This base class is for filter elements that process data. Elements that are suitable for implementation using #GstBaseTransform 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.