ffmpeg中AVOption的实现分析

[时间:2017-10] [状态:Open]
[关键词:ffmpeg,avutil,AVOption]

0 引言

AVOptions提供了一种通用的options机制,可以用于任意特定结构的对象。
本文将先介绍下AVOption的实现,然后整理下如何使用AVOption。(我使用的版本是ffmpeg3.4)

1 AVOption实现细节

这部分可能会很枯燥,多数是ffmpeg源码的粘贴和整理,如果读者对此不感兴趣可以调到下一部分
AVOption的定义如下:

enum AVOptionType{
    AV_OPT_TYPE_FLAGS,
    AV_OPT_TYPE_INT,
    AV_OPT_TYPE_INT64,
    AV_OPT_TYPE_DOUBLE,
    AV_OPT_TYPE_FLOAT,
    AV_OPT_TYPE_STRING,
    AV_OPT_TYPE_RATIONAL,
    AV_OPT_TYPE_BINARY,  ///< offset must point to a pointer immediately followed by an int for the length
    AV_OPT_TYPE_DICT,
    AV_OPT_TYPE_UINT64,
    AV_OPT_TYPE_CONST = 128,
    AV_OPT_TYPE_IMAGE_SIZE = MKBETAG('S','I','Z','E'), ///< offset must point to two consecutive integers
    AV_OPT_TYPE_PIXEL_FMT  = MKBETAG('P','F','M','T'),
    AV_OPT_TYPE_SAMPLE_FMT = MKBETAG('S','F','M','T'),
    AV_OPT_TYPE_VIDEO_RATE = MKBETAG('V','R','A','T'), ///< offset must point to AVRational
    AV_OPT_TYPE_DURATION   = MKBETAG('D','U','R',' '),
    AV_OPT_TYPE_COLOR      = MKBETAG('C','O','L','R'),
    AV_OPT_TYPE_CHANNEL_LAYOUT = MKBETAG('C','H','L','A'),
    AV_OPT_TYPE_BOOL           = MKBETAG('B','O','O','L'),
};

typedef struct AVOption {
    const char *name;

    /* short English help text */
    const char *help;

    /* option对应的偏移地址和类型 */
    int offset;
    enum AVOptionType type;

    /* 共用体,实际存储数据 */
    union {
        int64_t i64;
        double dbl;
        const char *str;
        /* TODO those are unused now */
        AVRational q;
    } default_val;
    double min;                 ///< minimum valid value for the option
    double max;                 ///< maximum valid value for the option

    int flags;// option类型属性的标志
    const char *unit;
} AVOption;

初始化函数av_opt_set_defaultsav_opt_set_defaults2

void av_opt_set_defaults(void *s)
{
    av_opt_set_defaults2(s, 0, 0);
}
// 这里的s是包含AVOptions成员的结构体指针
void av_opt_set_defaults2(void *s, int mask, int flags)
{
    const AVOption *opt = NULL;
	// 遍历AVOption中所有数据
    while ((opt = av_opt_next(s, opt))) {
        void *dst = ((uint8_t*)s) + opt->offset;

        if ((opt->flags & mask) != flags)
            continue;

        if (opt->flags & AV_OPT_FLAG_READONLY)
            continue;
		// 根据类型赋值
        switch (opt->type) {
            case AV_OPT_TYPE_CONST:
                /* Nothing to be done here */
                break;
            case AV_OPT_TYPE_BOOL:
            case AV_OPT_TYPE_FLAGS:
            case AV_OPT_TYPE_INT:
            case AV_OPT_TYPE_INT64:
            case AV_OPT_TYPE_UINT64:
            case AV_OPT_TYPE_DURATION:
            case AV_OPT_TYPE_CHANNEL_LAYOUT:
            case AV_OPT_TYPE_PIXEL_FMT:
            case AV_OPT_TYPE_SAMPLE_FMT:
                write_number(s, opt, dst, 1, 1, opt->default_val.i64);
                break;
            case AV_OPT_TYPE_DOUBLE:
            case AV_OPT_TYPE_FLOAT: {
                double val;
                val = opt->default_val.dbl;
                write_number(s, opt, dst, val, 1, 1);
            }
            break;
            case AV_OPT_TYPE_RATIONAL: {
                AVRational val;
                val = av_d2q(opt->default_val.dbl, INT_MAX);
                write_number(s, opt, dst, 1, val.den, val.num);
            }
            break;
            case AV_OPT_TYPE_COLOR:
                set_string_color(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_STRING:
                set_string(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_IMAGE_SIZE:
                set_string_image_size(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_VIDEO_RATE:
                set_string_video_rate(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_BINARY:
                set_string_binary(s, opt, opt->default_val.str, dst);
                break;
            case AV_OPT_TYPE_DICT:
                /* Cannot set defaults for these types */
            break;
        default:
            av_log(s, AV_LOG_DEBUG, "AVOption type %d of option %s not implemented yet\n",
                   opt->type, opt->name);
        }
    }
}

初始化的基本逻辑是扫描AVOptions数组的所有成员,然后按照类型依次初始化。其中最主要的是av_opt_next函数。其实现如下:

// 该函数通过直接在AVOptions数组上顺序+1实现遍历
const AVOption *av_opt_next(const void *obj, const AVOption *last)
{
    const AVClass *class;
    if (!obj)
        return NULL;
    class = *(const AVClass**)obj;
    if (!last && class && class->option && class->option[0].name)
        return class->option;
    if (last && last[1].name)
        return ++last;
    return NULL;
}

AVOptions数组必须以NULL结束,否则这个函数可能就出现死循环了。
接下来是保存数据的函数,write_number,其实现的代码如下:

static int write_number(void *obj, const AVOption *o, void *dst, double num, int den, int64_t intnum)
{
    switch (o->type) {
    case AV_OPT_TYPE_PIXEL_FMT:
        *(enum AVPixelFormat *)dst = llrint(num / den) * intnum;
        break;
    case AV_OPT_TYPE_SAMPLE_FMT:
        *(enum AVSampleFormat *)dst = llrint(num / den) * intnum;
        break;
    case AV_OPT_TYPE_BOOL:
    case AV_OPT_TYPE_FLAGS:
    case AV_OPT_TYPE_INT:
        *(int *)dst = llrint(num / den) * intnum;
        break;
    case AV_OPT_TYPE_DURATION:
    case AV_OPT_TYPE_CHANNEL_LAYOUT:
    case AV_OPT_TYPE_INT64:{
        double d = num / den;
        if (intnum == 1 && d == (double)INT64_MAX) {
            *(int64_t *)dst = INT64_MAX;
        } else
            *(int64_t *)dst = llrint(d) * intnum;
        break;}
    case AV_OPT_TYPE_UINT64:{
        double d = num / den;
        // We must special case uint64_t here as llrint() does not support values
        // outside the int64_t range and there is no portable function which does
        // "INT64_MAX + 1ULL" is used as it is representable exactly as IEEE double
        // while INT64_MAX is not
        if (intnum == 1 && d == (double)UINT64_MAX) {
            *(uint64_t *)dst = UINT64_MAX;
        } else if (d > INT64_MAX + 1ULL) {
            *(uint64_t *)dst = (llrint(d - (INT64_MAX + 1ULL)) + (INT64_MAX + 1ULL))*intnum;
        } else {
            *(uint64_t *)dst = llrint(d) * intnum;
        }
        break;}
    case AV_OPT_TYPE_FLOAT:
        *(float *)dst = num * intnum / den;
        break;
    case AV_OPT_TYPE_DOUBLE:
        *(double    *)dst = num * intnum / den;
        break;
    case AV_OPT_TYPE_RATIONAL:
    case AV_OPT_TYPE_VIDEO_RATE:
        if ((int) num == num)
            *(AVRational *)dst = (AVRational) { num *intnum, den };
        else
            *(AVRational *)dst = av_d2q(num * intnum / den, 1 << 24);
        break;
    default:
        return AVERROR(EINVAL);
    }
    return 0;
}

write_number仅支持数字形式的参数。其他格式需要单独处理。下面是其他格式的保存函数:

// AV_OPT_TYPE_COLOR:
static int set_string_color(void *obj, const AVOption *o, const char *val, uint8_t *dst)
{
    int ret;

    if (!val) {
        return 0;
    } else {
        ret = av_parse_color(dst, val, -1, obj);
        if (ret < 0)
            av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as color\n", val);
        return ret;
    }
    return 0;
}
// AV_OPT_TYPE_STRING:
static int set_string(void *obj, const AVOption *o, const char *val, uint8_t **dst)
{
    av_freep(dst);
    *dst = av_strdup(val);
    return *dst ? 0 : AVERROR(ENOMEM);
}
// AV_OPT_TYPE_IMAGE_SIZE:
static int set_string_image_size(void *obj, const AVOption *o, const char *val, int *dst)
{
    int ret;

    if (!val || !strcmp(val, "none")) {
        dst[0] =
        dst[1] = 0;
        return 0;
    }
    ret = av_parse_video_size(dst, dst + 1, val);
    if (ret < 0)
        av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as image size\n", val);
    return ret;
}
// AV_OPT_TYPE_VIDEO_RATE:
static int set_string_video_rate(void *obj, const AVOption *o, const char *val, AVRational *dst)
{
    int ret;
    if (!val) {
        ret = AVERROR(EINVAL);
    } else {
        ret = av_parse_video_rate(dst, val);
    }
    if (ret < 0)
        av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as video rate\n", val);
    return ret;
}
// AV_OPT_TYPE_BINARY:
static int set_string_binary(void *obj, const AVOption *o, const char *val, uint8_t **dst)
{
    int *lendst = (int *)(dst + 1);
    uint8_t *bin, *ptr;
    int len;

    av_freep(dst);
    *lendst = 0;

    if (!val || !(len = strlen(val)))
        return 0;

    if (len & 1)
        return AVERROR(EINVAL);
    len /= 2;

    ptr = bin = av_malloc(len);
    if (!ptr)
        return AVERROR(ENOMEM);
    while (*val) {
        int a = hexchar2int(*val++);
        int b = hexchar2int(*val++);
        if (a < 0 || b < 0) {
            av_free(bin);
            return AVERROR(EINVAL);
        }
        *ptr++ = (a << 4) | b;
    }
    *dst    = bin;
    *lendst = len;

    return 0;
}

当然,AVOptions还有两个构造函数,有兴趣的可以看看ffmpeg中的源代码,其声明如下:

int av_set_options_string(void *ctx, const char *opts,
                          const char *key_val_sep, const char *pairs_sep);
int av_opt_set_from_string(void *ctx, const char *opts,
                           const char *const *shorthand,
                           const char *key_val_sep, const char *pairs_sep);

销毁函数av_opt_free

该函数的实现主要依赖于av_opt_next,遍历所有AVOptions,释放初始化时申请的内存。

void av_opt_free(void *obj)
{
    const AVOption *o = NULL;
    while ((o = av_opt_next(obj, o))) {
        switch (o->type) {
        case AV_OPT_TYPE_STRING:
        case AV_OPT_TYPE_BINARY:
            av_freep((uint8_t *)obj + o->offset);
            break;

        case AV_OPT_TYPE_DICT:
            av_dict_free((AVDictionary **)(((uint8_t *)obj) + o->offset));
            break;

        default:
            break;
        }
    }
}

属性查找av_opt_findav_opt_find2

属性查找依赖于遍历的接口函数,其具体实现如下:

const AVOption *av_opt_find(void *obj, const char *name, const char *unit,
                            int opt_flags, int search_flags)
{
    return av_opt_find2(obj, name, unit, opt_flags, search_flags, NULL);
}

const AVOption *av_opt_find2(void *obj, const char *name, const char *unit,
                             int opt_flags, int search_flags, void **target_obj)
{
    const AVClass  *c;
    const AVOption *o = NULL;

    if(!obj)
        return NULL;

    c= *(AVClass**)obj;

    if (!c)
        return NULL;

	// 判断是否需要检查子对象属性	
    if (search_flags & AV_OPT_SEARCH_CHILDREN) {
        if (search_flags & AV_OPT_SEARCH_FAKE_OBJ) { // 假对象
            const AVClass *child = NULL;
            while (child = av_opt_child_class_next(c, child))
                if (o = av_opt_find2(&child, name, unit, opt_flags, search_flags, NULL))
                    return o;
        } else {
            void *child = NULL;
            while (child = av_opt_child_next(obj, child))
                if (o = av_opt_find2(child, name, unit, opt_flags, search_flags, target_obj))
                    return o;
        }
    }
	// 遍历当前AVClass的所有属性(这里的主要区别是遍历函数不同)
    while (o = av_opt_next(obj, o)) {
        if (!strcmp(o->name, name) && (o->flags & opt_flags) == opt_flags &&
            ((!unit && o->type != AV_OPT_TYPE_CONST) ||
             (unit  && o->type == AV_OPT_TYPE_CONST && o->unit && !strcmp(o->unit, unit)))) {
            if (target_obj) {
                if (!(search_flags & AV_OPT_SEARCH_FAKE_OBJ))
                    *target_obj = obj;
                else
                    *target_obj = NULL;
            }
            return o;
        }
    }
    return NULL;
}

AVOptions遍历

av_opt_next在前面的初始化函数中介绍过。这里重点关注后边两个:av_opt_child_next和av_opt_child_class_next。
对应的实现代码如下:

// 该函数直接通过AVClass->child_next函数指针访问子对象
void *av_opt_child_next(void *obj, void *prev)
{
    const AVClass *c = *(AVClass **)obj;
    if (c->child_next)
        return c->child_next(obj, prev);
    return NULL;
}
// 该函数直接通过AVClass->child_class_next函数指针访问子类
const AVClass *av_opt_child_class_next(const AVClass *parent, const AVClass *prev)
{
    if (parent->child_class_next)
        return parent->child_class_next(prev);
    return NULL;
}

属性设置

属性设置函数比较多,这里依次查看该实现:

int av_opt_set(void *obj, const char *name, const char *val, int search_flags)
{
    int ret = 0;
    void *dst, *target_obj;
	// 根据name查找对应的AVOption
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;
    if (!val && (o->type != AV_OPT_TYPE_STRING &&
                 o->type != AV_OPT_TYPE_PIXEL_FMT && o->type != AV_OPT_TYPE_SAMPLE_FMT &&
                 o->type != AV_OPT_TYPE_IMAGE_SIZE && o->type != AV_OPT_TYPE_VIDEO_RATE &&
                 o->type != AV_OPT_TYPE_DURATION && o->type != AV_OPT_TYPE_COLOR &&
                 o->type != AV_OPT_TYPE_CHANNEL_LAYOUT && o->type != AV_OPT_TYPE_BOOL))
        return AVERROR(EINVAL);

    if (o->flags & AV_OPT_FLAG_READONLY)
        return AVERROR(EINVAL);

    dst = ((uint8_t *)target_obj) + o->offset;
    switch (o->type) { // 根据AVOPtion的类型设置该参数
    case AV_OPT_TYPE_BOOL:
        return set_string_bool(obj, o, val, dst);//这个在初始化函数中有介绍
    case AV_OPT_TYPE_STRING:
        return set_string(obj, o, val, dst); //这个在初始化函数中有介绍
    case AV_OPT_TYPE_BINARY:
        return set_string_binary(obj, o, val, dst);//这个在初始化函数中有介绍
    case AV_OPT_TYPE_FLAGS:
    case AV_OPT_TYPE_INT:
    case AV_OPT_TYPE_INT64:
    case AV_OPT_TYPE_UINT64:
    case AV_OPT_TYPE_FLOAT:
    case AV_OPT_TYPE_DOUBLE:
    case AV_OPT_TYPE_RATIONAL:
        return set_string_number(obj, target_obj, o, val, dst);
    case AV_OPT_TYPE_IMAGE_SIZE:
        return set_string_image_size(obj, o, val, dst);
    case AV_OPT_TYPE_VIDEO_RATE: {
        AVRational tmp;
        ret = set_string_video_rate(obj, o, val, &tmp);
        if (ret < 0)
            return ret;
        return write_number(obj, o, dst, 1, tmp.den, tmp.num);
    }
    case AV_OPT_TYPE_PIXEL_FMT:
        return set_string_pixel_fmt(obj, o, val, dst);
    case AV_OPT_TYPE_SAMPLE_FMT:
        return set_string_sample_fmt(obj, o, val, dst);
    case AV_OPT_TYPE_DURATION:
        if (!val) {
            *(int64_t *)dst = 0;
            return 0;
        } else {
            if ((ret = av_parse_time(dst, val, 1)) < 0)
                av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as duration\n", val);
            return ret;
        }
        break;
    case AV_OPT_TYPE_COLOR:
        return set_string_color(obj, o, val, dst);
    case AV_OPT_TYPE_CHANNEL_LAYOUT:
        if (!val || !strcmp(val, "none")) {
            *(int64_t *)dst = 0;
        } else {
            int64_t cl = av_get_channel_layout(val);
            if (!cl) {
                av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\" as channel layout\n", val);
                ret = AVERROR(EINVAL);
            }
            *(int64_t *)dst = cl;
            return ret;
        }
        break;
    }

    av_log(obj, AV_LOG_ERROR, "Invalid option type.\n");
    return AVERROR(EINVAL);
}

下面看一下其中具体调用的函数实现细节(其中多数函数在初始化一节中一节介绍了,这里不做重复)

static int read_number(const AVOption *o, const void *dst, double *num, int *den, int64_t *intnum)
{
    switch (o->type) {
    case AV_OPT_TYPE_FLAGS:
        *intnum = *(unsigned int*)dst;
        return 0;
    case AV_OPT_TYPE_PIXEL_FMT:
        *intnum = *(enum AVPixelFormat *)dst;
        return 0;
    case AV_OPT_TYPE_SAMPLE_FMT:
        *intnum = *(enum AVSampleFormat *)dst;
        return 0;
    case AV_OPT_TYPE_BOOL:
    case AV_OPT_TYPE_INT:
        *intnum = *(int *)dst;
        return 0;
    case AV_OPT_TYPE_CHANNEL_LAYOUT:
    case AV_OPT_TYPE_DURATION:
    case AV_OPT_TYPE_INT64:
    case AV_OPT_TYPE_UINT64:
        *intnum = *(int64_t *)dst;
        return 0;
    case AV_OPT_TYPE_FLOAT:
        *num = *(float *)dst;
        return 0;
    case AV_OPT_TYPE_DOUBLE:
        *num = *(double *)dst;
        return 0;
    case AV_OPT_TYPE_RATIONAL:
        *intnum = ((AVRational *)dst)->num;
        *den    = ((AVRational *)dst)->den;
        return 0;
    case AV_OPT_TYPE_CONST:
        *num = o->default_val.dbl;
        return 0;
    }
    return AVERROR(EINVAL);
}

#define DEFAULT_NUMVAL(opt) ((opt->type == AV_OPT_TYPE_INT64 || \
                              opt->type == AV_OPT_TYPE_UINT64 || \
                              opt->type == AV_OPT_TYPE_CONST || \
                              opt->type == AV_OPT_TYPE_FLAGS || \
                              opt->type == AV_OPT_TYPE_INT)     \
                             ? opt->default_val.i64             \
                             : opt->default_val.dbl)
static int set_string_number(void *obj, void *target_obj, const AVOption *o, const char *val, void *dst)
{
    int ret = 0;
    int num, den;
    char c;

    if (sscanf(val, "%d%*1[:/]%d%c", &num, &den, &c) == 2) {
        if ((ret = write_number(obj, o, dst, 1, den, num)) >= 0)
            return ret;
        ret = 0;
    }
	// 下面支持表达式的处理逻辑
    for (;;) {
        int i = 0;
        char buf[256];
        int cmd = 0;
        double d;
        int64_t intnum = 1;

        if (o->type == AV_OPT_TYPE_FLAGS) {
            if (*val == '+' || *val == '-')
                cmd = *(val++);
            for (; i < sizeof(buf) - 1 && val[i] && val[i] != '+' && val[i] != '-'; i++)
                buf[i] = val[i];
            buf[i] = 0;
        }

        {
            const AVOption *o_named = av_opt_find(target_obj, i ? buf : val, o->unit, 0, 0);
            int res;
            int ci = 0;
            double const_values[64];
            const char * const_names[64];
            if (o_named && o_named->type == AV_OPT_TYPE_CONST)
                d = DEFAULT_NUMVAL(o_named);
            else {
                if (o->unit) {
                    for (o_named = NULL; o_named = av_opt_next(target_obj, o_named); ) {
                        if (o_named->type == AV_OPT_TYPE_CONST &&
                            o_named->unit &&
                            !strcmp(o_named->unit, o->unit)) {
                            if (ci + 6 >= FF_ARRAY_ELEMS(const_values)) {
                                av_log(obj, AV_LOG_ERROR, "const_values array too small for %s\n", o->unit);
                                return AVERROR_PATCHWELCOME;
                            }
                            const_names [ci  ] = o_named->name;
                            const_values[ci++] = DEFAULT_NUMVAL(o_named);
                        }
                    }
                }
                const_names [ci  ] = "default";
                const_values[ci++] = DEFAULT_NUMVAL(o);
                const_names [ci  ] = "max";
                const_values[ci++] = o->max;
                const_names [ci  ] = "min";
                const_values[ci++] = o->min;
                const_names [ci  ] = "none";
                const_values[ci++] = 0;
                const_names [ci  ] = "all";
                const_values[ci++] = ~0;
                const_names [ci] = NULL;
                const_values[ci] = 0;

                res = av_expr_parse_and_eval(&d, i ? buf : val, const_names,
                                            const_values, NULL, NULL, NULL, NULL, NULL, 0, obj);
                if (res < 0) {
                    av_log(obj, AV_LOG_ERROR, "Unable to parse option value \"%s\"\n", val);
                    return res;
                }
            }
        }
        if (o->type == AV_OPT_TYPE_FLAGS) {
            read_number(o, dst, NULL, NULL, &intnum);
            if (cmd == '+')
                d = intnum | (int64_t)d;
            else if (cmd == '-')
                d = intnum &~(int64_t)d;
        }

        if ((ret = write_number(obj, o, dst, d, 1, 1)) < 0)
            return ret;
        val += i;
        if (!i || !*val)
            return 0;
    }
}

从上面一部分代码可以看出av_opt_set函数仅仅是根据类型进行数据区域赋值。下面的函数是针对特定类型的赋值:

// int
int av_opt_set_int(void *obj, const char *name, int64_t val, int search_flags)
{
    return set_number(obj, name, 1, 1, val, search_flags);
}
// double
int av_opt_set_double(void *obj, const char *name, double val, int search_flags)
{
    return set_number(obj, name, val, 1, 1, search_flags);
}
// AVRational
int av_opt_set_q(void *obj, const char *name, AVRational val, int search_flags)
{
    return set_number(obj, name, val.num, val.den, 1, search_flags);
}
// binary
int av_opt_set_bin(void *obj, const char *name, const uint8_t *val, int len, int search_flags)
{
    void *target_obj;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
    uint8_t *ptr;
    uint8_t **dst;
    int *lendst;

    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;

    if (o->type != AV_OPT_TYPE_BINARY || o->flags & AV_OPT_FLAG_READONLY)
        return AVERROR(EINVAL);

    ptr = len ? av_malloc(len) : NULL;
    if (len && !ptr)
        return AVERROR(ENOMEM);

    dst    = (uint8_t **)(((uint8_t *)target_obj) + o->offset);
    lendst = (int *)(dst + 1);

    av_free(*dst);
    *dst    = ptr;
    *lendst = len;
    if (len)
        memcpy(ptr, val, len);

    return 0;
}
// image_size,字符格式"1024x768"
int av_opt_set_image_size(void *obj, const char *name, int w, int h, int search_flags)
{
    void *target_obj;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);

    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;
    if (o->type != AV_OPT_TYPE_IMAGE_SIZE) {
        av_log(obj, AV_LOG_ERROR,
               "The value set by option '%s' is not an image size.\n", o->name);
        return AVERROR(EINVAL);
    }
    if (w<0 || h<0) {
        av_log(obj, AV_LOG_ERROR,
               "Invalid negative size value %dx%d for size '%s'\n", w, h, o->name);
        return AVERROR(EINVAL);
    }
    *(int *)(((uint8_t *)target_obj)             + o->offset) = w;
    *(int *)(((uint8_t *)target_obj+sizeof(int)) + o->offset) = h;
    return 0;
}
// 视频码率
int av_opt_set_video_rate(void *obj, const char *name, AVRational val, int search_flags)
{
    void *target_obj;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);

    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;
    if (o->type != AV_OPT_TYPE_VIDEO_RATE) {
        av_log(obj, AV_LOG_ERROR,
               "The value set by option '%s' is not a video rate.\n", o->name);
        return AVERROR(EINVAL);
    }
    if (val.num <= 0 || val.den <= 0)
        return AVERROR(EINVAL);
    return set_number(obj, name, val.num, val.den, 1, search_flags);
}
// 图像采样格式
int av_opt_set_pixel_fmt(void *obj, const char *name, enum AVPixelFormat fmt, int search_flags)
{
    return set_format(obj, name, fmt, search_flags, AV_OPT_TYPE_PIXEL_FMT, "pixel", AV_PIX_FMT_NB);
}
// 音频采样格式
int av_opt_set_sample_fmt(void *obj, const char *name, enum AVSampleFormat fmt, int search_flags)
{
    return set_format(obj, name, fmt, search_flags, AV_OPT_TYPE_SAMPLE_FMT, "sample", AV_SAMPLE_FMT_NB);
}
// channel_layout
int av_opt_set_channel_layout(void *obj, const char *name, int64_t cl, int search_flags)
{
    void *target_obj;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);

    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;
    if (o->type != AV_OPT_TYPE_CHANNEL_LAYOUT) {
        av_log(obj, AV_LOG_ERROR,
               "The value set by option '%s' is not a channel layout.\n", o->name);
        return AVERROR(EINVAL);
    }
    *(int64_t *)(((uint8_t *)target_obj) + o->offset) = cl;
    return 0;
}
// 设置为AVDictionary
int av_opt_set_dict_val(void *obj, const char *name, const AVDictionary *val,
                        int search_flags)
{
    void *target_obj;
    AVDictionary **dst;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);

    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;
    if (o->flags & AV_OPT_FLAG_READONLY)
        return AVERROR(EINVAL);

    dst = (AVDictionary **)(((uint8_t *)target_obj) + o->offset);
    av_dict_free(dst);
    av_dict_copy(dst, val, 0);

    return 0;
}

属性获取

属性获取和属性设置是对应的,对应函数一般是成对存在,比如av_opt_get/av_opt_set。下面先查看av_opt_get代码:

int av_opt_get(void *obj, const char *name, int search_flags, uint8_t **out_val)
{
    void *dst, *target_obj;
	// 通过name查找对应的AVOption
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
    uint8_t *bin, buf[128];
    int len, i, ret;
    int64_t i64;

    if (!o || !target_obj || (o->offset<=0 && o->type != AV_OPT_TYPE_CONST))
        return AVERROR_OPTION_NOT_FOUND;

    dst = (uint8_t *)target_obj + o->offset;

    buf[0] = 0;
    switch (o->type) { // 根据实际类型返回参数
	// 对于整型参数,可以直接通过snprintf格式化之
    case AV_OPT_TYPE_BOOL:
        ret = snprintf(buf, sizeof(buf), "%s", (char *)av_x_if_null(get_bool_name(*(int *)dst), "invalid"));
        break;
    case AV_OPT_TYPE_FLAGS:
        ret = snprintf(buf, sizeof(buf), "0x%08X", *(int *)dst);
        break;
    case AV_OPT_TYPE_INT:
        ret = snprintf(buf, sizeof(buf), "%d", *(int *)dst);
        break;
    case AV_OPT_TYPE_INT64:
        ret = snprintf(buf, sizeof(buf), "%"PRId64, *(int64_t *)dst);
        break;
    case AV_OPT_TYPE_UINT64:
        ret = snprintf(buf, sizeof(buf), "%"PRIu64, *(uint64_t *)dst);
        break;
    case AV_OPT_TYPE_FLOAT:
        ret = snprintf(buf, sizeof(buf), "%f", *(float *)dst);
        break;
    case AV_OPT_TYPE_DOUBLE:
        ret = snprintf(buf, sizeof(buf), "%f", *(double *)dst);
        break;
    case AV_OPT_TYPE_VIDEO_RATE:
    case AV_OPT_TYPE_RATIONAL:
        ret = snprintf(buf, sizeof(buf), "%d/%d", ((AVRational *)dst)->num, ((AVRational *)dst)->den);
        break;
    case AV_OPT_TYPE_CONST:
        ret = snprintf(buf, sizeof(buf), "%f", o->default_val.dbl);
        break;
    case AV_OPT_TYPE_STRING: // 字符串可以直接复制
        if (*(uint8_t **)dst) {
            *out_val = av_strdup(*(uint8_t **)dst);
        } else if (search_flags & AV_OPT_ALLOW_NULL) {
            *out_val = NULL;
            return 0;
        } else {
            *out_val = av_strdup("");
        }
        return *out_val ? 0 : AVERROR(ENOMEM);
    case AV_OPT_TYPE_BINARY: // 二进制文件需要特殊处理
        if (!*(uint8_t **)dst && (search_flags & AV_OPT_ALLOW_NULL)) {
            *out_val = NULL;
            return 0;
        }
        len = *(int *)(((uint8_t *)dst) + sizeof(uint8_t *));
        if ((uint64_t)len * 2 + 1 > INT_MAX)
            return AVERROR(EINVAL);
        if (!(*out_val = av_malloc(len * 2 + 1)))
            return AVERROR(ENOMEM);
        if (!len) {
            *out_val[0] = '\0';
            return 0;
        }
        bin = *(uint8_t **)dst;
        for (i = 0; i < len; i++)
            snprintf(*out_val + i * 2, 3, "%02X", bin[i]);
        return 0;
    case AV_OPT_TYPE_IMAGE_SIZE:
        ret = snprintf(buf, sizeof(buf), "%dx%d", ((int *)dst)[0], ((int *)dst)[1]);
        break;
    case AV_OPT_TYPE_PIXEL_FMT:
        ret = snprintf(buf, sizeof(buf), "%s", (char *)av_x_if_null(av_get_pix_fmt_name(*(enum AVPixelFormat *)dst), "none"));
        break;
    case AV_OPT_TYPE_SAMPLE_FMT:
        ret = snprintf(buf, sizeof(buf), "%s", (char *)av_x_if_null(av_get_sample_fmt_name(*(enum AVSampleFormat *)dst), "none"));
        break;
    case AV_OPT_TYPE_DURATION:
        i64 = *(int64_t *)dst;
        format_duration(buf, sizeof(buf), i64);
        ret = strlen(buf); // no overflow possible, checked by an assert
        break;
    case AV_OPT_TYPE_COLOR:
        ret = snprintf(buf, sizeof(buf), "0x%02x%02x%02x%02x",
                       (int)((uint8_t *)dst)[0], (int)((uint8_t *)dst)[1],
                       (int)((uint8_t *)dst)[2], (int)((uint8_t *)dst)[3]);
        break;
    case AV_OPT_TYPE_CHANNEL_LAYOUT:
        i64 = *(int64_t *)dst;
        ret = snprintf(buf, sizeof(buf), "0x%"PRIx64, i64);
        break;
    default:
        return AVERROR(EINVAL);
    }

    if (ret >= sizeof(buf))
        return AVERROR(EINVAL);
    *out_val = av_strdup(buf);
    return *out_val ? 0 : AVERROR(ENOMEM);
}

上面只有一个函数没有实现format_duration,其他代码如下:

static void format_duration(char *buf, size_t size, int64_t d)
{
    char *e;
	// 主要是为了时长的特殊的格式化输出
    av_assert0(size >= 25);
    if (d < 0 && d != INT64_MIN) {
        *(buf++) = '-';
        size--;
        d = -d;
    }
    if (d == INT64_MAX)
        snprintf(buf, size, "INT64_MAX");
    else if (d == INT64_MIN)
        snprintf(buf, size, "INT64_MIN");
    else if (d > (int64_t)3600*1000000)
        snprintf(buf, size, "%"PRId64":%02d:%02d.%06d", d / 3600000000,
                 (int)((d / 60000000) % 60),
                 (int)((d / 1000000) % 60),
                 (int)(d % 1000000));
    else if (d > 60*1000000)
        snprintf(buf, size, "%d:%02d.%06d",
                 (int)(d / 60000000),
                 (int)((d / 1000000) % 60),
                 (int)(d % 1000000));
    else
        snprintf(buf, size, "%d.%06d",
                 (int)(d / 1000000),
                 (int)(d % 1000000));
    e = buf + strlen(buf);
    while (e > buf && e[-1] == '0')
        *(--e) = 0;
    if (e > buf && e[-1] == '.')
        *(--e) = 0;
}

接下来是对应的不同类型的AVOption的获取,其基本逻辑是对av_opt_get的拆分,代码如下:

// 查找数字格式
static int get_number(void *obj, const char *name, const AVOption **o_out, double *num, int *den, int64_t *intnum,
                      int search_flags)
{
    void *dst, *target_obj;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
    if (!o || !target_obj)
        goto error;

    dst = ((uint8_t *)target_obj) + o->offset;

    if (o_out) *o_out= o;

    return read_number(o, dst, num, den, intnum);

error:
    *den    =
    *intnum = 0;
    return -1;
}

int av_opt_get_int(void *obj, const char *name, int search_flags, int64_t *out_val)
{
    int64_t intnum = 1;
    double num = 1;
    int ret, den = 1;

    if ((ret = get_number(obj, name, NULL, &num, &den, &intnum, search_flags)) < 0)
        return ret;
    *out_val = num * intnum / den;
    return 0;
}

int av_opt_get_double(void *obj, const char *name, int search_flags, double *out_val)
{
    int64_t intnum = 1;
    double num = 1;
    int ret, den = 1;

    if ((ret = get_number(obj, name, NULL, &num, &den, &intnum, search_flags)) < 0)
        return ret;
    *out_val = num * intnum / den;
    return 0;
}

int av_opt_get_q(void *obj, const char *name, int search_flags, AVRational *out_val)
{
    int64_t intnum = 1;
    double num = 1;
    int ret, den = 1;

    if ((ret = get_number(obj, name, NULL, &num, &den, &intnum, search_flags)) < 0)
        return ret;

    if (num == 1.0 && (int)intnum == intnum)
        *out_val = (AVRational){intnum, den};
    else
        *out_val = av_d2q(num*intnum/den, 1<<24);
    return 0;
}

int av_opt_get_image_size(void *obj, const char *name, int search_flags, int *w_out, int *h_out)
{
    void *dst, *target_obj;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;
    if (o->type != AV_OPT_TYPE_IMAGE_SIZE) {
        av_log(obj, AV_LOG_ERROR,
               "The value for option '%s' is not an image size.\n", name);
        return AVERROR(EINVAL);
    }

    dst = ((uint8_t*)target_obj) + o->offset;
    if (w_out) *w_out = *(int *)dst;
    if (h_out) *h_out = *((int *)dst+1);
    return 0;
}

int av_opt_get_video_rate(void *obj, const char *name, int search_flags, AVRational *out_val)
{
    int64_t intnum = 1;
    double     num = 1;
    int   ret, den = 1;

    if ((ret = get_number(obj, name, NULL, &num, &den, &intnum, search_flags)) < 0)
        return ret;

    if (num == 1.0 && (int)intnum == intnum)
        *out_val = (AVRational) { intnum, den };
    else
        *out_val = av_d2q(num * intnum / den, 1 << 24);
    return 0;
}

static int get_format(void *obj, const char *name, int search_flags, int *out_fmt,
                      enum AVOptionType type, const char *desc)
{
    void *dst, *target_obj;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;
    if (o->type != type) {
        av_log(obj, AV_LOG_ERROR,
               "The value for option '%s' is not a %s format.\n", desc, name);
        return AVERROR(EINVAL);
    }

    dst = ((uint8_t*)target_obj) + o->offset;
    *out_fmt = *(int *)dst;
    return 0;
}

int av_opt_get_pixel_fmt(void *obj, const char *name, int search_flags, enum AVPixelFormat *out_fmt)
{
    return get_format(obj, name, search_flags, out_fmt, AV_OPT_TYPE_PIXEL_FMT, "pixel");
}

int av_opt_get_sample_fmt(void *obj, const char *name, int search_flags, enum AVSampleFormat *out_fmt)
{
    return get_format(obj, name, search_flags, out_fmt, AV_OPT_TYPE_SAMPLE_FMT, "sample");
}

int av_opt_get_channel_layout(void *obj, const char *name, int search_flags, int64_t *cl)
{
    void *dst, *target_obj;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);
    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;
    if (o->type != AV_OPT_TYPE_CHANNEL_LAYOUT) {
        av_log(obj, AV_LOG_ERROR,
               "The value for option '%s' is not a channel layout.\n", name);
        return AVERROR(EINVAL);
    }

    dst = ((uint8_t*)target_obj) + o->offset;
    *cl = *(int64_t *)dst;
    return 0;
}

int av_opt_get_dict_val(void *obj, const char *name, int search_flags, AVDictionary **out_val)
{
    void *target_obj;
    AVDictionary *src;
    const AVOption *o = av_opt_find2(obj, name, NULL, 0, search_flags, &target_obj);

    if (!o || !target_obj)
        return AVERROR_OPTION_NOT_FOUND;
    if (o->type != AV_OPT_TYPE_DICT)
        return AVERROR(EINVAL);

    src = *(AVDictionary **)(((uint8_t *)target_obj) + o->offset);
    av_dict_copy(out_val, src, 0);

    return 0;
}

这几个函数逻辑个对应的av_opt_set_xxx类似,从代码可以直接看出来。

// 该函数返回AVOption的地址
void *av_opt_ptr(const AVClass *class, void *obj, const char *name)
{
    const AVOption *opt= av_opt_find2(&class, name, NULL, 0, AV_OPT_SEARCH_FAKE_OBJ, NULL);
    if(!opt)
        return NULL;
    return (uint8_t*)obj + opt->offset;
}

杂项

此类函数可能就是部分工具函数。我们逐个分析下:

int av_opt_flag_is_set(void *obj, const char *field_name, const char *flag_name)
{
	// 依次查找各部分,然后确定是否已经设置该flag
    const AVOption *field = av_opt_find(obj, field_name, NULL, 0, 0);
    const AVOption *flag  = av_opt_find(obj, flag_name,
                                        field ? field->unit : NULL, 0, 0);
    int64_t res;

    if (!field || !flag || flag->type != AV_OPT_TYPE_CONST ||
        av_opt_get_int(obj, field_name, 0, &res) < 0)
        return 0;
    return res & flag->default_val.i64;
}

下面几个函数是使用宏统一定义的:

int av_opt_eval_flags (void *obj, const AVOption *o, const char *val, int        *flags_out);
int av_opt_eval_int   (void *obj, const AVOption *o, const char *val, int        *int_out);
int av_opt_eval_int64 (void *obj, const AVOption *o, const char *val, int64_t    *int64_out);
int av_opt_eval_float (void *obj, const AVOption *o, const char *val, float      *float_out);
int av_opt_eval_double(void *obj, const AVOption *o, const char *val, double     *double_out);
int av_opt_eval_q     (void *obj, const AVOption *o, const char *val, AVRational *q_out);

#define OPT_EVAL_NUMBER(name, opttype, vartype)                         \
int av_opt_eval_ ## name(void *obj, const AVOption *o,                  \
                         const char *val, vartype *name ## _out)        \
{                                                                       \
    if (!o || o->type != opttype || o->flags & AV_OPT_FLAG_READONLY)    \
        return AVERROR(EINVAL);                                         \
    return set_string_number(obj, obj, o, val, name ## _out);           \
}

OPT_EVAL_NUMBER(flags,  AV_OPT_TYPE_FLAGS,    int)
OPT_EVAL_NUMBER(int,    AV_OPT_TYPE_INT,      int)
OPT_EVAL_NUMBER(int64,  AV_OPT_TYPE_INT64,    int64_t)
OPT_EVAL_NUMBER(float,  AV_OPT_TYPE_FLOAT,    float)
OPT_EVAL_NUMBER(double, AV_OPT_TYPE_DOUBLE,   double)
OPT_EVAL_NUMBER(q,      AV_OPT_TYPE_RATIONAL, AVRational)
// 最后直接调用set_string_number函数,这个函数在上文中有提及

av_opt_copy函数完成AVOption的复制工作。下面看看代码,原则上应该跟初始化函数类似。

int av_opt_copy(void *dst, const void *src)
{
    const AVOption *o = NULL;
    const AVClass *c;
    int ret = 0;

    if (!src)
        return AVERROR(EINVAL);

    c = *(AVClass **)src;
    if (!c || c != *(AVClass **)dst)
        return AVERROR(EINVAL);

	// 遍历+按照类型复制
    while ((o = av_opt_next(src, o))) {
        void *field_dst = (uint8_t *)dst + o->offset;
        void *field_src = (uint8_t *)src + o->offset;
        uint8_t **field_dst8 = (uint8_t **)field_dst;
        uint8_t **field_src8 = (uint8_t **)field_src;

        if (o->type == AV_OPT_TYPE_STRING) {
            if (*field_dst8 != *field_src8)
                av_freep(field_dst8);
            *field_dst8 = av_strdup(*field_src8);
            if (*field_src8 && !*field_dst8)
                ret = AVERROR(ENOMEM);
        } else if (o->type == AV_OPT_TYPE_BINARY) {
            int len = *(int *)(field_src8 + 1);
            if (*field_dst8 != *field_src8)
                av_freep(field_dst8);
            *field_dst8 = av_memdup(*field_src8, len);
            if (len && !*field_dst8) {
                ret = AVERROR(ENOMEM);
                len = 0;
            }
            *(int *)(field_dst8 + 1) = len;
        } else if (o->type == AV_OPT_TYPE_CONST) {
            // do nothing
        } else if (o->type == AV_OPT_TYPE_DICT) {
            AVDictionary **sdict = (AVDictionary **) field_src;
            AVDictionary **ddict = (AVDictionary **) field_dst;
            if (*sdict != *ddict)
                av_dict_free(ddict);
            *ddict = NULL;
            av_dict_copy(ddict, *sdict, 0);
            if (av_dict_count(*sdict) != av_dict_count(*ddict))
                ret = AVERROR(ENOMEM);
        } else {
            int size = opt_size(o->type); // 返回特定类型的长度
            if (size < 0)
                ret = size;
            else
                memcpy(field_dst, field_src, size);
        }
    }
    return ret;
}
static int opt_size(enum AVOptionType type)
{
    switch(type) {
    case AV_OPT_TYPE_BOOL:
    case AV_OPT_TYPE_INT:
    case AV_OPT_TYPE_FLAGS:
        return sizeof(int);
    case AV_OPT_TYPE_DURATION:
    case AV_OPT_TYPE_CHANNEL_LAYOUT:
    case AV_OPT_TYPE_INT64:
    case AV_OPT_TYPE_UINT64:
        return sizeof(int64_t);
    case AV_OPT_TYPE_DOUBLE:
        return sizeof(double);
    case AV_OPT_TYPE_FLOAT:
        return sizeof(float);
    case AV_OPT_TYPE_STRING:
        return sizeof(uint8_t*);
    case AV_OPT_TYPE_VIDEO_RATE:
    case AV_OPT_TYPE_RATIONAL:
        return sizeof(AVRational);
    case AV_OPT_TYPE_BINARY:
        return sizeof(uint8_t*) + sizeof(int);
    case AV_OPT_TYPE_IMAGE_SIZE:
        return sizeof(int[2]);
    case AV_OPT_TYPE_PIXEL_FMT:
        return sizeof(enum AVPixelFormat);
    case AV_OPT_TYPE_SAMPLE_FMT:
        return sizeof(enum AVSampleFormat);
    case AV_OPT_TYPE_COLOR:
        return 4;
    }
    return AVERROR(EINVAL);
}

最后三个值是判断是否是默认值、以及序列化输出。

int av_opt_is_set_to_default(void *obj, const AVOption *o)
{
    int64_t i64;
    double d, d2;
    float f;
    AVRational q;
    int ret, w, h;
    char *str;
    void *dst;

    if (!o || !obj)
        return AVERROR(EINVAL);

    dst = ((uint8_t*)obj) + o->offset;
	// 基本思路是根据类型,判断当前属性值和默认值是否一致。
    switch (o->type) {
    case AV_OPT_TYPE_CONST:
        return 1;
    case AV_OPT_TYPE_BOOL:
    case AV_OPT_TYPE_FLAGS:
    case AV_OPT_TYPE_PIXEL_FMT:
    case AV_OPT_TYPE_SAMPLE_FMT:
    case AV_OPT_TYPE_INT:
    case AV_OPT_TYPE_CHANNEL_LAYOUT:
    case AV_OPT_TYPE_DURATION:
    case AV_OPT_TYPE_INT64:
    case AV_OPT_TYPE_UINT64:
        read_number(o, dst, NULL, NULL, &i64); // 数值类型直接读取
        return o->default_val.i64 == i64;// 判断默认值是否一致
    case AV_OPT_TYPE_STRING:
        str = *(char **)dst;
        if (str == o->default_val.str) //2 NULLs
            return 1;
        if (!str || !o->default_val.str) //1 NULL
            return 0;
        return !strcmp(str, o->default_val.str);
    case AV_OPT_TYPE_DOUBLE:
        read_number(o, dst, &d, NULL, NULL);
        return o->default_val.dbl == d;
    case AV_OPT_TYPE_FLOAT:
        read_number(o, dst, &d, NULL, NULL);
        f = o->default_val.dbl;
        d2 = f;
        return d2 == d;
    case AV_OPT_TYPE_RATIONAL:
        q = av_d2q(o->default_val.dbl, INT_MAX);
        return !av_cmp_q(*(AVRational*)dst, q);
    case AV_OPT_TYPE_BINARY: {
        struct {
            uint8_t *data;
            int size;
        } tmp = {0};
        int opt_size = *(int *)((void **)dst + 1);
        void *opt_ptr = *(void **)dst;
        if (!opt_size && (!o->default_val.str || !strlen(o->default_val.str)))
            return 1;
        if (!opt_size ||  !o->default_val.str || !strlen(o->default_val.str ))
            return 0;
        if (opt_size != strlen(o->default_val.str) / 2)
            return 0;
        ret = set_string_binary(NULL, NULL, o->default_val.str, &tmp.data);
        if (!ret)
            ret = !memcmp(opt_ptr, tmp.data, tmp.size);
        av_free(tmp.data);
        return ret;
    }
    case AV_OPT_TYPE_DICT:
        /* Binary and dict have not default support yet. Any pointer is not default. */
        return !!(*(void **)dst);
    case AV_OPT_TYPE_IMAGE_SIZE:
        if (!o->default_val.str || !strcmp(o->default_val.str, "none"))
            w = h = 0;
        else if ((ret = av_parse_video_size(&w, &h, o->default_val.str)) < 0)
            return ret;
        return (w == *(int *)dst) && (h == *((int *)dst+1));
    case AV_OPT_TYPE_VIDEO_RATE:
        q = (AVRational){0, 0};
        if (o->default_val.str) {
            if ((ret = av_parse_video_rate(&q, o->default_val.str)) < 0)
                return ret;
        }
        return !av_cmp_q(*(AVRational*)dst, q);
    case AV_OPT_TYPE_COLOR: {
        uint8_t color[4] = {0, 0, 0, 0};
        if (o->default_val.str) {
            if ((ret = av_parse_color(color, o->default_val.str, -1, NULL)) < 0)
                return ret;
        }
        return !memcmp(color, dst, sizeof(color));
    }
    default:
        av_log(obj, AV_LOG_WARNING, "Not supported option type: %d, option name: %s\n", o->type, o->name);
        break;
    }
    return AVERROR_PATCHWELCOME;
}

int av_opt_is_set_to_default_by_name(void *obj, const char *name, int search_flags)
{
    const AVOption *o;
    void *target;
    if (!obj)
        return AVERROR(EINVAL);
	// 先查找,然后对比是否是默认值
    o = av_opt_find2(obj, name, NULL, 0, search_flags, &target);
    if (!o)
        return AVERROR_OPTION_NOT_FOUND;
    return av_opt_is_set_to_default(target, o);
}

int av_opt_serialize(void *obj, int opt_flags, int flags, char **buffer,
                     const char key_val_sep, const char pairs_sep)
{
    const AVOption *o = NULL;
    uint8_t *buf;
    AVBPrint bprint;
    int ret, cnt = 0;
    const char special_chars[] = {pairs_sep, key_val_sep, '\0'};

    if (pairs_sep == '\0' || key_val_sep == '\0' || pairs_sep == key_val_sep ||
        pairs_sep == '\\' || key_val_sep == '\\') {
        av_log(obj, AV_LOG_ERROR, "Invalid separator(s) found.");
        return AVERROR(EINVAL);
    }

    if (!obj || !buffer)
        return AVERROR(EINVAL);

    *buffer = NULL;
    av_bprint_init(&bprint, 64, AV_BPRINT_SIZE_UNLIMITED);
	
	// 序列化的基本思路是遍历 
    while (o = av_opt_next(obj, o)) {
        if (o->type == AV_OPT_TYPE_CONST)
            continue;
        if ((flags & AV_OPT_SERIALIZE_OPT_FLAGS_EXACT) && o->flags != opt_flags)
            continue;
        else if (((o->flags & opt_flags) != opt_flags))
            continue;
        if (flags & AV_OPT_SERIALIZE_SKIP_DEFAULTS && av_opt_is_set_to_default(obj, o) > 0)
            continue;
        if ((ret = av_opt_get(obj, o->name, 0, &buf)) < 0) {
            av_bprint_finalize(&bprint, NULL);
            return ret;
        }
        if (buf) {
            if (cnt++)
                av_bprint_append_data(&bprint, &pairs_sep, 1);
            av_bprint_escape(&bprint, o->name, special_chars, AV_ESCAPE_MODE_BACKSLASH, 0);
            av_bprint_append_data(&bprint, &key_val_sep, 1);
            av_bprint_escape(&bprint, buf, special_chars, AV_ESCAPE_MODE_BACKSLASH, 0);
            av_freep(&buf);
        }
    }
    av_bprint_finalize(&bprint, buffer);
    return 0;
}

2 AVOption的用法

只要是支持AVOption的对象都可以使用上面提供的接口。支持AVOption的结构体或对象的第一个数据域必须是一个指向AVClass对象的指针,其中该AVClass的对象的option域必须设置为一个以NULL结束的AVOption数组。

通常可以使用av_opt_next()编译一个对象的全部选项,也可以使用av_opt_find()通过指定选项名称查找特定选项。
也可以通过av_opt_child_class_next()av_opt_child_next()两种不同的遍历子对象属性的方式。

对于读取和设置AVOption可以通过av_opt_set()av_opt_get,也可以通过更详细的区分的av_opt_set_xxx和av_opt_get_xxx。

3 如何实现支持AVOption的对象

按照之前的AVOption的接口要求,下面给出一个简单的支持AVOption属性的结构体,首先给出一个示例结构体的定义,代码如下:

struct zyj_object {
    const AVClass *class;
    int      int_opt;
    char    *str_opt;
    uint8_t *bin_opt;
    int      bin_len;
	struct child_object * child;
};
static const AVOption zyj_object_options[] = {
  { "example_int", "This is a test option of int type.", offsetof(zyj_object, int_opt),
    AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX },
  { "example_str", "This is a test option of string type.", offsetof(zyj_object, str_opt),
    AV_OPT_TYPE_STRING },
  { "example_bin", "This is a test option of binary type.", offsetof(zyj_object, bin_opt),
    AV_OPT_TYPE_BINARY },
  { NULL },
};
static const AVClass zyj_object_class = {
    .class_name = "zyj class",
    .item_name  = av_default_item_name,
    .option     = zyj_object_options,
    .version    = LIBAVUTIL_VERSION_INT,
};

这样一个结构体可以使用av_opt_set_defaults()将AVOption所有成员初始化为默认值。下面我们看一下如何创建和销毁zyj_object,代码如下:

zyj_object *alloc_zyj_object(void)
{
    zyj_object *ret = av_mallocz(sizeof(*ret));
    ret->class = &zyj_object_class;
    av_opt_set_defaults(ret);
    return ret;
}
void free_zyj_object(zyj_object **foo)
{
    av_opt_free(*foo);
    av_freep(foo);
}

特例一:嵌套AVOption实现

像AVCodecContext一样,其中包含了libavcodec导出的通用选项,同时它的priv_data成员包含了codec相关的选项。在这种情况下,需要设置父结构体来导出子选项。也就是实现在父结构体的AVClass中实现AVClass.child_next()和AVClass.child_class_next()。这里给出一个简单的嵌套示例:

typedef struct child_object {
    AVClass *class;
    int flags_opt;
} child_struct;
static const AVOption child_opts[] = {
    { "test_flags", "This is a test option of flags type.",
      offsetof(child_struct, flags_opt), AV_OPT_TYPE_FLAGS, { .i64 = 0 }, INT_MIN, INT_MAX },
    { NULL },
};
static const AVClass child_class = {
    .class_name = "child class",
    .item_name  = av_default_item_name,
    .option     = child_opts,
    .version    = LIBAVUTIL_VERSION_INT,
};
void *child_next(void *obj, void *prev)
{
    zyj_object *t = obj;
    if (!prev && t->child_struct)
        return t->child_struct;
    return NULL
}
const AVClass child_class_next(const AVClass *prev)
{
    return prev ? NULL : &child_class;
}

// 这个是更新
static const AVClass zyj_object_class = {
    .class_name = "zyj class",
    .item_name  = av_default_item_name,
    .option     = zyj_object_options,
	.child_next = child_next,
	.child_class_next = child_class_next,
    .version    = LIBAVUTIL_VERSION_INT,
};

命名常量

AVOption支持命名常量。使用AV_OPT_TYPE_CONST可以声明常量,但是必须给定默认的初始值。示例代码如下:

{ "flag1", "This is a flag with value 16", 0, AV_OPT_TYPE_CONST, { .i64 = 16 }, 0, 0, "test_unit" },

4 小结

本文可能涉及比较多的源代码,是对libavutil/opt.c的整理及摘取,通过梳理相关实现代码,让我基本理解AVOption的实现逻辑,值得后续更深入的使用作为参考。


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本文作者:Tocy e-mail: zyvj@qq.com
版权所有@2015-2020,请勿用于商业用途,转载请注明原文地址。本人保留所有权利。
posted @ 2017-11-30 22:33  Tocy  阅读(1188)  评论(0编辑  收藏  举报