter.unmap.
Other methods such as gstbase.adapter.Adapter.take and gstbase.adapter.Adapter.takeBuffer combine gstbase.adapter.Adapter.map and gstbase.adapter.Adapter.unmap in one method and are potentially more convenient for some use cases.
For example, a sink pad's chain function that needs to pass data to a library in 512-byte chunks could be implemented like this:
For another example, a simple element inside GStreamer that uses #GstAdapter is the libvisual element.
An element using #GstAdapter in its sink pad chain function should ensure that when the FLUSH_STOP event is received, that any queued data is cleared using gstbase.adapter.Adapter.clear. Data should also be cleared or processed on EOS and when changing state from gst.types.State.Paused to gst.types.State.Ready.
Also check the GST_BUFFER_FLAG_DISCONT flag on the buffer. Some elements might need to clear the adapter after a discontinuity.
The adapter will keep track of the timestamps of the buffers that were pushed. The last seen timestamp before the current position can be queried with gstbase.adapter.Adapter.prevPts. This function can optionally return the number of bytes between the start of the buffer that carried the timestamp and the current adapter position. The distance is useful when dealing with, for example, raw audio samples because it allows you to calculate the timestamp of the current adapter position by using the last seen timestamp and the amount of bytes since. Additionally, the gstbase.adapter.Adapter.prevPtsAtOffset can be used to determine the last seen timestamp at a particular offset in the adapter.
The adapter will also keep track of the offset of the buffers (#GST_BUFFER_OFFSET) that were pushed. The last seen offset before the current position can be queried with gstbase.adapter.Adapter.prevOffset. This function can optionally return the number of bytes between the start of the buffer that carried the offset and the current adapter position.
Additionally the adapter also keeps track of the PTS, DTS and buffer offset at the last discontinuity, which can be retrieved with gstbase.adapter.Adapter.ptsAtDiscont, gstbase.adapter.Adapter.dtsAtDiscont and gstbase.adapter.Adapter.offsetAtDiscont. The number of bytes that were consumed since then can be queried with gstbase.adapter.Adapter.distanceFromDiscont.
A last thing to note is that while #GstAdapter is pretty optimized, merging buffers still might be an operation that requires a malloc() and memcpy() operation, and these operations are not the fastest. Because of this, some functions like gstbase.adapter.Adapter.availableFast are provided to help speed up such cases should you want to. To avoid repeated memory allocations, gstbase.adapter.Adapter.copy can be used to copy data into a (statically allocated) user provided buffer.
#GstAdapter is not MT safe. All operations on an adapter must be serialized by the caller. This is not normally a problem, however, as the normal use case of #GstAdapter is inside one pad's chain function, in which case access is serialized via the pad's STREAM_LOCK.
Note that gstbase.adapter.Adapter.push takes ownership of the buffer passed. Use gst_buffer_ref() before pushing it into the adapter if you still want to access the buffer later. The adapter will never modify the data in the buffer pushed in it.