g GObject.InitiallyUnowned GObject.InitiallyUnowned Gst.Object Gst.Object GObject.InitiallyUnowned->Gst.Object GObject.Object GObject.Object GObject.Object->GObject.InitiallyUnowned Gst.Element Gst.Element GstBase.BaseTransform GstBase.BaseTransform Gst.Element->GstBase.BaseTransform Gst.Object->Gst.Element



Inherited:Gst.Element (77), Gst.Object (27), GObject.Object (38)
Structs:Gst.ElementClass (10), GObject.ObjectClass (5)
  get_allocator ()
  get_buffer_pool ()
  is_in_place ()
  is_passthrough ()
  is_qos_enabled ()
  reconfigure_sink ()
  reconfigure_src ()
  set_gap_aware (gap_aware)
  set_in_place (in_place)
  set_passthrough (passthrough)
  set_prefer_passthrough (prefer_passthrough)
  set_qos_enabled (enabled)
  update_qos (proportion, diff, timestamp)
  update_src_caps (updated_caps)

Virtual Methods

Inherited:Gst.Element (16), Gst.Object (1), GObject.Object (7)
  do_accept_caps (direction, caps)
  do_before_transform (buffer)
  do_copy_metadata (input, outbuf)
  do_decide_allocation (query)
  do_filter_meta (query, api, params)
  do_fixate_caps (direction, caps, othercaps)
  do_generate_output ()
  do_get_unit_size (caps)
  do_prepare_output_buffer (input)
  do_propose_allocation (decide_query, query)
  do_query (direction, query)
  do_set_caps (incaps, outcaps)
  do_sink_event (event)
  do_src_event (event)
  do_start ()
  do_stop ()
  do_submit_input_buffer (is_discont, input)
  do_transform (inbuf, outbuf)
  do_transform_caps (direction, caps, filter)
  do_transform_ip (buf)
  do_transform_meta (outbuf, meta, inbuf)
  do_transform_size (direction, caps, size, othercaps)


Inherited:Gst.Object (2)
Name Type Flags Short Description
qos bool r/w Handle Quality-of-Service events


Inherited:Gst.Element (3), Gst.Object (1), GObject.Object (1)
Name Type Access Description
element Gst.Element r  
have_segment bool r  
queued_buf Gst.Buffer r  
segment Gst.Segment r  
sinkpad Gst.Pad r  
srcpad Gst.Pad r  

Class Details

class GstBase.BaseTransform(**kwargs)

This base class is for filter elements that process data. Elements that are suitable for implementation using GstBase.BaseTransform are ones where the size and caps of the output is known entirely from the input caps and buffer sizes. These include elements that directly transform one buffer into another, modify the contents of a buffer in-place, as well as elements that collate multiple input buffers into one output buffer, or that expand one input buffer into multiple output buffers. See below for more concrete use cases.

It provides for:

  • one sinkpad and one srcpad
  • Possible formats on sink and source pad implemented with custom transform_caps function. By default uses same format on sink and source.
  • Handles state changes
  • Does flushing
  • Push mode
  • Pull mode if the sub-class transform can operate on arbitrary data
Use Cases
Passthrough mode
  • Element has no interest in modifying the buffer. It may want to inspect it, in which case the element should have a transform_ip function. If there is no transform_ip function in passthrough mode, the buffer is pushed intact.
  • The GstBase.BaseTransformClass.passthrough_on_same_caps variable will automatically set/unset passthrough based on whether the element negotiates the same caps on both pads.
  • GstBase.BaseTransformClass.passthrough_on_same_caps on an element that doesn’t implement a transform_caps function is useful for elements that only inspect data (such as level)
  • Example elements
    • Level
    • Videoscale, audioconvert, videoconvert, audioresample in certain modes.
Modifications in-place - input buffer and output buffer are the same thing.
  • The element must implement a transform_ip function.
  • Output buffer size must <= input buffer size
  • If the always_in_place flag is set, non-writable buffers will be copied and passed to the transform_ip function, otherwise a new buffer will be created and the transform function called.
  • Incoming writable buffers will be passed to the transform_ip function immediately.
  • only implementing transform_ip and not transform implies always_in_place = True
    • Example elements:
      • Volume
      • Audioconvert in certain modes (signed/unsigned conversion)
      • videoconvert in certain modes (endianness swapping)
Modifications only to the caps/metadata of a buffer
  • The element does not require writable data, but non-writable buffers should be subbuffered so that the meta-information can be replaced.
  • Elements wishing to operate in this mode should replace the prepare_output_buffer method to create subbuffers of the input buffer and set always_in_place to True
  • Example elements
    • Capsfilter when setting caps on outgoing buffers that have none.
    • identity when it is going to re-timestamp buffers by datarate.
Normal mode
  • always_in_place flag is not set, or there is no transform_ip function
  • Element will receive an input buffer and output buffer to operate on.
  • Output buffer is allocated by calling the prepare_output_buffer function.
  • Example elements:
    • Videoscale, videoconvert, audioconvert when doing scaling/conversions
Special output buffer allocations
  • Elements which need to do special allocation of their output buffers beyond allocating output buffers via the negotiated allocator or buffer pool should implement the prepare_output_buffer method.
  • Example elements:
    • efence
Sub-class settable flags on GstBase.BaseTransform
  • passthrough
    • Implies that in the current configuration, the sub-class is not interested in modifying the buffers.
    • Elements which are always in passthrough mode whenever the same caps has been negotiated on both pads can set the class variable passthrough_on_same_caps to have this behaviour automatically.
  • always_in_place
    • Determines whether a non-writable buffer will be copied before passing to the transform_ip function.
    • Implied True if no transform function is implemented.
    • Implied False if ONLY transform function is implemented.
allocator:the Gst.Allocator used
params:the Gst.AllocationParams of allocator
Return type:(allocator: Gst.Allocator, params: Gst.AllocationParams)

Lets GstBase.BaseTransform sub-classes to know the memory allocator used by the base class and its params.

Unref the allocator after use it.

Returns:the instance of the Gst.BufferPool used by self; free it after use it
Return type:Gst.BufferPool
Returns:True is the transform is configured in in_place mode.

MT safe.

Return type:bool

See if self is configured as a in_place transform.

Returns:True is the transform is configured in passthrough mode.

MT safe.

Return type:bool

See if self is configured as a passthrough transform.

Returns:True if QoS is enabled.

MT safe.

Return type:bool

Queries if the transform will handle QoS.


Instructs self to request renegotiation upstream. This function is typically called after properties on the transform were set that influence the input format.


Instructs self to renegotiate a new downstream transform on the next buffer. This function is typically called after properties on the transform were set that influence the output format.

Parameters:gap_aware (bool) – New state

If gap_aware is False (the default), output buffers will have the Gst.BufferFlags.GAP flag unset.

If set to True, the element must handle output buffers with this flag set correctly, i.e. it can assume that the buffer contains neutral data but must unset the flag if the output is no neutral data.

MT safe.

Parameters:in_place (bool) – Boolean value indicating that we would like to operate on in_place buffers.

Determines whether a non-writable buffer will be copied before passing to the transform_ip function.

  • Always True if no transform function is implemented.
  • Always False if ONLY transform function is implemented.

MT safe.

Parameters:passthrough (bool) – boolean indicating passthrough mode.

Set passthrough mode for this filter by default. This is mostly useful for filters that do not care about negotiation.

Always True for filters which don’t implement either a transform or transform_ip method.

MT safe.

Parameters:prefer_passthrough (bool) – New state

If prefer_passthrough is True (the default), self will check and prefer passthrough caps from the list of caps returned by the transform_caps vmethod.

If set to False, the element must order the caps returned from the transform_caps function in such a way that the preferred format is first in the list. This can be interesting for transforms that can do passthrough transforms but prefer to do something else, like a capsfilter.

MT safe.

New in version 1.0.1.

Parameters:enabled (bool) – new state

Enable or disable QoS handling in the transform.

MT safe.

update_qos(proportion, diff, timestamp)[source]
  • proportion (float) – the proportion
  • diff (int) – the diff against the clock
  • timestamp (int) – the timestamp of the buffer generating the QoS expressed in running_time.

Set the QoS parameters in the transform. This function is called internally when a QOS event is received but subclasses can provide custom information when needed.

MT safe.

Parameters:updated_caps (Gst.Caps) – An updated version of the srcpad caps to be pushed downstream
Returns:True if the caps could be send downstream False otherwise
Return type:bool

Updates the srcpad caps and send the caps downstream. This function can be used by subclasses when they have already negotiated their caps but found a change in them (or computed new informations). This way, they can notify downstream about that change without loosing any buffer.

New in version 1.6.

do_accept_caps(direction, caps) virtual
Return type:


do_before_transform(buffer) virtual
Parameters:buffer (Gst.Buffer) –
do_copy_metadata(input, outbuf) virtual
Return type:


do_decide_allocation(query) virtual
Parameters:query (Gst.Query) –
Return type:bool
do_filter_meta(query, api, params) virtual
Return type:


do_fixate_caps(direction, caps, othercaps) virtual
Return type:


do_generate_output() virtual
Return type:(Gst.FlowReturn, outbuf: Gst.Buffer)
do_get_unit_size(caps) virtual
Parameters:caps (Gst.Caps) –
Return type:(bool, size: int)
do_prepare_output_buffer(input) virtual
Parameters:input (Gst.Buffer) –
Return type:(Gst.FlowReturn, outbuf: Gst.Buffer)
do_propose_allocation(decide_query, query) virtual
Return type:


do_query(direction, query) virtual
Return type:


do_set_caps(incaps, outcaps) virtual
Return type:


do_sink_event(event) virtual
Parameters:event (Gst.Event) –
Return type:bool
do_src_event(event) virtual
Parameters:event (Gst.Event) –
Return type:bool
do_start() virtual
Return type:bool
do_stop() virtual
Return type:bool
do_submit_input_buffer(is_discont, input) virtual
Return type:


do_transform(inbuf, outbuf) virtual
Return type:


do_transform_caps(direction, caps, filter) virtual
Return type:


do_transform_ip(buf) virtual
Parameters:buf (Gst.Buffer) –
Return type:Gst.FlowReturn
do_transform_meta(outbuf, meta, inbuf) virtual
Return type:


do_transform_size(direction, caps, size, othercaps) virtual
Return type:

(bool, othersize: int)

Property Details

Default Value:False

Handle Quality-of-Service events