ffmpeg 中 结构体初始化 的那些花样

https://blog.csdn.net/ericbar/article/details/79567108
https://blog.csdn.net/ericbar/article/details/79567108

花样1:
AVRational 是一个表示有理数的结构体。

/**
 * Rational number (pair of numerator and denominator).
 */
typedef struct AVRational{
    int num; ///< Numerator
    int den; ///< Denominator
} AVRational;

其实当num=1，den=1000的时候pts的时间单位就相当于毫秒 1/1000秒
其实当num=1，den=1000000的时候pts的时间单位就相当于微秒 1/1000000秒

AVRational 最常用的初始化套路是这样的:

    enc_ctx->time_base = (AVRational){1,25};  // meaningless

这种方式的特例:

/**
 * Internal time base represented as integer
 */

#define AV_TIME_BASE            1000000

/**
 * Internal time base represented as fractional value
 */

#define AV_TIME_BASE_Q          (AVRational){1, AV_TIME_BASE}

花样2 (清晰):

                        fs->time_base.num = 1;
                        fs->time_base.den = AV_TIME_BASE;

花样3:

static const AVRational avf_time_base_q = {
    .num = 1,
    .den = avf_time_base
};

AVPacket packet = { .data = NULL, .size = 0 };

再比如:

AVFilter的定义是这样的:

/**
 * 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 inputs, terminated by a zeroed element.
     *
     * 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 outputs, terminated by a zeroed element.
     *
     * 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;

    ...

    int (*activate)(AVFilterContext *ctx);
} AVFilter;

初始化的套路是这样的，这也是最具c风格的初始化:

AVFilter ff_vf_fps = {
    .name        = "fps",
    .description = NULL_IF_CONFIG_SMALL("Force constant framerate."),
    .init        = init,
    .uninit      = uninit,
    .priv_size   = sizeof(FPSContext),
    .priv_class  = &fps_class,
    .activate    = activate,
    .inputs      = avfilter_vf_fps_inputs,
    .outputs     = avfilter_vf_fps_outputs,
};

References:

https://blog.csdn.net/ericbar/article/details/79567108
https://blog.51cto.com/xiacaojun/1958680