这个对不同编码还有不同含义的
对h.264，它的含义在这里：
https://trac.ffmpeg.org/wiki/Encode/H.264
对h.265
https://trac.ffmpeg.org/wiki/Encode/H.265
在这里可以看到
https://x265.readthedocs.io/en/stable/presets.html
文章目录
20230816
FFmpeg av_opt_set()函数使用方法详解
FFmpeg是一款强大的开源多媒体处理库，提供了丰富的API供开发者调用。在这篇文章中，我们将重点探讨其中的av_opt_set()函数，此函数是FFmpeg中的选项设置API。
1. av_opt_set()函数简介
在开始之前，首先理解av_opt_set()函数的基本概念和定义非常关键。av_opt_set()函数是FFmpeg库的一部分，它的主要作用是设置选项值。具体来说，这个函数可以设置对象（如编码器、解码器等）的各种属性。
int av_opt_set(void *obj, const char *name, const char *val, int search_flags);
参数解释：
obj: 需要设置选项的对象。name: 要设置的选项名称。val: 设置的选项值。search_flags: 搜索标志，通常为0。
返回值：
如果设置成功，函数返回0，否则返回负值。
2. av_opt_set()函数使用示例
下面的代码示例展示了如何使用av_opt_set()函数。
#include <libavutil/opt.h>
#include <libavcodec/avcodec.h>
// 创建一个编码器上下文
AVCodecContext *avctx = avcodec_alloc_context3(NULL);
// 使用av_opt_set()函数设置选项
av_opt_set(avctx, "b", "2.5M", 0);
在这个示例中，av_opt_set()被用来设置编码器上下文avctx的比特率为"2.5M"。
3. av_opt_set()函数错误处理
当调用av_opt_set()时，可能会遇到错误。以下代码片段演示了如何进行错误处理。
// 使用av_opt_set()函数设置选项
if (av_opt_set(avctx, "b", "2.5M", 0) < 0) {
fprintf(stderr, "Error setting option 'b'\n");
return -1;
}
在上述代码中，如果av_opt_set()返回负值，我们会输出错误信息并返回-1表示出错。
4. 参考文献
- FFmpeg官方文档：libavutil documentation
- StackOverflow讨论：“How to use av_opt_set() in FFmpeg?”
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