Transform elements

Transform elements transform input buffers to output buffers based on the sink and source caps.

An important requirement for a transform is that the output caps are completely defined by the input caps and vice versa. This means that a typical decoder element can NOT be implemented with a transform element, this is because the output caps like width and height of the decompressed video frame, for example, are encoded in the stream and thus not defined by the input caps.

Typical transform elements include:

audio convertors (audioconvert, audioresample,…)
video convertors (colorspace, videoscale, …)
filters (capsfilter, volume, colorbalance, …)

The implementation of the transform element has to take care of the following things:

efficient negotiation both up and downstream
efficient buffer alloc and other buffer management

Some transform elements can operate in different modes:

passthrough (no changes are done on the input buffers)
in-place (changes made directly to the incoming buffers without requiring a copy or new buffer allocation)
metadata changes only

Depending on the mode of operation the buffer allocation strategy might change.

The transform element should at any point be able to renegotiate sink and src caps as well as change the operation mode.

In addition, the transform element will typically take care of the following things as well:

flushing, seeking
state changes
timestamping, this is typically done by copying the input timestamps to the output buffers but subclasses should be able to override this.
QoS, avoiding calls to the subclass transform function
handle scheduling issues such as push and pull based operation.

In the next sections, we will describe the behaviour of the transform element in each of the above use cases. We focus mostly on the buffer allocation strategies and caps negotiation.

Processing

A transform has 2 main processing functions:

transform(): Transform the input buffer to the output buffer. The output buffer is guaranteed to be writable and different from the input buffer.
transform_ip(): Transform the input buffer in-place. The input buffer is writable and of bigger or equal size than the output buffer.

A transform can operate in the following modes:

passthrough: The element will not make changes to the buffers, buffers are pushed straight through, caps on both sides need to be the same. The element can optionally implement a transform_ip() function to take a look at the data, the buffer does not have to be writable.
in-place: Changes can be made to the input buffer directly to obtain the output buffer. The transform must implement a transform_ip() function.
copy-transform: The transform is performed by copying and transforming the input buffer to a new output buffer. The transform must implement a transform() function.

When no transform() function is provided, only in-place and passthrough operation is allowed, this means that source and destination caps must be equal or that the source buffer size is bigger or equal than the destination buffer.

When no transform_ip() function is provided, only passthrough and copy-transforms are supported. Providing this function is an optimisation that can avoid a buffer copy.

When no functions are provided, we can only process in passthrough mode.

Negotiation

Typical (re)negotiation of the transform element in push mode always goes from sink to src, this means triggers the following sequence:

the sinkpad receives a new caps event.
the transform function figures out what it can convert these caps to.
try to see if we can configure the caps unmodified on the peer. We need to do this because we prefer to not do anything.
the transform configures itself to transform from the new sink caps to the target src caps
the transform processes and sets the output caps on the src pad

We call this downstream negotiation (DN) and it goes roughly like this:

          sinkpad              transform               srcpad
CAPS event   |                    |                      |
------------>|  find_transform()  |                      |
             |------------------->|                      |
             |                    |       CAPS event     |
             |                    |--------------------->|
             | <configure caps> <-|                      |

These steps configure the element for a transformation from the input caps to the output caps.

The transform has 3 function to perform the negotiation:

transform_caps(): Transform the caps on a certain pad to all the possible supported caps on the other pad. The input caps are guaranteed to be a simple caps with just one structure. The caps do not have to be fixed.
fixate_caps(): Given a caps on one pad, fixate the caps on the other pad. The target caps are writable.
set_caps(): Configure the transform for a transformation between src caps and dest caps. Both caps are guaranteed to be fixed caps.

If no transform_caps() is defined, we can only perform the identity transform, by default.

If no set_caps() is defined, we don’t care about caps. In that case we also assume nothing is going to write to the buffer and we don’t enforce a writable buffer for the transform_ip() function, when present.

One common function that we need for the transform element is to find the best transform from one format (src) to another (dest). Some requirements of this function are:

has a fixed src caps
finds a fixed dest caps that the transform element can transform to
the dest caps are compatible and can be accepted by peer elements
the transform function prefers to make src caps == dest caps
the transform function can optionally fixate dest caps.

The find_transform() function goes like this:

start from src aps, these caps are fixed.
check if the caps are acceptable for us as src caps. This is usually enforced by the padtemplate of the element.
calculate all caps we can transform too with transform_caps()
if the original caps are a subset of the transforms, try to see if the the caps are acceptable for the peer. If this is possible, we can perform passthrough and make src == dest. This is performed by simply calling gst_pad_peer_query_accept_caps().
if the caps are not fixed, we need to fixate it, start by taking the peer caps and intersect with them.
for each of the transformed caps retrieved with transform_caps():
try to fixate the caps with fixate_caps()
if the caps are fixated, check if the peer accepts them with _peer_query_accept_caps(), if the peer accepts, we have found a dest caps.
if we run out of caps, we fail to find a transform.
if we found a destination caps, configure the transform with set_caps().

After this negotiation process, the transform element is usually in a steady state. We can identify these steady states:

src and sink pads both have the same caps. Note that when the caps are equal on both pads, the input and output buffers automatically have the same size. The element can operate on the buffers in the following ways: (Same caps, SC)

passthrough: buffers are inspected but no metadata or buffer data is changed. The input buffers don’t need to be writable. The input buffer is simply pushed out again without modifications. (SCP)

          sinkpad              transform               srcpad
  chain()    |                    |                      |
------------>|   handle_buffer()  |                      |
             |------------------->|      pad_push()      |
             |                    |--------------------->|
             |                    |                      |

in-place: buffers are modified in-place, this means that the input buffer is modified to produce a new output buffer. This requires the input buffer to be writable. If the input buffer is not writable, a new buffer has to be allocated from the bufferpool. (SCI)

          sinkpad              transform               srcpad
  chain()    |                    |                      |
------------>|   handle_buffer()  |                      |
             |------------------->|                      |
             |                    |   [!writable]        |
             |                    |   alloc buffer       |
             |                  .-|                      |
             |  <transform_ip>  | |                      |
             |                  '>|                      |
             |                    |      pad_push()      |
             |                    |--------------------->|
             |                    |                      |

copy transform: a new output buffer is allocate from the bufferpool and data from the input buffer is transformed into the output buffer. (SCC)

          sinkpad              transform               srcpad
  chain()    |                    |                      |
------------>|   handle_buffer()  |                      |
             |------------------->|                      |
             |                    |     alloc buffer     |
             |                  .-|                      |
             |     <transform>  | |                      |
             |                  '>|                      |
             |                    |      pad_push()      |
             |                    |--------------------->|
             |                    |                      |

src and sink pads have different caps. The element can operate on the buffers in the following way: (Different Caps, DC)

in-place: input buffers are modified in-place. This means that the input buffer has a size that is larger or equal to the output size. The input buffer will be resized to the size of the output buffer. If the input buffer is not writable or the output size is bigger than the input size, we need to pad-alloc a new buffer. (DCI)

          sinkpad              transform               srcpad
  chain()    |                    |                      |
------------>|   handle_buffer()  |                      |
             |------------------->|                      |
             |                    | [!writable || !size] |
             |                    |     alloc buffer     |
             |                  .-|                      |
             |  <transform_ip>  | |                      |
             |                  '>|                      |
             |                    |      pad_push()      |
             |                    |--------------------->|
             |                    |                      |

copy transform: a new output buffer is allocated and the data from the input buffer is transformed into the output buffer. The flow is exactly the same as the case with the same-caps negotiation. (DCC)

We can immediately observe that the copy transform states will need to allocate a new buffer from the bufferpool. When the transform element is receiving a non-writable buffer in the in-place state, it will also need to perform an allocation. There is no reason why the passthrough state would perform an allocation.

This steady state changes when one of the following actions occur:

the sink pad receives new caps, this triggers the above downstream renegotation process, see above for the flow.
the transform element wants to renegotiate (because of changed properties, for example). This essentially clears the current steady state and triggers the downstream and upstream renegotiation process. This situation also happens when a RECONFIGURE event was received on the transform srcpad.

Allocation

After the transform element is configured with caps, a bufferpool needs to be negotiated to perform the allocation of buffers. We have 2 cases:

The element is operating in passthrough we don’t need to allocate a buffer in the transform element.
The element is not operating in passthrough and needs to allocation an output buffer.

In case 1, we don’t query and configure a pool. We let upstream decide if it wants to use a bufferpool and then we will proxy the bufferpool from downstream to upstream.

In case 2, we query and set a bufferpool on the srcpad that will be used for doing the allocations.

In order to perform allocation, we need to be able to get the size of the output buffer after the transform. We need additional function to retrieve the size. There are two functions:

transform_size(): Given a caps and a size on one pad, and a caps on the other pad, calculate the size of the other buffer. This function is able to perform all size transforms and is the preferred method of transforming a size.
get_unit_size(): When the input size and output size are always a multiple of each other (audio conversion, ..) we can define a more simple get_unit_size() function. The transform will use this function to get the same amount of units in the source and destination buffers. For performance reasons, the mapping between caps and size is kept in a cache.

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