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doc: update filter_design.txt.

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Nicolas George 7 years ago
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  1. 251
      doc/filter_design.txt

251
doc/filter_design.txt
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@ -5,7 +5,7 @@ This document explains guidelines that should be observed (or ignored with
good reason) when writing filters for libavfilter.
In this document, the word “frame” indicates either a video frame or a group
of audio samples, as stored in an AVFilterBuffer structure.
of audio samples, as stored in an AVFrame structure.
Format negotiation
@ -35,32 +35,31 @@ Format negotiation
to set the formats supported on another.
Buffer references ownership and permissions
===========================================
Frame references ownership and permissions
==========================================
Principle
---------
Audio and video data are voluminous; the buffer and buffer reference
Audio and video data are voluminous; the frame and frame reference
mechanism is intended to avoid, as much as possible, expensive copies of
that data while still allowing the filters to produce correct results.
The data is stored in buffers represented by AVFilterBuffer structures.
They must not be accessed directly, but through references stored in
AVFilterBufferRef structures. Several references can point to the
same buffer; the buffer is automatically deallocated once all
corresponding references have been destroyed.
The data is stored in buffers represented by AVFrame structures.
Several references can point to the same frame buffer; the buffer is
automatically deallocated once all corresponding references have been
destroyed.
The characteristics of the data (resolution, sample rate, etc.) are
stored in the reference; different references for the same buffer can
show different characteristics. In particular, a video reference can
point to only a part of a video buffer.
A reference is usually obtained as input to the start_frame or
filter_frame method or requested using the ff_get_video_buffer or
ff_get_audio_buffer functions. A new reference on an existing buffer can
be created with the avfilter_ref_buffer. A reference is destroyed using
the avfilter_unref_bufferp function.
A reference is usually obtained as input to the filter_frame method or
requested using the ff_get_video_buffer or ff_get_audio_buffer
functions. A new reference on an existing buffer can be created with
av_frame_ref(). A reference is destroyed using
the av_frame_free() function.
Reference ownership
-------------------
@ -73,17 +72,13 @@ Buffer references ownership and permissions
Here are the (fairly obvious) rules for reference ownership:
* A reference received by the filter_frame method (or its start_frame
deprecated version) belongs to the corresponding filter.
* A reference received by the filter_frame method belongs to the
corresponding filter.
Special exception: for video references: the reference may be used
internally for automatic copying and must not be destroyed before
end_frame; it can be given away to ff_start_frame.
* A reference passed to ff_filter_frame is given away and must no longer
be used.
* A reference passed to ff_filter_frame (or the deprecated
ff_start_frame) is given away and must no longer be used.
* A reference created with avfilter_ref_buffer belongs to the code that
* A reference created with av_frame_ref() belongs to the code that
created it.
* A reference obtained with ff_get_video_buffer or ff_get_audio_buffer
@ -95,89 +90,32 @@ Buffer references ownership and permissions
Link reference fields
---------------------
The AVFilterLink structure has a few AVFilterBufferRef fields. The
cur_buf and out_buf were used with the deprecated
start_frame/draw_slice/end_frame API and should no longer be used.
src_buf and partial_buf are used by libavfilter internally
and must not be accessed by filters.
Reference permissions
---------------------
The AVFilterBufferRef structure has a perms field that describes what
the code that owns the reference is allowed to do to the buffer data.
Different references for the same buffer can have different permissions.
For video filters that implement the deprecated
start_frame/draw_slice/end_frame API, the permissions only apply to the
parts of the buffer that have already been covered by the draw_slice
method.
The value is a binary OR of the following constants:
* AV_PERM_READ: the owner can read the buffer data; this is essentially
always true and is there for self-documentation.
* AV_PERM_WRITE: the owner can modify the buffer data.
* AV_PERM_PRESERVE: the owner can rely on the fact that the buffer data
will not be modified by previous filters.
* AV_PERM_REUSE: the owner can output the buffer several times, without
modifying the data in between.
* AV_PERM_REUSE2: the owner can output the buffer several times and
modify the data in between (useless without the WRITE permissions).
* AV_PERM_ALIGN: the owner can access the data using fast operations
that require data alignment.
The READ, WRITE and PRESERVE permissions are about sharing the same
buffer between several filters to avoid expensive copies without them
doing conflicting changes on the data.
The REUSE and REUSE2 permissions are about special memory for direct
rendering. For example a buffer directly allocated in video memory must
not modified once it is displayed on screen, or it will cause tearing;
it will therefore not have the REUSE2 permission.
The ALIGN permission is about extracting part of the buffer, for
copy-less padding or cropping for example.
The AVFilterLink structure has a few AVFrame fields.
References received on input pads are guaranteed to have all the
permissions stated in the min_perms field and none of the permissions
stated in the rej_perms.
partial_buf is used by libavfilter internally and must not be accessed
by filters.
References obtained by ff_get_video_buffer and ff_get_audio_buffer are
guaranteed to have at least all the permissions requested as argument.
fifo contains frames queued in the filter's input. They belong to the
framework until they are taken by the filter.
References created by avfilter_ref_buffer have the same permissions as
the original reference minus the ones explicitly masked; the mask is
usually ~0 to keep the same permissions.
Filters should remove permissions on reference they give to output
whenever necessary. It can be automatically done by setting the
rej_perms field on the output pad.
Here are a few guidelines corresponding to common situations:
Reference permissions
---------------------
* Filters that modify and forward their frame (like drawtext) need the
WRITE permission.
Since the same frame data can be shared by several frames, modifying may
have unintended consequences. A frame is considered writable if only one
reference to it exists. The code owning that reference it then allowed
to modify the data.
* Filters that read their input to produce a new frame on output (like
scale) need the READ permission on input and must request a buffer
with the WRITE permission.
A filter can check if a frame is writable by using the
av_frame_is_writable() function.
* Filters that intend to keep a reference after the filtering process
is finished (after filter_frame returns) must have the PRESERVE
permission on it and remove the WRITE permission if they create a new
reference to give it away.
A filter can ensure that a frame is writable at some point of the code
by using the ff_inlink_make_frame_writable() function. It will duplicate
the frame if needed.
* Filters that intend to modify a reference they have kept after the end
of the filtering process need the REUSE2 permission and must remove
the PRESERVE permission if they create a new reference to give it
away.
A filter can ensure that the frame passed to the filter_frame() callback
is writable by setting the needs_writable flag on the corresponding
input pad. It does not apply to the activate() callback.
Frame scheduling
@ -189,11 +127,100 @@ Frame scheduling
Simple filters that output one frame for each input frame should not have
to worry about it.
There are two design for filters: one using the filter_frame() and
request_frame() callbacks and the other using the activate() callback.
The design using filter_frame() and request_frame() is legacy, but it is
suitable for filters that have a single input and process one frame at a
time. New filters with several inputs, that treat several frames at a time
or that require a special treatment at EOF should probably use the design
using activate().
activate
--------
This method is called when something must be done in a filter; the
definition of that "something" depends on the semantic of the filter.
The callback must examine the status of the filter's links and proceed
accordingly.
The status of output links is stored in the frame_wanted_out, status_in
and status_out fields and tested by the ff_outlink_frame_wanted()
function. If this function returns true, then the processing requires a
frame on this link and the filter is expected to make efforts in that
direction.
The status of input links is stored by the status_in, fifo and
status_out fields; they must not be accessed directly. The fifo field
contains the frames that are queued in the input for processing by the
filter. The status_in and status_out fields contains the queued status
(EOF or error) of the link; status_in is a status change that must be
taken into account after all frames in fifo have been processed;
status_out is the status that have been taken into account, it is final
when it is not 0.
The typical task of an activate callback is to first check the backward
status of output links, and if relevant forward it to the corresponding
input. Then, if relevant, for each input link: test the availability of
frames in fifo and process them; if no frame is available, test and
acknowledge a change of status using ff_inlink_acknowledge_status(); and
forward the result (frame or status change) to the corresponding input.
If nothing is possible, test the status of outputs and forward it to the
corresponding input(s). If still not possible, return FFERROR_NOT_READY.
If the filters stores internally one or a few frame for some input, it
can consider them to be part of the FIFO and delay acknowledging a
status change accordingly.
Example code:
ret = ff_outlink_get_status(outlink);
if (ret) {
ff_inlink_set_status(inlink, ret);
return 0;
}
if (priv->next_frame) {
/* use it */
return 0;
}
ret = ff_inlink_consume_frame(inlink, &frame);
if (ret < 0)
return ret;
if (ret) {
/* use it */
return 0;
}
ret = ff_inlink_acknowledge_status(inlink, &status, &pts);
if (ret) {
/* flush */
ff_outlink_set_status(outlink, status, pts);
return 0;
}
if (ff_outlink_frame_wanted(outlink)) {
ff_inlink_request_frame(inlink);
return 0;
}
return FFERROR_NOT_READY;
The exact code depends on how similar the /* use it */ blocks are and
how related they are to the /* flush */ block, and needs to apply these
operations to the correct inlink or outlink if there are several.
Macros are available to factor that when no extra processing is needed:
FF_FILTER_FORWARD_STATUS_BACK(outlink, inlink);
FF_FILTER_FORWARD_STATUS_ALL(outlink, filter);
FF_FILTER_FORWARD_STATUS(inlink, outlink);
FF_FILTER_FORWARD_STATUS_ALL(inlink, filter);
FF_FILTER_FORWARD_WANTED(outlink, inlink);
filter_frame
------------
This method is called when a frame is pushed to the filter's input. It
can be called at any time except in a reentrant way.
For filters that do not use the activate() callback, this method is
called when a frame is pushed to the filter's input. It can be called at
any time except in a reentrant way.
If the input frame is enough to produce output, then the filter should
push the output frames on the output link immediately.
@ -222,9 +249,10 @@ Frame scheduling
request_frame
-------------
This method is called when a frame is wanted on an output.
For filters that do not use the activate() callback, this method is
called when a frame is wanted on an output.
For an input, it should directly call filter_frame on the corresponding
For a source, it should directly call filter_frame on the corresponding
output.
For a filter, if there are queued frames already ready, one of these
@ -254,16 +282,7 @@ Frame scheduling
}
return 0;
Note that, except for filters that can have queued frames, request_frame
does not push frames: it requests them to its input, and as a reaction,
the filter_frame method possibly will be called and do the work.
Legacy API
==========
Until libavfilter 3.23, the filter_frame method was split:
- for video filters, it was made of start_frame, draw_slice (that could be
called several times on distinct parts of the frame) and end_frame;
- for audio filters, it was called filter_samples.
Note that, except for filters that can have queued frames and sources,
request_frame does not push frames: it requests them to its input, and
as a reaction, the filter_frame method possibly will be called and do
the work.

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