mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-03 16:09:39 +00:00
308 lines
13 KiB
Text
308 lines
13 KiB
Text
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_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_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.
|
|
|