This base class is for filter elements that process data.
Elements that are suitable for implementation using Transform 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
typeof (unichar2) Use Cases
typeof (unichar2)typeof
(unichar2) 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_capss variable will automatically set/unset passthrough based on whether
the element negotiates the same caps on both pads.
* GstBaseTransformClass.passthrough_on_same_capss 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.
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(unichar2) 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)
typeof (unichar2)typeof
(unichar2) 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.
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(unichar2) 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
typeof (unichar2)typeof
(unichar2) 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
typeof (unichar2) 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.