ffmpeg中filter_query_formats主要起一个pix fmt引用指定的功能。
下下结论：

先看几个结构体定义：

//删除了一些与本次分析不必要的成员
struct AVFilterLink {
    AVFilterContext *src;       ///< source filter
    AVFilterPad *srcpad;        ///< output pad on the source filter

    AVFilterContext *dst;       ///< dest filter
    AVFilterPad *dstpad;        ///< input pad on the dest filter

    enum AVMediaType type;      ///< filter media type

    /* These parameters apply only to video */
    int w;                      ///< agreed upon image width
    int h;                      ///< agreed upon image height
    AVRational sample_aspect_ratio; ///< agreed upon sample aspect ratio
    /* These parameters apply only to audio */
    uint64_t channel_layout;    ///< channel layout of current buffer (see libavutil/channel_layout.h)
    int sample_rate;            ///< samples per second

    int format;                 ///< agreed upon media format

    /*****************************************************************
     * 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.
     *****************************************************************
     */

    /**
     * Lists of supported formats / etc. supported by the input filter.
     */
    AVFilterFormatsConfig incfg;

    /**
     * Graph the filter belongs to.
     */
    struct AVFilterGraph *graph;
    ...
};

结构体：AVFilterFormatsConfig

typedef struct AVFilterFormatsConfig {

    /**
     * List of supported formats (pixel or sample).
     */
    AVFilterFormats *formats;

    /**
     * Lists of supported sample rates, only for audio.
     */
    AVFilterFormats  *samplerates;

    /**
     * Lists of supported channel layouts, only for audio.
     */
    AVFilterChannelLayouts  *channel_layouts;

} AVFilterFormatsConfig;

再来看函数：

static int filter_query_formats(AVFilterContext *ctx)
{
    int ret, i;
    AVFilterFormats *formats;
    AVFilterChannelLayouts *chlayouts;
    AVFilterFormats *samplerates;
    enum AVMediaType type = ctx->inputs  && ctx->inputs [0] ? ctx->inputs [0]->type :
                            ctx->outputs && ctx->outputs[0] ? ctx->outputs[0]->type :
                            AVMEDIA_TYPE_VIDEO;

    if ((ret = ctx->filter->query_formats(ctx)) < 0) {
        if (ret != AVERROR(EAGAIN))
            av_log(ctx, AV_LOG_ERROR, "Query format failed for '%s': %s\n",
                   ctx->name, av_err2str(ret));
        return ret;
    }
    ret = filter_check_formats(ctx);
    if (ret < 0)
        return ret;

    for (i = 0; i < ctx->nb_inputs; i++)
        sanitize_channel_layouts(ctx, ctx->inputs[i]->outcfg.channel_layouts);
    for (i = 0; i < ctx->nb_outputs; i++)
        sanitize_channel_layouts(ctx, ctx->outputs[i]->incfg.channel_layouts);
//如果query_formats函数有，那么这里其实什么也不做
    formats = ff_all_formats(type);
    if ((ret = ff_set_common_formats(ctx, formats)) < 0)
        return ret;
    if (type == AVMEDIA_TYPE_AUDIO) {
        samplerates = ff_all_samplerates();
        if ((ret = ff_set_common_samplerates(ctx, samplerates)) < 0)
            return ret;
        chlayouts = ff_all_channel_layouts();
        if ((ret = ff_set_common_channel_layouts(ctx, chlayouts)) < 0)
            return ret;
    }
    return 0;
}

核心函数：ff_set_common_formats

int ff_set_common_formats(AVFilterContext *ctx, AVFilterFormats *formats)
{
    SET_COMMON_FORMATS(ctx, formats,
                       ff_formats_ref, ff_formats_unref);
}

看宏定义：

#define SET_COMMON_FORMATS(ctx, fmts, ref_fn, unref_fn)             \
    int count = 0, i;                                               \
                                                                    \
    if (!fmts)                                                      \
        return AVERROR(ENOMEM);                                     \
                                                                    \
    for (i = 0; i < ctx->nb_inputs; i++) {                          \
        if (ctx->inputs[i] && !ctx->inputs[i]->outcfg.fmts) {       \
            int ret = ref_fn(fmts, &ctx->inputs[i]->outcfg.fmts);   \
            if (ret < 0) {                                          \
                return ret;                                         \
            }                                                       \
            count++;                                                \
        }                                                           \
    }                                                               \
    for (i = 0; i < ctx->nb_outputs; i++) {                         \
        if (ctx->outputs[i] && !ctx->outputs[i]->incfg.fmts) {      \
            int ret = ref_fn(fmts, &ctx->outputs[i]->incfg.fmts);   \
            if (ret < 0) {                                          \
                return ret;                                         \
            }                                                       \
            count++;                                                \
        }                                                           \
    }                                                               \
                                                                    \
    if (!count) {                                                   \
        unref_fn(&fmts);                                            \
    }                                                               \
                                                                    \
    return 0;

接着看ref

int ff_formats_ref(AVFilterFormats *f, AVFilterFormats **ref)
{
    FORMATS_REF(f, ref, ff_formats_unref);
}

#define FORMATS_REF(f, ref, unref_fn)                                           \
    void *tmp;                                                                  \
                                                                                \
    if (!f)                                                                     \
        return AVERROR(ENOMEM);                                                 \
                                                                                \
    tmp = av_realloc_array(f->refs, sizeof(*f->refs), f->refcount + 1);         \
    if (!tmp) {                                                                 \
        unref_fn(&f);                                                           \
        return AVERROR(ENOMEM);                                                 \
    }                                                                           \
    f->refs = tmp;                                                              \
    f->refs[f->refcount++] = ref;                                               \
    *ref = f;                                                                   \
    return 0

主要看关键的三行代码：

    f->refs = tmp;                                                              \
    f->refs[f->refcount++] = ref;                                               \
    *ref = f;  

这就是最开始图片指示的互相引用。