FFmpeg的AVFilter框架总成AVFilter-AVFilterContext

毫无疑问，还是和前面的一样一个context和一个包含有回调函数指针的插件结构体，想要实现自己的插件，主要实现里面的回调函数就可以了，当然，AVFilter比其它模块稍微复杂一点还要牵扯到其它一些辅助模块，在其它章节介绍

下面是关键函数调用图：

/**
 * Add a frame to the buffer source.
 *
 * @param ctx   an instance of the buffersrc filter
 * @param frame frame to be added. If the frame is reference counted, this
 * function will make a new reference to it. Otherwise the frame data will be
 * copied.
 *
 * @return 0 on success, a negative AVERROR on error
 *
 * This function is equivalent to av_buffersrc_add_frame_flags() with the
 * AV_BUFFERSRC_FLAG_KEEP_REF flag.
 */
av_warn_unused_result
int av_buffersrc_write_frame(AVFilterContext *ctx, const AVFrame *frame);

/**
 * Add a frame to the buffer source.
 *
 * @param ctx   an instance of the buffersrc filter
 * @param frame frame to be added. If the frame is reference counted, this
 * function will take ownership of the reference(s) and reset the frame.
 * Otherwise the frame data will be copied. If this function returns an error,
 * the input frame is not touched.
 *
 * @return 0 on success, a negative AVERROR on error.
 *
 * @note the difference between this function and av_buffersrc_write_frame() is
 * that av_buffersrc_write_frame() creates a new reference to the input frame,
 * while this function takes ownership of the reference passed to it.
 *
 * This function is equivalent to av_buffersrc_add_frame_flags() without the
 * AV_BUFFERSRC_FLAG_KEEP_REF flag.
 */
av_warn_unused_result
int av_buffersrc_add_frame(AVFilterContext *ctx, AVFrame *frame);
/**
 * Get a frame with filtered data from sink and put it in frame.
 *
 * @param ctx pointer to a context of a buffersink or abuffersink AVFilter.
 * @param frame pointer to an allocated frame that will be filled with data.
 *              The data must be freed using av_frame_unref() / av_frame_free()
 *
 * @return
 *         - >= 0 if a frame was successfully returned.
 *         - AVERROR(EAGAIN) if no frames are available at this point; more
 *           input frames must be added to the filtergraph to get more output.
 *         - AVERROR_EOF if there will be no more output frames on this sink.
 *         - A different negative AVERROR code in other failure cases.
 */
int av_buffersink_get_frame(AVFilterContext *ctx, AVFrame *frame);

结构体

/** An instance of a filter */
struct AVFilterContext {
    const AVClass *av_class;        ///< needed for av_log() and filters common options

    const AVFilter *filter;         ///< the AVFilter of which this is an instance

    char *name;                     ///< name of this filter instance

    AVFilterPad   *input_pads;      ///< array of input pads
    AVFilterLink **inputs;          ///< array of pointers to input links
    unsigned    nb_inputs;          ///< number of input pads

    AVFilterPad   *output_pads;     ///< array of output pads
    AVFilterLink **outputs;         ///< array of pointers to output links
    unsigned    nb_outputs;         ///< number of output pads

    void *priv;                     ///< private data for use by the filter

    struct AVFilterGraph *graph;    ///< filtergraph this filter belongs to

    /**
     * Type of multithreading being allowed/used. A combination of
     * AVFILTER_THREAD_* flags.
     *
     * May be set by the caller before initializing the filter to forbid some
     * or all kinds of multithreading for this filter. The default is allowing
     * everything.
     *
     * When the filter is initialized, this field is combined using bit AND with
     * AVFilterGraph.thread_type to get the final mask used for determining
     * allowed threading types. I.e. a threading type needs to be set in both
     * to be allowed.
     *
     * After the filter is initialized, libavfilter sets this field to the
     * threading type that is actually used (0 for no multithreading).
     */
    int thread_type;

    /**
     * An opaque struct for libavfilter internal use.
     */
    AVFilterInternal *internal;

    struct AVFilterCommand *command_queue;

    char *enable_str;               ///< enable expression string
    void *enable;                   ///< parsed expression (AVExpr*)
    double *var_values;             ///< variable values for the enable expression
    int is_disabled;                ///< the enabled state from the last expression evaluation

    /**
     * For filters which will create hardware frames, sets the device the
     * filter should create them in.  All other filters will ignore this field:
     * in particular, a filter which consumes or processes hardware frames will
     * instead use the hw_frames_ctx field in AVFilterLink to carry the
     * hardware context information.
     */
    AVBufferRef *hw_device_ctx;

    /**
     * Max number of threads allowed in this filter instance.
     * If <= 0, its value is ignored.
     * Overrides global number of threads set per filter graph.
     */
    int nb_threads;

    /**
     * Ready status of the filter.
     * A non-0 value means that the filter needs activating;
     * a higher value suggests a more urgent activation.
     */
    unsigned ready;

    /**
     * Sets the number of extra hardware frames which the filter will
     * allocate on its output links for use in following filters or by
     * the caller.
     *
     * Some hardware filters require all frames that they will use for
     * output to be defined in advance before filtering starts.  For such
     * filters, any hardware frame pools used for output must therefore be
     * of fixed size.  The extra frames set here are on top of any number
     * that the filter needs internally in order to operate normally.
     *
     * This field must be set before the graph containing this filter is
     * configured.
     */
    int extra_hw_frames;
};

下面结构体种含有若干回调，可以实现，不过实现的时候要注意一些特殊返回，用指定的函数去结束函数。

/**
 * Filter definition. This defines the pads a filter contains, and all the
 * callback functions used to interact with the filter.
 */
typedef struct AVFilter {
    /**
     * Filter name. Must be non-NULL and unique among filters.
     */
    const char *name;

    /**
     * A description of the filter. May be NULL.
     *
     * You should use the NULL_IF_CONFIG_SMALL() macro to define it.
     */
    const char *description;

    /**
     * List of static inputs.
     *
     * NULL if there are no (static) inputs. Instances of filters with
     * AVFILTER_FLAG_DYNAMIC_INPUTS set may have more inputs than present in
     * this list.
     */
    const AVFilterPad *inputs;

    /**
     * List of static outputs.
     *
     * NULL if there are no (static) outputs. Instances of filters with
     * AVFILTER_FLAG_DYNAMIC_OUTPUTS set may have more outputs than present in
     * this list.
     */
    const AVFilterPad *outputs;

    /**
     * A class for the private data, used to declare filter private AVOptions.
     * This field is NULL for filters that do not declare any options.
     *
     * If this field is non-NULL, the first member of the filter private data
     * must be a pointer to AVClass, which will be set by libavfilter generic
     * code to this class.
     */
    const AVClass *priv_class;

    /**
     * A combination of AVFILTER_FLAG_*
     */
    int flags;

    /*****************************************************************
     * All fields below this line are not part of the public API. They
     * may not be used outside of libavfilter and can be changed and
     * removed at will.
     * New public fields should be added right above.
     *****************************************************************
     */

    /**
     * The number of entries in the list of inputs.
     */
    uint8_t nb_inputs;

    /**
     * The number of entries in the list of outputs.
     */
    uint8_t nb_outputs;

    /**
     * This field determines the state of the formats union.
     * It is an enum FilterFormatsState value.
     */
    uint8_t formats_state;

    /**
     * Filter pre-initialization function
     *
     * This callback will be called immediately after the filter context is
     * allocated, to allow allocating and initing sub-objects.
     *
     * If this callback is not NULL, the uninit callback will be called on
     * allocation failure.
     *
     * @return 0 on success,
     *         AVERROR code on failure (but the code will be
     *           dropped and treated as ENOMEM by the calling code)
     */
    int (*preinit)(AVFilterContext *ctx);

    /**
     * Filter initialization function.
     *
     * This callback will be called only once during the filter lifetime, after
     * all the options have been set, but before links between filters are
     * established and format negotiation is done.
     *
     * Basic filter initialization should be done here. Filters with dynamic
     * inputs and/or outputs should create those inputs/outputs here based on
     * provided options. No more changes to this filter's inputs/outputs can be
     * done after this callback.
     *
     * This callback must not assume that the filter links exist or frame
     * parameters are known.
     *
     * @ref AVFilter.uninit "uninit" is guaranteed to be called even if
     * initialization fails, so this callback does not have to clean up on
     * failure.
     *
     * @return 0 on success, a negative AVERROR on failure
     */
    int (*init)(AVFilterContext *ctx);

    /**
     * Should be set instead of @ref AVFilter.init "init" by the filters that
     * want to pass a dictionary of AVOptions to nested contexts that are
     * allocated during init.
     *
     * On return, the options dict should be freed and replaced with one that
     * contains all the options which could not be processed by this filter (or
     * with NULL if all the options were processed).
     *
     * Otherwise the semantics is the same as for @ref AVFilter.init "init".
     */
    int (*init_dict)(AVFilterContext *ctx, AVDictionary **options);

    /**
     * Filter uninitialization function.
     *
     * Called only once right before the filter is freed. Should deallocate any
     * memory held by the filter, release any buffer references, etc. It does
     * not need to deallocate the AVFilterContext.priv memory itself.
     *
     * This callback may be called even if @ref AVFilter.init "init" was not
     * called or failed, so it must be prepared to handle such a situation.
     */
    void (*uninit)(AVFilterContext *ctx);

    /**
     * The state of the following union is determined by formats_state.
     * See the documentation of enum FilterFormatsState in internal.h.
     */
    union {
        /**
         * Query formats supported by the filter on its inputs and outputs.
         *
         * This callback is called after the filter is initialized (so the inputs
         * and outputs are fixed), shortly before the format negotiation. This
         * callback may be called more than once.
         *
         * This callback must set AVFilterLink.outcfg.formats on every input link
         * and AVFilterLink.incfg.formats on every output link to a list of
         * pixel/sample formats that the filter supports on that link. For audio
         * links, this filter must also set @ref AVFilterLink.incfg.samplerates
         * "in_samplerates" / @ref AVFilterLink.outcfg.samplerates "out_samplerates"
         * and @ref AVFilterLink.incfg.channel_layouts "in_channel_layouts" /
         * @ref AVFilterLink.outcfg.channel_layouts "out_channel_layouts" analogously.
         *
         * This callback must never be NULL if the union is in this state.
         *
         * @return zero on success, a negative value corresponding to an
         * AVERROR code otherwise
         */
        int (*query_func)(AVFilterContext *);
        /**
         * A pointer to an array of admissible pixel formats delimited
         * by AV_PIX_FMT_NONE. The generic code will use this list
         * to indicate that this filter supports each of these pixel formats,
         * provided that all inputs and outputs use the same pixel format.
         *
         * This list must never be NULL if the union is in this state.
         * The type of all inputs and outputs of filters using this must
         * be AVMEDIA_TYPE_VIDEO.
         */
        const enum AVPixelFormat *pixels_list;
        /**
         * Analogous to pixels, but delimited by AV_SAMPLE_FMT_NONE
         * and restricted to filters that only have AVMEDIA_TYPE_AUDIO
         * inputs and outputs.
         *
         * In addition to that the generic code will mark all inputs
         * and all outputs as supporting all sample rates and every
         * channel count and channel layout, as long as all inputs
         * and outputs use the same sample rate and channel count/layout.
         */
        const enum AVSampleFormat *samples_list;
        /**
         * Equivalent to { pix_fmt, AV_PIX_FMT_NONE } as pixels_list.
         */
        enum AVPixelFormat  pix_fmt;
        /**
         * Equivalent to { sample_fmt, AV_SAMPLE_FMT_NONE } as samples_list.
         */
        enum AVSampleFormat sample_fmt;
    } formats;

    int priv_size;      ///< size of private data to allocate for the filter

    int flags_internal; ///< Additional flags for avfilter internal use only.

    /**
     * Make the filter instance process a command.
     *
     * @param cmd    the command to process, for handling simplicity all commands must be alphanumeric only
     * @param arg    the argument for the command
     * @param res    a buffer with size res_size where the filter(s) can return a response. This must not change when the command is not supported.
     * @param flags  if AVFILTER_CMD_FLAG_FAST is set and the command would be
     *               time consuming then a filter should treat it like an unsupported command
     *
     * @returns >=0 on success otherwise an error code.
     *          AVERROR(ENOSYS) on unsupported commands
     */
    int (*process_command)(AVFilterContext *, const char *cmd, const char *arg, char *res, int res_len, int flags);

    /**
     * Filter activation function.
     *
     * Called when any processing is needed from the filter, instead of any
     * filter_frame and request_frame on pads.
     *
     * The function must examine inlinks and outlinks and perform a single
     * step of processing. If there is nothing to do, the function must do
     * nothing and not return an error. If more steps are or may be
     * possible, it must use ff_filter_set_ready() to schedule another
     * activation.
     */
    int (*activate)(AVFilterContext *ctx);
} AVFilter;

函数

/**
 * Iterate over all registered filters.
 *
 * @param opaque a pointer where libavfilter will store the iteration state. Must
 *               point to NULL to start the iteration.
 *
 * @return the next registered filter or NULL when the iteration is
 *         finished
 */
const AVFilter *av_filter_iterate(void **opaque);

/**
 * Get a filter definition matching the given name.
 *
 * @param name the filter name to find
 * @return     the filter definition, if any matching one is registered.
 *             NULL if none found.
 */
const AVFilter *avfilter_get_by_name(const char *name);

/**
 * Initialize a filter with the supplied parameters.
 *
 * @param ctx  uninitialized filter context to initialize
 * @param args Options to initialize the filter with. This must be a
 *             ':'-separated list of options in the 'key=value' form.
 *             May be NULL if the options have been set directly using the
 *             AVOptions API or there are no options that need to be set.
 * @return 0 on success, a negative AVERROR on failure
 */
int avfilter_init_str(AVFilterContext *ctx, const char *args);

/**
 * Initialize a filter with the supplied dictionary of options.
 *
 * @param ctx     uninitialized filter context to initialize
 * @param options An AVDictionary filled with options for this filter. On
 *                return this parameter will be destroyed and replaced with
 *                a dict containing options that were not found. This dictionary
 *                must be freed by the caller.
 *                May be NULL, then this function is equivalent to
 *                avfilter_init_str() with the second parameter set to NULL.
 * @return 0 on success, a negative AVERROR on failure
 *
 * @note This function and avfilter_init_str() do essentially the same thing,
 * the difference is in manner in which the options are passed. It is up to the
 * calling code to choose whichever is more preferable. The two functions also
 * behave differently when some of the provided options are not declared as
 * supported by the filter. In such a case, avfilter_init_str() will fail, but
 * this function will leave those extra options in the options AVDictionary and
 * continue as usual.
 */
int avfilter_init_dict(AVFilterContext *ctx, AVDictionary **options);

/**
 * Free a filter context. This will also remove the filter from its
 * filtergraph's list of filters.
 *
 * @param filter the filter to free
 */
void avfilter_free(AVFilterContext *filter);

/**
 * Insert a filter in the middle of an existing link.
 *
 * @param link the link into which the filter should be inserted
 * @param filt the filter to be inserted
 * @param filt_srcpad_idx the input pad on the filter to connect
 * @param filt_dstpad_idx the output pad on the filter to connect
 * @return     zero on success
 */
int avfilter_insert_filter(AVFilterLink *link, AVFilterContext *filt,
                           unsigned filt_srcpad_idx, unsigned filt_dstpad_idx);

案例

最后面还是贴个案例吧,想要说明一点的是，avfilter的最开始和结尾的filter一定要是

abuffer ：it will be used for feeding the data into the graph
abuffersink：it will be used to get the filtered data out of the graph

下面这个案例非常具有代表性，真正作用的就一个filter“volume”。都是采用手动链接，链接后通过函数avfilter_graph_config再重新去协商，这个阶段会往pipeline张自动增加一些filter，最常用的就是csc,颜色空间转换，如果上下两个filter的pix_fmt不一致，那么就会自动增加一个filter。

其实就是这些函数族

int avfilter_graph_config(AVFilterGraph *graphctx, void *log_ctx)
{
    int ret;

    if ((ret = graph_check_validity(graphctx, log_ctx)))
        return ret;
    if ((ret = graph_config_formats(graphctx, log_ctx)))
        return ret;
    if ((ret = graph_config_links(graphctx, log_ctx)))
        return ret;
    if ((ret = graph_check_links(graphctx, log_ctx)))
        return ret;
    if ((ret = graph_config_pointers(graphctx, log_ctx)))
        return ret;

    return 0;
}

下面是代码全貌：

/*
 * copyright (c) 2013 Andrew Kelley
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

/**
 * @file
 * libavfilter API usage example.
 *
 * @example filter_audio.c
 * This example will generate a sine wave audio,
 * pass it through a simple filter chain, and then compute the MD5 checksum of
 * the output data.
 *
 * The filter chain it uses is:
 * (input) -> abuffer -> volume -> aformat -> abuffersink -> (output)
 *
 * abuffer: This provides the endpoint where you can feed the decoded samples.
 * volume: In this example we hardcode it to 0.90.
 * aformat: This converts the samples to the samplefreq, channel layout,
 *          and sample format required by the audio device.
 * abuffersink: This provides the endpoint where you can read the samples after
 *              they have passed through the filter chain.
 */

#include <inttypes.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

#include "libavutil/channel_layout.h"
#include "libavutil/md5.h"
#include "libavutil/mem.h"
#include "libavutil/opt.h"
#include "libavutil/samplefmt.h"

#include "libavfilter/avfilter.h"
#include "libavfilter/buffersink.h"
#include "libavfilter/buffersrc.h"

#define INPUT_SAMPLERATE     48000
#define INPUT_FORMAT         AV_SAMPLE_FMT_FLTP
#define INPUT_CHANNEL_LAYOUT AV_CH_LAYOUT_5POINT0

#define VOLUME_VAL 0.90

static int init_filter_graph(AVFilterGraph **graph, AVFilterContext **src,
                             AVFilterContext **sink)
{
    AVFilterGraph *filter_graph;
    AVFilterContext *abuffer_ctx;
    const AVFilter  *abuffer;
    AVFilterContext *volume_ctx;
    const AVFilter  *volume;
    AVFilterContext *aformat_ctx;
    const AVFilter  *aformat;
    AVFilterContext *abuffersink_ctx;
    const AVFilter  *abuffersink;

    AVDictionary *options_dict = NULL;
    uint8_t options_str[1024];
    uint8_t ch_layout[64];

    int err;

    /* Create a new filtergraph, which will contain all the filters. */
    filter_graph = avfilter_graph_alloc();
    if (!filter_graph) {
        fprintf(stderr, "Unable to create filter graph.\n");
        return AVERROR(ENOMEM);
    }

    /* Create the abuffer filter;
     * it will be used for feeding the data into the graph. */
    abuffer = avfilter_get_by_name("abuffer");
    if (!abuffer) {
        fprintf(stderr, "Could not find the abuffer filter.\n");
        return AVERROR_FILTER_NOT_FOUND;
    }

    abuffer_ctx = avfilter_graph_alloc_filter(filter_graph, abuffer, "src");
    if (!abuffer_ctx) {
        fprintf(stderr, "Could not allocate the abuffer instance.\n");
        return AVERROR(ENOMEM);
    }

    /* Set the filter options through the AVOptions API. */
    av_get_channel_layout_string(ch_layout, sizeof(ch_layout), 0, INPUT_CHANNEL_LAYOUT);
    av_opt_set    (abuffer_ctx, "channel_layout", ch_layout,                            AV_OPT_SEARCH_CHILDREN);
    av_opt_set    (abuffer_ctx, "sample_fmt",     av_get_sample_fmt_name(INPUT_FORMAT), AV_OPT_SEARCH_CHILDREN);
    av_opt_set_q  (abuffer_ctx, "time_base",      (AVRational){ 1, INPUT_SAMPLERATE },  AV_OPT_SEARCH_CHILDREN);
    av_opt_set_int(abuffer_ctx, "sample_rate",    INPUT_SAMPLERATE,                     AV_OPT_SEARCH_CHILDREN);

    /* Now initialize the filter; we pass NULL options, since we have already
     * set all the options above. */
    err = avfilter_init_str(abuffer_ctx, NULL);
    if (err < 0) {
        fprintf(stderr, "Could not initialize the abuffer filter.\n");
        return err;
    }

    /* Create volume filter. */
    volume = avfilter_get_by_name("volume");
    if (!volume) {
        fprintf(stderr, "Could not find the volume filter.\n");
        return AVERROR_FILTER_NOT_FOUND;
    }

    volume_ctx = avfilter_graph_alloc_filter(filter_graph, volume, "volume");
    if (!volume_ctx) {
        fprintf(stderr, "Could not allocate the volume instance.\n");
        return AVERROR(ENOMEM);
    }

    /* A different way of passing the options is as key/value pairs in a
     * dictionary. */
    av_dict_set(&options_dict, "volume", AV_STRINGIFY(VOLUME_VAL), 0);
    err = avfilter_init_dict(volume_ctx, &options_dict);
    av_dict_free(&options_dict);
    if (err < 0) {
        fprintf(stderr, "Could not initialize the volume filter.\n");
        return err;
    }

    /* Create the aformat filter;
     * it ensures that the output is of the format we want. */
    aformat = avfilter_get_by_name("aformat");
    if (!aformat) {
        fprintf(stderr, "Could not find the aformat filter.\n");
        return AVERROR_FILTER_NOT_FOUND;
    }

    aformat_ctx = avfilter_graph_alloc_filter(filter_graph, aformat, "aformat");
    if (!aformat_ctx) {
        fprintf(stderr, "Could not allocate the aformat instance.\n");
        return AVERROR(ENOMEM);
    }

    /* A third way of passing the options is in a string of the form
     * key1=value1:key2=value2.... */
    snprintf(options_str, sizeof(options_str),
             "sample_fmts=%s:sample_rates=%d:channel_layouts=0x%"PRIx64,
             av_get_sample_fmt_name(AV_SAMPLE_FMT_S16), 44100,
             (uint64_t)AV_CH_LAYOUT_STEREO);
    err = avfilter_init_str(aformat_ctx, options_str);
    if (err < 0) {
        av_log(NULL, AV_LOG_ERROR, "Could not initialize the aformat filter.\n");
        return err;
    }

    /* Finally create the abuffersink filter;
     * it will be used to get the filtered data out of the graph. */
    abuffersink = avfilter_get_by_name("abuffersink");
    if (!abuffersink) {
        fprintf(stderr, "Could not find the abuffersink filter.\n");
        return AVERROR_FILTER_NOT_FOUND;
    }

    abuffersink_ctx = avfilter_graph_alloc_filter(filter_graph, abuffersink, "sink");
    if (!abuffersink_ctx) {
        fprintf(stderr, "Could not allocate the abuffersink instance.\n");
        return AVERROR(ENOMEM);
    }

    /* This filter takes no options. */
    err = avfilter_init_str(abuffersink_ctx, NULL);
    if (err < 0) {
        fprintf(stderr, "Could not initialize the abuffersink instance.\n");
        return err;
    }

    /* Connect the filters;
     * in this simple case the filters just form a linear chain. */
    err = avfilter_link(abuffer_ctx, 0, volume_ctx, 0);
    if (err >= 0)
        err = avfilter_link(volume_ctx, 0, aformat_ctx, 0);
    if (err >= 0)
        err = avfilter_link(aformat_ctx, 0, abuffersink_ctx, 0);
    if (err < 0) {
        fprintf(stderr, "Error connecting filters\n");
        return err;
    }

    /* Configure the graph. */
    err = avfilter_graph_config(filter_graph, NULL);
    if (err < 0) {
        av_log(NULL, AV_LOG_ERROR, "Error configuring the filter graph\n");
        return err;
    }

    *graph = filter_graph;
    *src   = abuffer_ctx;
    *sink  = abuffersink_ctx;

    return 0;
}

/* Do something useful with the filtered data: this simple
 * example just prints the MD5 checksum of each plane to stdout. */
static int process_output(struct AVMD5 *md5, AVFrame *frame)
{
    int planar     = av_sample_fmt_is_planar(frame->format);
    int channels   = av_get_channel_layout_nb_channels(frame->channel_layout);
    int planes     = planar ? channels : 1;
    int bps        = av_get_bytes_per_sample(frame->format);
    int plane_size = bps * frame->nb_samples * (planar ? 1 : channels);
    int i, j;

    for (i = 0; i < planes; i++) {
        uint8_t checksum[16];

        av_md5_init(md5);
        av_md5_sum(checksum, frame->extended_data[i], plane_size);

        fprintf(stdout, "plane %d: 0x", i);
        for (j = 0; j < sizeof(checksum); j++)
            fprintf(stdout, "%02X", checksum[j]);
        fprintf(stdout, "\n");
    }
    fprintf(stdout, "\n");

    return 0;
}

/* Construct a frame of audio data to be filtered;
 * this simple example just synthesizes a sine wave. */
static int get_input(AVFrame *frame, int frame_num)
{
    int err, i, j;

#define FRAME_SIZE 1024

    /* Set up the frame properties and allocate the buffer for the data. */
    frame->sample_rate    = INPUT_SAMPLERATE;
    frame->format         = INPUT_FORMAT;
    frame->channel_layout = INPUT_CHANNEL_LAYOUT;
    frame->nb_samples     = FRAME_SIZE;
    frame->pts            = frame_num * FRAME_SIZE;

    err = av_frame_get_buffer(frame, 0);
    if (err < 0)
        return err;

    /* Fill the data for each channel. */
    for (i = 0; i < 5; i++) {
        float *data = (float*)frame->extended_data[i];

        for (j = 0; j < frame->nb_samples; j++)
            data[j] = sin(2 * M_PI * (frame_num + j) * (i + 1) / FRAME_SIZE);
    }

    return 0;
}

int main(int argc, char *argv[])
{
    struct AVMD5 *md5;
    AVFilterGraph *graph;
    AVFilterContext *src, *sink;
    AVFrame *frame;
    uint8_t errstr[1024];
    float duration;
    int err, nb_frames, i;

    if (argc < 2) {
        fprintf(stderr, "Usage: %s <duration>\n", argv[0]);
        return 1;
    }

    duration  = atof(argv[1]);
    nb_frames = duration * INPUT_SAMPLERATE / FRAME_SIZE;
    if (nb_frames <= 0) {
        fprintf(stderr, "Invalid duration: %s\n", argv[1]);
        return 1;
    }

    /* Allocate the frame we will be using to store the data. */
    frame  = av_frame_alloc();
    if (!frame) {
        fprintf(stderr, "Error allocating the frame\n");
        return 1;
    }

    md5 = av_md5_alloc();
    if (!md5) {
        fprintf(stderr, "Error allocating the MD5 context\n");
        return 1;
    }

    /* Set up the filtergraph. */
    err = init_filter_graph(&graph, &src, &sink);
    if (err < 0) {
        fprintf(stderr, "Unable to init filter graph:");
        goto fail;
    }

    /* the main filtering loop */
    for (i = 0; i < nb_frames; i++) {
        /* get an input frame to be filtered */
        err = get_input(frame, i);
        if (err < 0) {
            fprintf(stderr, "Error generating input frame:");
            goto fail;
        }

        /* Send the frame to the input of the filtergraph. */
        err = av_buffersrc_add_frame(src, frame);
        if (err < 0) {
            av_frame_unref(frame);
            fprintf(stderr, "Error submitting the frame to the filtergraph:");
            goto fail;
        }

        /* Get all the filtered output that is available. */
        while ((err = av_buffersink_get_frame(sink, frame)) >= 0) {
            /* now do something with our filtered frame */
            err = process_output(md5, frame);
            if (err < 0) {
                fprintf(stderr, "Error processing the filtered frame:");
                goto fail;
            }
            av_frame_unref(frame);
        }

        if (err == AVERROR(EAGAIN)) {
            /* Need to feed more frames in. */
            continue;
        } else if (err == AVERROR_EOF) {
            /* Nothing more to do, finish. */
            break;
        } else if (err < 0) {
            /* An error occurred. */
            fprintf(stderr, "Error filtering the data:");
            goto fail;
        }
    }

    avfilter_graph_free(&graph);
    av_frame_free(&frame);
    av_freep(&md5);

    return 0;

fail:
    av_strerror(err, errstr, sizeof(errstr));
    fprintf(stderr, "%s\n", errstr);
    return 1;
}