Novice learning FFmpeg - to achieve controlled by API Filter call chain

Although by declaring [x][y]avfilter=a=x:b=y;avfilter=xxxcreate a usable way Filtercall chain, and in most cases this way is reliable and practical. But if you want to 精细化manage AVFilterthe call chain, such as dynamically generated based on certain conditions AVFilter Graph. This declarative approach is less flexible (can also be dynamically assembled to determine if the string through, if you really like this string declaratively, not far suggest you read on the).

First quick brush up on how to create a AVFilter Graph.

             +-------+             +---------------------+         +---------------+
             |buffer |             |Filter ..... Filter N|         |   buffersink  |
 ----------> |    |output|------>|input|            |output|---> |input|           |-------->
             +-------+             +---------------------+         +---------------+

Creating Trilogy:

1. Initialization bufferand buffersink.
2. Other filter initialization
3. Filter Graph set of Input and Output.

Wherein bufferand buffersinkrepresent the Graphstart and end.

Then quickly package argsis movie=t.png[wm];[in][wm]overlay=10:20[out]such a filter-complex commands. And by avfilter_graph_parse_ptrcompleting the initialization of the intermediate filter,

Finally, specify the input and each filter output, even get a graph.

Well, let's look how the API 精细化generation AVFilter Graph. Generates the following Graph:

             +-------+             +---------------------+         +---------------+
             |buffer |             |       Filter        |         |   buffersink  |
 ----------> |    |output|------>|input|    Fade      |output|---> |input|           |-------->
             +-------+             +---------------------+         +---------------+

First, initialize each AVFilter. All initialization AVFilter are two steps can be simplified as:

• Through avfilter_get_by_nameto find the specified AVFilter
• By avfilter_graph_create_filterinitializing AVFilterContext

The same AVcodecand AVCodecContextthe same relationship, perform all rely AVFilter the corresponding AVFilterContext (ffmpeg in development, Each component corresponds to a context manager, context manager of the various parameters and invoke the component package execution).

By avfilter_get_by_nameAfter generating AVFilter instance, followed by need to call avfilter_graph_create_filterinitialize context manager.

According to the following procedure, the initialization sequence three AVFilter:

buffer_src = avfilter_get_by_name("buffer");
ret = avfilter_graph_create_filter(&buffersrc_ctx, buffer_src, "in", args, NULL, filter_graph);

The focus here talk about avfilter_graph_create_filter. Here is the function prototype:

int avfilter_graph_create_filter(AVFilterContext **filt_ctx, const AVFilter *filt,
                                 const char *name, const char *args, void *opaque,
                                 AVFilterGraph *graph_ctx);

filt_ctx indicates that the AVFilter context manager.

AVFilter name is indicated in the Graph name in this Jiaosha name is not important but must be unique. For example Fade AVFilterit can be called fade1, fade2or ifadethe like.

args is the argument of this AVFilter, pay attention to this parameter is only AVFilter, not the entire graph parameters. Let's use Fadean example, args will be t=in:st=3:d=3.

generally opaque to NULL on it.

After completion of the initial three AVFilter, entered into the focus of this document: avfilter_link.

int avfilter_link(  AVFilterContext *   src,
                    unsigned    srcpad,
                    AVFilterContext *   dst,
                    unsigned    dstpad
)

avfilter_linkThey were used to link two AVFilter (Satisfy legendary). srcAnd dstrepresent the source and destination Filter Filter. srcpadIt represents srcN-th output terminal, dstpadshowing dstthe N-th input terminal. If not well understood, direct look at the following chart:

    +-------------+                  +-------------+
    |  src      srcpad 1 ----->   dstpad 3    dst  |
    |           srcpad 2 ----->   dstpad 2         |
    |           srcpad 3 ----->   dstpad 1         |
    +-------------+                  +-------------+

The figure assumptions srcand dstrespectively have three outputs and three inputs (not all avfilter have so many inputs and outputs, just like a fade, but there are multiple overlay).

And srcpadand dstpadrepresented is the output / input number. If the bufferfirst output terminal corresponding to fadethe first input terminal. Then it would look like this:

avfilter_link(buffersrc_ctx, 0, ifade_ctx, 0);

Then fadethe first output terminal corresponding to buffersinkthe input, so write:

avfilter_link(ifade_ctx, 0, buffersink_ctx, 0);

The so-called 精细化is here embodied by the code determination logic may be a combination of different dynamic AVFilterdifferent build Filter Graph. And it may also be a combination of different input / output terminal.

The reference code sample ifilter. Also refer FFmpeg-Go-Server (attempt to provide a restful API for ffmpeg web server).