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1 Filter design
2 =============
4 This document explains guidelines that should be observed (or ignored with
5 good reason) when writing filters for libavfilter.
7 In this document, the word “frame” indicates either a video frame or a group
8 of audio samples, as stored in an AVFrame structure.
11 Format negotiation
12 ==================
14 The query_formats method should set, for each input and each output links,
15 the list of supported formats.
17 For video links, that means pixel format. For audio links, that means
18 channel layout, sample format (the sample packing is implied by the sample
19 format) and sample rate.
21 The lists are not just lists, they are references to shared objects. When
22 the negotiation mechanism computes the intersection of the formats
23 supported at each end of a link, all references to both lists are replaced
24 with a reference to the intersection. And when a single format is
25 eventually chosen for a link amongst the remaining list, again, all
26 references to the list are updated.
28 That means that if a filter requires that its input and output have the
29 same format amongst a supported list, all it has to do is use a reference
30 to the same list of formats.
32 query_formats can leave some formats unset and return AVERROR(EAGAIN) to
33 cause the negotiation mechanism to try again later. That can be used by
34 filters with complex requirements to use the format negotiated on one link
35 to set the formats supported on another.
38 Frame references ownership and permissions
39 ==========================================
41 Principle
42 ---------
44 Audio and video data are voluminous; the frame and frame reference
45 mechanism is intended to avoid, as much as possible, expensive copies of
46 that data while still allowing the filters to produce correct results.
48 The data is stored in buffers represented by AVFrame structures.
49 Several references can point to the same frame buffer; the buffer is
50 automatically deallocated once all corresponding references have been
51 destroyed.
53 The characteristics of the data (resolution, sample rate, etc.) are
54 stored in the reference; different references for the same buffer can
55 show different characteristics. In particular, a video reference can
56 point to only a part of a video buffer.
58 A reference is usually obtained as input to the filter_frame method or
59 requested using the ff_get_video_buffer or ff_get_audio_buffer
60 functions. A new reference on an existing buffer can be created with
61 av_frame_ref(). A reference is destroyed using
62 the av_frame_free() function.
64 Reference ownership
65 -------------------
67 At any time, a reference “belongs” to a particular piece of code,
68 usually a filter. With a few caveats that will be explained below, only
69 that piece of code is allowed to access it. It is also responsible for
70 destroying it, although this is sometimes done automatically (see the
71 section on link reference fields).
73 Here are the (fairly obvious) rules for reference ownership:
75 * A reference received by the filter_frame method belongs to the
76 corresponding filter.
78 * A reference passed to ff_filter_frame is given away and must no longer
79 be used.
81 * A reference created with av_frame_ref() belongs to the code that
82 created it.
84 * A reference obtained with ff_get_video_buffer or ff_get_audio_buffer
85 belongs to the code that requested it.
87 * A reference given as return value by the get_video_buffer or
88 get_audio_buffer method is given away and must no longer be used.
90 Link reference fields
91 ---------------------
93 The AVFilterLink structure has a few AVFrame fields.
95 partial_buf is used by libavfilter internally and must not be accessed
96 by filters.
98 fifo contains frames queued in the filter's input. They belong to the
99 framework until they are taken by the filter.
101 Reference permissions
102 ---------------------
104 Since the same frame data can be shared by several frames, modifying may
105 have unintended consequences. A frame is considered writable if only one
106 reference to it exists. The code owning that reference it then allowed
107 to modify the data.
109 A filter can check if a frame is writable by using the
110 av_frame_is_writable() function.
112 A filter can ensure that a frame is writable at some point of the code
113 by using the ff_inlink_make_frame_writable() function. It will duplicate
114 the frame if needed.
116 A filter can ensure that the frame passed to the filter_frame() callback
117 is writable by setting the needs_writable flag on the corresponding
118 input pad. It does not apply to the activate() callback.
121 Frame scheduling
122 ================
124 The purpose of these rules is to ensure that frames flow in the filter
125 graph without getting stuck and accumulating somewhere.
127 Simple filters that output one frame for each input frame should not have
128 to worry about it.
130 There are two design for filters: one using the filter_frame() and
131 request_frame() callbacks and the other using the activate() callback.
133 The design using filter_frame() and request_frame() is legacy, but it is
134 suitable for filters that have a single input and process one frame at a
135 time. New filters with several inputs, that treat several frames at a time
136 or that require a special treatment at EOF should probably use the design
137 using activate().
139 activate
140 --------
142 This method is called when something must be done in a filter; the
143 definition of that "something" depends on the semantic of the filter.
145 The callback must examine the status of the filter's links and proceed
146 accordingly.
148 The status of output links is stored in the frame_wanted_out, status_in
149 and status_out fields and tested by the ff_outlink_frame_wanted()
150 function. If this function returns true, then the processing requires a
151 frame on this link and the filter is expected to make efforts in that
152 direction.
154 The status of input links is stored by the status_in, fifo and
155 status_out fields; they must not be accessed directly. The fifo field
156 contains the frames that are queued in the input for processing by the
157 filter. The status_in and status_out fields contains the queued status
158 (EOF or error) of the link; status_in is a status change that must be
159 taken into account after all frames in fifo have been processed;
160 status_out is the status that have been taken into account, it is final
161 when it is not 0.
163 The typical task of an activate callback is to first check the backward
164 status of output links, and if relevant forward it to the corresponding
165 input. Then, if relevant, for each input link: test the availability of
166 frames in fifo and process them; if no frame is available, test and
167 acknowledge a change of status using ff_inlink_acknowledge_status(); and
168 forward the result (frame or status change) to the corresponding input.
169 If nothing is possible, test the status of outputs and forward it to the
170 corresponding input(s). If still not possible, return FFERROR_NOT_READY.
172 If the filters stores internally one or a few frame for some input, it
173 can consider them to be part of the FIFO and delay acknowledging a
174 status change accordingly.
176 Example code:
178 ret = ff_outlink_get_status(outlink);
179 if (ret) {
180 ff_inlink_set_status(inlink, ret);
181 return 0;
182 }
183 if (priv->next_frame) {
184 /* use it */
185 return 0;
186 }
187 ret = ff_inlink_consume_frame(inlink, &frame);
188 if (ret < 0)
189 return ret;
190 if (ret) {
191 /* use it */
192 return 0;
193 }
194 ret = ff_inlink_acknowledge_status(inlink, &status, &pts);
195 if (ret) {
196 /* flush */
197 ff_outlink_set_status(outlink, status, pts);
198 return 0;
199 }
200 if (ff_outlink_frame_wanted(outlink)) {
201 ff_inlink_request_frame(inlink);
202 return 0;
203 }
204 return FFERROR_NOT_READY;
206 The exact code depends on how similar the /* use it */ blocks are and
207 how related they are to the /* flush */ block, and needs to apply these
208 operations to the correct inlink or outlink if there are several.
210 Macros are available to factor that when no extra processing is needed:
212 FF_FILTER_FORWARD_STATUS_BACK(outlink, inlink);
213 FF_FILTER_FORWARD_STATUS_ALL(outlink, filter);
214 FF_FILTER_FORWARD_STATUS(inlink, outlink);
215 FF_FILTER_FORWARD_STATUS_ALL(inlink, filter);
216 FF_FILTER_FORWARD_WANTED(outlink, inlink);
218 filter_frame
219 ------------
221 For filters that do not use the activate() callback, this method is
222 called when a frame is pushed to the filter's input. It can be called at
223 any time except in a reentrant way.
225 If the input frame is enough to produce output, then the filter should
226 push the output frames on the output link immediately.
228 As an exception to the previous rule, if the input frame is enough to
229 produce several output frames, then the filter needs output only at
230 least one per link. The additional frames can be left buffered in the
231 filter; these buffered frames must be flushed immediately if a new input
232 produces new output.
234 (Example: frame rate-doubling filter: filter_frame must (1) flush the
235 second copy of the previous frame, if it is still there, (2) push the
236 first copy of the incoming frame, (3) keep the second copy for later.)
238 If the input frame is not enough to produce output, the filter must not
239 call request_frame to get more. It must just process the frame or queue
240 it. The task of requesting more frames is left to the filter's
241 request_frame method or the application.
243 If a filter has several inputs, the filter must be ready for frames
244 arriving randomly on any input. Therefore, any filter with several inputs
245 will most likely require some kind of queuing mechanism. It is perfectly
246 acceptable to have a limited queue and to drop frames when the inputs
247 are too unbalanced.
249 request_frame
250 -------------
252 For filters that do not use the activate() callback, this method is
253 called when a frame is wanted on an output.
255 For a source, it should directly call filter_frame on the corresponding
256 output.
258 For a filter, if there are queued frames already ready, one of these
259 frames should be pushed. If not, the filter should request a frame on
260 one of its inputs, repeatedly until at least one frame has been pushed.
262 Return values:
263 if request_frame could produce a frame, or at least make progress
264 towards producing a frame, it should return 0;
265 if it could not for temporary reasons, it should return AVERROR(EAGAIN);
266 if it could not because there are no more frames, it should return
267 AVERROR_EOF.
269 The typical implementation of request_frame for a filter with several
270 inputs will look like that:
272 if (frames_queued) {
273 push_one_frame();
274 return 0;
275 }
276 input = input_where_a_frame_is_most_needed();
277 ret = ff_request_frame(input);
278 if (ret == AVERROR_EOF) {
279 process_eof_on_input();
280 } else if (ret < 0) {
281 return ret;
282 }
283 return 0;
285 Note that, except for filters that can have queued frames and sources,
286 request_frame does not push frames: it requests them to its input, and
287 as a reaction, the filter_frame method possibly will be called and do
288 the work.