BaseTransform

class BaseTransform(**properties: Any)

Superclasses: Element, Object, InitiallyUnowned, Object

This base class is for filter elements that process data. Elements that are suitable for implementation using 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 GstBaseTransformClass.passthrough_on_same_caps variable will automatically set/unset passthrough based on whether the element negotiates the same caps on both pads.

  • GstBaseTransformClass.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 GstBaseTransform

  • 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.

Methods

class BaseTransform
get_allocator() → tuple[Allocator, AllocationParams]

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

Unref the allocator after use.

get_buffer_pool() → BufferPool | None
is_in_place() → bool

See if trans is configured as a in_place transform.

is_passthrough() → bool

See if trans is configured as a passthrough transform.

is_qos_enabled() → bool

Queries if the transform will handle QoS.

reconfigure() → bool

Negotiates src pad caps with downstream elements if the source pad is marked as needing reconfiguring. Unmarks GST_PAD_FLAG_NEED_RECONFIGURE in any case. But marks it again if negotiation fails.

Do not call this in the GstBaseTransformClass::transform or GstBaseTransformClass::transform_ip vmethod. Call this in GstBaseTransformClass::submit_input_buffer, GstBaseTransformClass::prepare_output_buffer or in GstBaseTransformClass::generate_output before any output buffer is allocated.

It will be default be called when handling an ALLOCATION query or at the very beginning of the default GstBaseTransformClass::submit_input_buffer implementation.

Added in version 1.18.

reconfigure_sink() → None

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

reconfigure_src() → None

Instructs trans 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.

set_gap_aware(gap_aware: bool) → None

If gap_aware is False (the default), output buffers will have the %GST_BUFFER_FLAG_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:

gap_aware – New state

set_in_place(in_place: bool) → None

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:

in_place – Boolean value indicating that we would like to operate on in_place buffers.

set_passthrough(passthrough: bool) → None

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 or generate_output method.

MT safe.

Parameters:

passthrough – boolean indicating passthrough mode.

set_prefer_passthrough(prefer_passthrough: bool) → None

If prefer_passthrough is True (the default), trans 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.

Added in version 1.0.1.

Parameters:

prefer_passthrough – New state

set_qos_enabled(enabled: bool) → None

Enable or disable QoS handling in the transform.

MT safe.

Parameters:

enabled – new state

update_qos(proportion: float, diff: int, timestamp: int) → None

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:

  • proportion – the proportion

  • diff – the diff against the clock

  • timestamp – the timestamp of the buffer generating the QoS expressed in running_time.

update_src_caps(updated_caps: Caps) → bool

Updates the srcpad caps and sends 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 information). This way, they can notify downstream about that change without losing any buffer.

Added in version 1.6.

Parameters:

updated_caps – An updated version of the srcpad caps to be pushed downstream

Properties

class BaseTransform
props.qos: bool

Virtual Methods

class BaseTransform
do_accept_caps(direction: PadDirection, caps: Caps) → bool
Parameters:

  • direction

  • caps

do_before_transform(buffer: Buffer) → None
Parameters:

buffer

do_copy_metadata(input: Buffer, outbuf: Buffer) → bool
Parameters:

  • input

  • outbuf

do_decide_allocation(query: Query) → bool
Parameters:

query

do_filter_meta(query: Query, api: type, params: Structure) → bool
Parameters:

  • query

  • api

  • params

do_fixate_caps(direction: PadDirection, caps: Caps, othercaps: Caps) → Caps
Parameters:

  • direction

  • caps

  • othercaps

do_generate_output() → tuple[FlowReturn, Buffer]
do_get_unit_size(caps: Caps) → tuple[bool, int]
Parameters:

caps

do_prepare_output_buffer(input: Buffer) → tuple[FlowReturn, Buffer]
Parameters:

input

do_propose_allocation(decide_query: Query, query: Query) → bool
Parameters:

  • decide_query

  • query

do_query(direction: PadDirection, query: Query) → bool
Parameters:

  • direction

  • query

do_set_caps(incaps: Caps, outcaps: Caps) → bool
Parameters:

  • incaps

  • outcaps

do_sink_event(event: Event) → bool
Parameters:

event

do_src_event(event: Event) → bool
Parameters:

event

do_start() → bool
do_stop() → bool
do_submit_input_buffer(is_discont: bool, input: Buffer) → FlowReturn
Parameters:

  • is_discont

  • input

do_transform(inbuf: Buffer, outbuf: Buffer) → FlowReturn
Parameters:

  • inbuf

  • outbuf

do_transform_caps(direction: PadDirection, caps: Caps, filter: Caps) → Caps
Parameters:

  • direction

  • caps

  • filter

do_transform_ip(buf: Buffer) → FlowReturn
Parameters:

buf

do_transform_meta(outbuf: Buffer, meta: Meta, inbuf: Buffer) → bool
Parameters:

  • outbuf

  • meta

  • inbuf

do_transform_size(direction: PadDirection, caps: Caps, size: int, othercaps: Caps) → tuple[bool, int]
Parameters:

  • direction

  • caps

  • size

  • othercaps

Fields

class BaseTransform
element
have_segment
priv
queued_buf
segment
sinkpad
srcpad