ijkplayer框架简析 -- avcodec_open2

avcodec_open2() 主要作用是初始化一个视音频编解码器的 AVCodecContext

avcodec_open2

int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options)
{
    int ret = 0;
    AVDictionary *tmp = NULL;
    const AVPixFmtDescriptor *pixdesc;

    if (avcodec_is_open(avctx))
        return 0;

    if ((!codec && !avctx->codec)) {
        av_log(avctx, AV_LOG_ERROR, "No codec provided to avcodec_open2()\n");
        return AVERROR(EINVAL);
    }
    if ((codec && avctx->codec && codec != avctx->codec)) {
        av_log(avctx, AV_LOG_ERROR, "This AVCodecContext was allocated for %s, "
                                    "but %s passed to avcodec_open2()\n", avctx->codec->name, codec->name);
        return AVERROR(EINVAL);
    }
    if (!codec)
        codec = avctx->codec;

    if (avctx->extradata_size < 0 || avctx->extradata_size >= FF_MAX_EXTRADATA_SIZE)
        return AVERROR(EINVAL);

    if (options)
        av_dict_copy(&tmp, *options, 0);

    ret = ff_lock_avcodec(avctx, codec);
    if (ret < 0)
        return ret;

    avctx->internal = av_mallocz(sizeof(AVCodecInternal));
    if (!avctx->internal) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    avctx->internal->pool = av_mallocz(sizeof(*avctx->internal->pool));
    if (!avctx->internal->pool) {
        ret = AVERROR(ENOMEM);
        goto free_and_end;
    }

    avctx->internal->to_free = av_frame_alloc();
    if (!avctx->internal->to_free) {
        ret = AVERROR(ENOMEM);
        goto free_and_end;
    }

    avctx->internal->compat_decode_frame = av_frame_alloc();
    if (!avctx->internal->compat_decode_frame) {
        ret = AVERROR(ENOMEM);
        goto free_and_end;
    }

    avctx->internal->buffer_frame = av_frame_alloc();
    if (!avctx->internal->buffer_frame) {
        ret = AVERROR(ENOMEM);
        goto free_and_end;
    }

    avctx->internal->buffer_pkt = av_packet_alloc();
    if (!avctx->internal->buffer_pkt) {
        ret = AVERROR(ENOMEM);
        goto free_and_end;
    }

    avctx->internal->ds.in_pkt = av_packet_alloc();
    if (!avctx->internal->ds.in_pkt) {
        ret = AVERROR(ENOMEM);
        goto free_and_end;
    }

    avctx->internal->last_pkt_props = av_packet_alloc();
    if (!avctx->internal->last_pkt_props) {
        ret = AVERROR(ENOMEM);
        goto free_and_end;
    }

    avctx->internal->skip_samples_multiplier = 1;

    if (codec->priv_data_size > 0) {
        if (!avctx->priv_data) {
            avctx->priv_data = av_mallocz(codec->priv_data_size);
            if (!avctx->priv_data) {
                ret = AVERROR(ENOMEM);
                goto end;
            }
            if (codec->priv_class) {
                *(const AVClass **)avctx->priv_data = codec->priv_class;
                av_opt_set_defaults(avctx->priv_data);
            }
        }
        if (codec->priv_class && (ret = av_opt_set_dict(avctx->priv_data, &tmp)) < 0)
            goto free_and_end;
    } else {
        avctx->priv_data = NULL;
    }
    // 将输入的 AVDictionary 形式的选项设置到 AVCodecContext
    if ((ret = av_opt_set_dict(avctx, &tmp)) < 0)
        goto free_and_end;

    if (avctx->codec_whitelist && av_match_list(codec->name, avctx->codec_whitelist, ',') <= 0) {
        av_log(avctx, AV_LOG_ERROR, "Codec (%s) not on whitelist \'%s\'\n", codec->name, avctx->codec_whitelist);
        ret = AVERROR(EINVAL);
        goto free_and_end;
    }

    // only call ff_set_dimensions() for non H.264/VP6F/DXV codecs so as not to overwrite previously setup dimensions
    if (!(avctx->coded_width && avctx->coded_height && avctx->width && avctx->height &&
          (avctx->codec_id == AV_CODEC_ID_H264 || avctx->codec_id == AV_CODEC_ID_VP6F || avctx->codec_id == AV_CODEC_ID_DXV))) {
    if (avctx->coded_width && avctx->coded_height)
        ret = ff_set_dimensions(avctx, avctx->coded_width, avctx->coded_height);
    else if (avctx->width && avctx->height)
        ret = ff_set_dimensions(avctx, avctx->width, avctx->height);
    if (ret < 0)
        goto free_and_end;
    }

    // 检查宽和高
    if ((avctx->coded_width || avctx->coded_height || avctx->width || avctx->height)
        && (  av_image_check_size2(avctx->coded_width, avctx->coded_height, avctx->max_pixels, AV_PIX_FMT_NONE, 0, avctx) < 0
           || av_image_check_size2(avctx->width,       avctx->height,       avctx->max_pixels, AV_PIX_FMT_NONE, 0, avctx) < 0)) {
        av_log(avctx, AV_LOG_WARNING, "Ignoring invalid width/height values\n");
        ff_set_dimensions(avctx, 0, 0);
    }

    // 检查宽高比
    if (avctx->width > 0 && avctx->height > 0) {
        if (av_image_check_sar(avctx->width, avctx->height,
                               avctx->sample_aspect_ratio) < 0) {
            av_log(avctx, AV_LOG_WARNING, "ignoring invalid SAR: %u/%u\n",
                   avctx->sample_aspect_ratio.num,
                   avctx->sample_aspect_ratio.den);
            avctx->sample_aspect_ratio = (AVRational){ 0, 1 };
        }
    }

    /* if the decoder init function was already called previously,
     * free the already allocated subtitle_header before overwriting it */
    if (av_codec_is_decoder(codec))
        av_freep(&avctx->subtitle_header);

    if (avctx->channels > FF_SANE_NB_CHANNELS) {
        ret = AVERROR(EINVAL);
        goto free_and_end;
    }

    avctx->codec = codec;
    if ((avctx->codec_type == AVMEDIA_TYPE_UNKNOWN || avctx->codec_type == codec->type) &&
        avctx->codec_id == AV_CODEC_ID_NONE) {
        avctx->codec_type = codec->type;
        avctx->codec_id   = codec->id;
    }
    if (avctx->codec_id != codec->id || (avctx->codec_type != codec->type
                                         && avctx->codec_type != AVMEDIA_TYPE_ATTACHMENT)) {
        av_log(avctx, AV_LOG_ERROR, "Codec type or id mismatches\n");
        ret = AVERROR(EINVAL);
        goto free_and_end;
    }
    avctx->frame_number = 0;
    avctx->codec_descriptor = avcodec_descriptor_get(avctx->codec_id);

    // 检查编码器是否出于“实验”阶段
    if ((avctx->codec->capabilities & AV_CODEC_CAP_EXPERIMENTAL) &&
        avctx->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL) {
        const char *codec_string = av_codec_is_encoder(codec) ? "encoder" : "decoder";
        AVCodec *codec2;
        av_log(avctx, AV_LOG_ERROR,
               "The %s '%s' is experimental but experimental codecs are not enabled, "
               "add '-strict %d' if you want to use it.\n",
               codec_string, codec->name, FF_COMPLIANCE_EXPERIMENTAL);
        codec2 = av_codec_is_encoder(codec) ? avcodec_find_encoder(codec->id) : avcodec_find_decoder(codec->id);
        if (!(codec2->capabilities & AV_CODEC_CAP_EXPERIMENTAL))
            av_log(avctx, AV_LOG_ERROR, "Alternatively use the non experimental %s '%s'.\n",
                codec_string, codec2->name);
        ret = AVERROR_EXPERIMENTAL;
        goto free_and_end;
    }

    if (avctx->codec_type == AVMEDIA_TYPE_AUDIO &&
        (!avctx->time_base.num || !avctx->time_base.den)) {
        avctx->time_base.num = 1;
        avctx->time_base.den = avctx->sample_rate;
    }

    if (!HAVE_THREADS)
        av_log(avctx, AV_LOG_WARNING, "Warning: not compiled with thread support, using thread emulation\n");

    if (CONFIG_FRAME_THREAD_ENCODER && av_codec_is_encoder(avctx->codec)) {
        ff_unlock_avcodec(codec); //we will instantiate a few encoders thus kick the counter to prevent false detection of a problem
        ret = ff_frame_thread_encoder_init(avctx, options ? *options : NULL);
        ff_lock_avcodec(avctx, codec);
        if (ret < 0)
            goto free_and_end;
    }

    if (HAVE_THREADS
        && !(avctx->internal->frame_thread_encoder && (avctx->active_thread_type&FF_THREAD_FRAME))) {
        ret = ff_thread_init(avctx);
        if (ret < 0) {
            goto free_and_end;
        }
    }
    if (!HAVE_THREADS && !(codec->capabilities & AV_CODEC_CAP_AUTO_THREADS))
        avctx->thread_count = 1;

    if (avctx->codec->max_lowres < avctx->lowres || avctx->lowres < 0) {
        av_log(avctx, AV_LOG_WARNING, "The maximum value for lowres supported by the decoder is %d\n",
               avctx->codec->max_lowres);
        avctx->lowres = avctx->codec->max_lowres;
    }

#if FF_API_VISMV
    if (avctx->debug_mv)
        av_log(avctx, AV_LOG_WARNING, "The 'vismv' option is deprecated, "
               "see the codecview filter instead.\n");
#endif

    // 检查输入参数是否符合【编码器】要求
    if (av_codec_is_encoder(avctx->codec)) {
        int i;
#if FF_API_CODED_FRAME
FF_DISABLE_DEPRECATION_WARNINGS
        avctx->coded_frame = av_frame_alloc();
        if (!avctx->coded_frame) {
            ret = AVERROR(ENOMEM);
            goto free_and_end;
        }
FF_ENABLE_DEPRECATION_WARNINGS
#endif

        if (avctx->time_base.num <= 0 || avctx->time_base.den <= 0) {
            av_log(avctx, AV_LOG_ERROR, "The encoder timebase is not set.\n");
            ret = AVERROR(EINVAL);
            goto free_and_end;
        }

        // 如果包含采样率参数（表明是音频），检查采样率是否符合要求
        if (avctx->codec->sample_fmts) {
            // 遍历编码器支持的所有采样率
            for (i = 0; avctx->codec->sample_fmts[i] != AV_SAMPLE_FMT_NONE; i++) {
                // 如果设置的采样率 == 编码器支持的采样率，跳出循环
                if (avctx->sample_fmt == avctx->codec->sample_fmts[i])
                    break;
                if (avctx->channels == 1 &&
                    av_get_planar_sample_fmt(avctx->sample_fmt) ==
                    av_get_planar_sample_fmt(avctx->codec->sample_fmts[i])) {
                    avctx->sample_fmt = avctx->codec->sample_fmts[i];
                    break;
                }
            }
            // 采样率取值是否正确
            if (avctx->codec->sample_fmts[i] == AV_SAMPLE_FMT_NONE) {
                char buf[128];
                snprintf(buf, sizeof(buf), "%d", avctx->sample_fmt);
                av_log(avctx, AV_LOG_ERROR, "Specified sample format %s is invalid or not supported\n",
                       (char *)av_x_if_null(av_get_sample_fmt_name(avctx->sample_fmt), buf));
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
        }
        // 像素格式
        if (avctx->codec->pix_fmts) {
            for (i = 0; avctx->codec->pix_fmts[i] != AV_PIX_FMT_NONE; i++)
                if (avctx->pix_fmt == avctx->codec->pix_fmts[i])
                    break;
            if (avctx->codec->pix_fmts[i] == AV_PIX_FMT_NONE
                && !((avctx->codec_id == AV_CODEC_ID_MJPEG || avctx->codec_id == AV_CODEC_ID_LJPEG)
                     && avctx->strict_std_compliance <= FF_COMPLIANCE_UNOFFICIAL)) {
                char buf[128];
                snprintf(buf, sizeof(buf), "%d", avctx->pix_fmt);
                av_log(avctx, AV_LOG_ERROR, "Specified pixel format %s is invalid or not supported\n",
                       (char *)av_x_if_null(av_get_pix_fmt_name(avctx->pix_fmt), buf));
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
            if (avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ420P ||
                avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ411P ||
                avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ422P ||
                avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ440P ||
                avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ444P)
                avctx->color_range = AVCOL_RANGE_JPEG;
        }
        // 采样率
        if (avctx->codec->supported_samplerates) {
            for (i = 0; avctx->codec->supported_samplerates[i] != 0; i++)
                if (avctx->sample_rate == avctx->codec->supported_samplerates[i])
                    break;
            if (avctx->codec->supported_samplerates[i] == 0) {
                av_log(avctx, AV_LOG_ERROR, "Specified sample rate %d is not supported\n",
                       avctx->sample_rate);
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
        }
        if (avctx->sample_rate < 0) {
            av_log(avctx, AV_LOG_ERROR, "Specified sample rate %d is not supported\n",
                    avctx->sample_rate);
            ret = AVERROR(EINVAL);
            goto free_and_end;
        }
        // 声道布局
        if (avctx->codec->channel_layouts) {
            if (!avctx->channel_layout) {
                av_log(avctx, AV_LOG_WARNING, "Channel layout not specified\n");
            } else {
                for (i = 0; avctx->codec->channel_layouts[i] != 0; i++)
                    if (avctx->channel_layout == avctx->codec->channel_layouts[i])
                        break;
                if (avctx->codec->channel_layouts[i] == 0) {
                    char buf[512];
                    av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);
                    av_log(avctx, AV_LOG_ERROR, "Specified channel layout '%s' is not supported\n", buf);
                    ret = AVERROR(EINVAL);
                    goto free_and_end;
                }
            }
        }
        // 声道
        if (avctx->channel_layout && avctx->channels) {
            int channels = av_get_channel_layout_nb_channels(avctx->channel_layout);
            if (channels != avctx->channels) {
                char buf[512];
                av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);
                av_log(avctx, AV_LOG_ERROR,
                       "Channel layout '%s' with %d channels does not match number of specified channels %d\n",
                       buf, channels, avctx->channels);
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
        } else if (avctx->channel_layout) {
            avctx->channels = av_get_channel_layout_nb_channels(avctx->channel_layout);
        }
        if (avctx->channels < 0) {
            av_log(avctx, AV_LOG_ERROR, "Specified number of channels %d is not supported\n",
                    avctx->channels);
            ret = AVERROR(EINVAL);
            goto free_and_end;
        }
        if(avctx->codec_type == AVMEDIA_TYPE_VIDEO) {
            pixdesc = av_pix_fmt_desc_get(avctx->pix_fmt);
            if (    avctx->bits_per_raw_sample < 0
                || (avctx->bits_per_raw_sample > 8 && pixdesc->comp[0].depth <= 8)) {
                av_log(avctx, AV_LOG_WARNING, "Specified bit depth %d not possible with the specified pixel formats depth %d\n",
                    avctx->bits_per_raw_sample, pixdesc->comp[0].depth);
                avctx->bits_per_raw_sample = pixdesc->comp[0].depth;
            }
            // 宽高
            if (avctx->width <= 0 || avctx->height <= 0) {
                av_log(avctx, AV_LOG_ERROR, "dimensions not set\n");
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
        }
        // 码率
        if (   (avctx->codec_type == AVMEDIA_TYPE_VIDEO || avctx->codec_type == AVMEDIA_TYPE_AUDIO)
            && avctx->bit_rate>0 && avctx->bit_rate<1000) {
            av_log(avctx, AV_LOG_WARNING, "Bitrate %"PRId64" is extremely low, maybe you mean %"PRId64"k\n", avctx->bit_rate, avctx->bit_rate);
        }

        if (!avctx->rc_initial_buffer_occupancy)
            avctx->rc_initial_buffer_occupancy = avctx->rc_buffer_size * 3LL / 4;

        if (avctx->ticks_per_frame && avctx->time_base.num &&
            avctx->ticks_per_frame > INT_MAX / avctx->time_base.num) {
            av_log(avctx, AV_LOG_ERROR,
                   "ticks_per_frame %d too large for the timebase %d/%d.",
                   avctx->ticks_per_frame,
                   avctx->time_base.num,
                   avctx->time_base.den);
            goto free_and_end;
        }

        if (avctx->hw_frames_ctx) {
            AVHWFramesContext *frames_ctx = (AVHWFramesContext*)avctx->hw_frames_ctx->data;
            if (frames_ctx->format != avctx->pix_fmt) {
                av_log(avctx, AV_LOG_ERROR,
                       "Mismatching AVCodecContext.pix_fmt and AVHWFramesContext.format\n");
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
            if (avctx->sw_pix_fmt != AV_PIX_FMT_NONE &&
                avctx->sw_pix_fmt != frames_ctx->sw_format) {
                av_log(avctx, AV_LOG_ERROR,
                       "Mismatching AVCodecContext.sw_pix_fmt (%s) "
                       "and AVHWFramesContext.sw_format (%s)\n",
                       av_get_pix_fmt_name(avctx->sw_pix_fmt),
                       av_get_pix_fmt_name(frames_ctx->sw_format));
                ret = AVERROR(EINVAL);
                goto free_and_end;
            }
            avctx->sw_pix_fmt = frames_ctx->sw_format;
        }
    }

    avctx->pts_correction_num_faulty_pts =
    avctx->pts_correction_num_faulty_dts = 0;
    avctx->pts_correction_last_pts =
    avctx->pts_correction_last_dts = INT64_MIN;

    if (   !CONFIG_GRAY && avctx->flags & AV_CODEC_FLAG_GRAY
        && avctx->codec_descriptor->type == AVMEDIA_TYPE_VIDEO)
        av_log(avctx, AV_LOG_WARNING,
               "gray decoding requested but not enabled at configuration time\n");

    // 一切检查都无误之后，调用编解码器初始化函数
    if (   avctx->codec->init && (!(avctx->active_thread_type&FF_THREAD_FRAME)
        || avctx->internal->frame_thread_encoder)) {
        ret = avctx->codec->init(avctx);
        if (ret < 0) {
            goto free_and_end;
        }
    }

    ret=0;

#if FF_API_AUDIOENC_DELAY
    if (av_codec_is_encoder(avctx->codec))
        avctx->delay = avctx->initial_padding;
#endif

    // 解码器的参数大部分都是由系统自动设定而不是由用户设定，因而不怎么需要检查
    if (av_codec_is_decoder(avctx->codec)) {
        if (!avctx->bit_rate)
            avctx->bit_rate = get_bit_rate(avctx);
        /* validate channel layout from the decoder */
        if (avctx->channel_layout) {
            int channels = av_get_channel_layout_nb_channels(avctx->channel_layout);
            if (!avctx->channels)
                avctx->channels = channels;
            else if (channels != avctx->channels) {
                char buf[512];
                av_get_channel_layout_string(buf, sizeof(buf), -1, avctx->channel_layout);
                av_log(avctx, AV_LOG_WARNING,
                       "Channel layout '%s' with %d channels does not match specified number of channels %d: "
                       "ignoring specified channel layout\n",
                       buf, channels, avctx->channels);
                avctx->channel_layout = 0;
            }
        }
        if (avctx->channels && avctx->channels < 0 ||
            avctx->channels > FF_SANE_NB_CHANNELS) {
            ret = AVERROR(EINVAL);
            goto free_and_end;
        }
        if (avctx->sub_charenc) {
            if (avctx->codec_type != AVMEDIA_TYPE_SUBTITLE) {
                av_log(avctx, AV_LOG_ERROR, "Character encoding is only "
                       "supported with subtitles codecs\n");
                ret = AVERROR(EINVAL);
                goto free_and_end;
            } else if (avctx->codec_descriptor->props & AV_CODEC_PROP_BITMAP_SUB) {
                av_log(avctx, AV_LOG_WARNING, "Codec '%s' is bitmap-based, "
                       "subtitles character encoding will be ignored\n",
                       avctx->codec_descriptor->name);
                avctx->sub_charenc_mode = FF_SUB_CHARENC_MODE_DO_NOTHING;
            } else {
                /* input character encoding is set for a text based subtitle
                 * codec at this point */
                if (avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_AUTOMATIC)
                    avctx->sub_charenc_mode = FF_SUB_CHARENC_MODE_PRE_DECODER;

                if (avctx->sub_charenc_mode == FF_SUB_CHARENC_MODE_PRE_DECODER) {
#if CONFIG_ICONV
                    iconv_t cd = iconv_open("UTF-8", avctx->sub_charenc);
                    if (cd == (iconv_t)-1) {
                        ret = AVERROR(errno);
                        av_log(avctx, AV_LOG_ERROR, "Unable to open iconv context "
                               "with input character encoding \"%s\"\n", avctx->sub_charenc);
                        goto free_and_end;
                    }
                    iconv_close(cd);
#else
                    av_log(avctx, AV_LOG_ERROR, "Character encoding subtitles "
                           "conversion needs a libavcodec built with iconv support "
                           "for this codec\n");
                    ret = AVERROR(ENOSYS);
                    goto free_and_end;
#endif
                }
            }
        }

#if FF_API_AVCTX_TIMEBASE
        if (avctx->framerate.num > 0 && avctx->framerate.den > 0)
            avctx->time_base = av_inv_q(av_mul_q(avctx->framerate, (AVRational){avctx->ticks_per_frame, 1}));
#endif
    }
    if (codec->priv_data_size > 0 && avctx->priv_data && codec->priv_class) {
        av_assert0(*(const AVClass **)avctx->priv_data == codec->priv_class);
    }

end:
    ff_unlock_avcodec(codec);
    if (options) {
        av_dict_free(options);
        *options = tmp;
    }

    return ret;
free_and_end:
    if (avctx->codec &&
        (avctx->codec->caps_internal & FF_CODEC_CAP_INIT_CLEANUP))
        avctx->codec->close(avctx);

    if (codec->priv_class && codec->priv_data_size)
        av_opt_free(avctx->priv_data);
    av_opt_free(avctx);

#if FF_API_CODED_FRAME
FF_DISABLE_DEPRECATION_WARNINGS
    av_frame_free(&avctx->coded_frame);
FF_ENABLE_DEPRECATION_WARNINGS
#endif

    av_dict_free(&tmp);
    av_freep(&avctx->priv_data);
    if (avctx->internal) {
        av_frame_free(&avctx->internal->to_free);
        av_frame_free(&avctx->internal->compat_decode_frame);
        av_frame_free(&avctx->internal->buffer_frame);
        av_packet_free(&avctx->internal->buffer_pkt);
        av_packet_free(&avctx->internal->last_pkt_props);

        av_packet_free(&avctx->internal->ds.in_pkt);

        av_freep(&avctx->internal->pool);
    }
    av_freep(&avctx->internal);
    avctx->codec = NULL;
    goto end;
}

其中最主要是调用了 AVCodec 的 init() 初始化，其他还做的有：

• 为各种结构体分配内存 av_malloc()
• 将输入的 AVDictionary 形式的选项设置到 AVCodecContext
• 一系列检查，比如采样率，宽高等
• 如果是编码器，检查输入参数是否符合编码器的要求

av_codec_is_encoder

当满足了 av_codec_is_encoder() 满足的情况下，进行了参数是否满足编码器规范要求的检查，如：

if (avctx->codec->pix_fmts) {
    for (i = 0; avctx->codec->pix_fmts[i] != AV_PIX_FMT_NONE; i++)
        if (avctx->pix_fmt == avctx->codec->pix_fmts[i])
            break;
    if (avctx->codec->pix_fmts[i] == AV_PIX_FMT_NONE
        && !((avctx->codec_id == AV_CODEC_ID_MJPEG || avctx->codec_id == AV_CODEC_ID_LJPEG)
             && avctx->strict_std_compliance <= FF_COMPLIANCE_UNOFFICIAL)) {
        char buf[128];
        snprintf(buf, sizeof(buf), "%d", avctx->pix_fmt);
        av_log(avctx, AV_LOG_ERROR, "Specified pixel format %s is invalid or not supported\n",
               (char *)av_x_if_null(av_get_pix_fmt_name(avctx->pix_fmt), buf));
        ret = AVERROR(EINVAL);
        goto free_and_end;
    }
    if (avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ420P ||
        avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ411P ||
        avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ422P ||
        avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ440P ||
        avctx->codec->pix_fmts[i] == AV_PIX_FMT_YUVJ444P)
        avctx->color_range = AVCOL_RANGE_JPEG;
}

将 AVCodecContext 中的 pix_fmt 与编码器 AVCodec 中的 pix_fmts 数组中的元素进行对比，其中 AVCodec 中的 pix_fmts 数组存储了该种编码器支持的像素格式，并且规定以 AV_PIX_FMT_NONE（AV_PIX_FMT_NONE 取值为 - 1）为结尾

static const enum AVPixelFormat pix_fmts_8bit[] = {
    AV_PIX_FMT_YUV420P,
    AV_PIX_FMT_YUVJ420P,
    AV_PIX_FMT_YUV422P,
    AV_PIX_FMT_YUVJ422P,
    AV_PIX_FMT_YUV444P,
    AV_PIX_FMT_YUVJ444P,
    AV_PIX_FMT_NV12,
    AV_PIX_FMT_NV16,
    AV_PIX_FMT_NONE
};

以 libx264 的 pix_fmts 数组为例，从 pix_fmts_8bit 的定义可以看出 libx264 主要支持的是以 YUV 为主的像素格式

avctx->codec->init

AVCodec ff_libx264_encoder = {
    .name             = "libx264",
    .long_name        = NULL_IF_CONFIG_SMALL("libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
    .type             = AVMEDIA_TYPE_VIDEO,
    .id               = AV_CODEC_ID_H264,
    .priv_data_size   = sizeof(X264Context),
    .init             = X264_init,
    .encode2          = X264_frame,
    .close            = X264_close,
    .capabilities     = AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AUTO_THREADS,
    .priv_class       = &x264_class,
    .defaults         = x264_defaults,
    .init_static_data = X264_init_static,
    .caps_internal    = FF_CODEC_CAP_INIT_THREADSAFE |
                        FF_CODEC_CAP_INIT_CLEANUP,
};

以 libx264 为例，init 方法对应的是 X264_init

static av_cold int X264_init(AVCodecContext *avctx)
{
    X264Context *x4 = avctx->priv_data;
    AVCPBProperties *cpb_props;
    int sw,sh;

    if (avctx->global_quality > 0)
        av_log(avctx, AV_LOG_WARNING, "-qscale is ignored, -crf is recommended.\n");

#if CONFIG_LIBX262_ENCODER
    if (avctx->codec_id == AV_CODEC_ID_MPEG2VIDEO) {
        x4->params.b_mpeg2 = 1;
        x264_param_default_mpeg2(&x4->params);
    } else
#endif
    x264_param_default(&x4->params);

    x4->params.b_deblocking_filter         = avctx->flags & AV_CODEC_FLAG_LOOP_FILTER;

    if (x4->preset || x4->tune)
        if (x264_param_default_preset(&x4->params, x4->preset, x4->tune) < 0) {
            int i;
            av_log(avctx, AV_LOG_ERROR, "Error setting preset/tune %s/%s.\n", x4->preset, x4->tune);
            av_log(avctx, AV_LOG_INFO, "Possible presets:");
            for (i = 0; x264_preset_names[i]; i++)
                av_log(avctx, AV_LOG_INFO, " %s", x264_preset_names[i]);
            av_log(avctx, AV_LOG_INFO, "\n");
            av_log(avctx, AV_LOG_INFO, "Possible tunes:");
            for (i = 0; x264_tune_names[i]; i++)
                av_log(avctx, AV_LOG_INFO, " %s", x264_tune_names[i]);
            av_log(avctx, AV_LOG_INFO, "\n");
            return AVERROR(EINVAL);
        }

    if (avctx->level > 0)
        x4->params.i_level_idc = avctx->level;

    x4->params.pf_log               = X264_log;
    x4->params.p_log_private        = avctx;
    x4->params.i_log_level          = X264_LOG_DEBUG;
    x4->params.i_csp                = convert_pix_fmt(avctx->pix_fmt);

    PARSE_X264_OPT("weightp", wpredp);

    if (avctx->bit_rate) {
        x4->params.rc.i_bitrate   = avctx->bit_rate / 1000;
        x4->params.rc.i_rc_method = X264_RC_ABR;
    }
    x4->params.rc.i_vbv_buffer_size = avctx->rc_buffer_size / 1000;
    x4->params.rc.i_vbv_max_bitrate = avctx->rc_max_rate    / 1000;
    x4->params.rc.b_stat_write      = avctx->flags & AV_CODEC_FLAG_PASS1;
    if (avctx->flags & AV_CODEC_FLAG_PASS2) {
        x4->params.rc.b_stat_read = 1;
    } else {
        if (x4->crf >= 0) {
            x4->params.rc.i_rc_method   = X264_RC_CRF;
            x4->params.rc.f_rf_constant = x4->crf;
        } else if (x4->cqp >= 0) {
            x4->params.rc.i_rc_method   = X264_RC_CQP;
            x4->params.rc.i_qp_constant = x4->cqp;
        }

        if (x4->crf_max >= 0)
            x4->params.rc.f_rf_constant_max = x4->crf_max;
    }

    if (avctx->rc_buffer_size && avctx->rc_initial_buffer_occupancy > 0 &&
        (avctx->rc_initial_buffer_occupancy <= avctx->rc_buffer_size)) {
        x4->params.rc.f_vbv_buffer_init =
            (float)avctx->rc_initial_buffer_occupancy / avctx->rc_buffer_size;
    }

    PARSE_X264_OPT("level", level);

    if (avctx->i_quant_factor > 0)
        x4->params.rc.f_ip_factor         = 1 / fabs(avctx->i_quant_factor);
    if (avctx->b_quant_factor > 0)
        x4->params.rc.f_pb_factor         = avctx->b_quant_factor;

#if FF_API_PRIVATE_OPT
FF_DISABLE_DEPRECATION_WARNINGS
    if (avctx->chromaoffset >= 0)
        x4->chroma_offset = avctx->chromaoffset;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    if (x4->chroma_offset >= 0)
        x4->params.analyse.i_chroma_qp_offset = x4->chroma_offset;

    if (avctx->gop_size >= 0)
        x4->params.i_keyint_max         = avctx->gop_size;
    if (avctx->max_b_frames >= 0)
        x4->params.i_bframe             = avctx->max_b_frames;

#if FF_API_PRIVATE_OPT
FF_DISABLE_DEPRECATION_WARNINGS
    if (avctx->scenechange_threshold >= 0)
        x4->scenechange_threshold = avctx->scenechange_threshold;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    if (x4->scenechange_threshold >= 0)
        x4->params.i_scenecut_threshold = x4->scenechange_threshold;

    if (avctx->qmin >= 0)
        x4->params.rc.i_qp_min          = avctx->qmin;
    if (avctx->qmax >= 0)
        x4->params.rc.i_qp_max          = avctx->qmax;
    if (avctx->max_qdiff >= 0)
        x4->params.rc.i_qp_step         = avctx->max_qdiff;
    if (avctx->qblur >= 0)
        x4->params.rc.f_qblur           = avctx->qblur;     /* temporally blur quants */
    if (avctx->qcompress >= 0)
        x4->params.rc.f_qcompress       = avctx->qcompress; /* 0.0 => cbr, 1.0 => constant qp */
    if (avctx->refs >= 0)
        x4->params.i_frame_reference    = avctx->refs;
    else if (x4->level) {
        int i;
        int mbn = AV_CEIL_RSHIFT(avctx->width, 4) * AV_CEIL_RSHIFT(avctx->height, 4);
        int level_id = -1;
        char *tail;
        int scale = X264_BUILD < 129 ? 384 : 1;

        if (!strcmp(x4->level, "1b")) {
            level_id = 9;
        } else if (strlen(x4->level) <= 3){
            level_id = av_strtod(x4->level, &tail) * 10 + 0.5;
            if (*tail)
                level_id = -1;
        }
        if (level_id <= 0)
            av_log(avctx, AV_LOG_WARNING, "Failed to parse level\n");

        for (i = 0; i<x264_levels[i].level_idc; i++)
            if (x264_levels[i].level_idc == level_id)
                x4->params.i_frame_reference = av_clip(x264_levels[i].dpb / mbn / scale, 1, x4->params.i_frame_reference);
    }

    if (avctx->trellis >= 0)
        x4->params.analyse.i_trellis    = avctx->trellis;
    if (avctx->me_range >= 0)
        x4->params.analyse.i_me_range   = avctx->me_range;
#if FF_API_PRIVATE_OPT
    FF_DISABLE_DEPRECATION_WARNINGS
    if (avctx->noise_reduction >= 0)
        x4->noise_reduction = avctx->noise_reduction;
    FF_ENABLE_DEPRECATION_WARNINGS
#endif
    if (x4->noise_reduction >= 0)
        x4->params.analyse.i_noise_reduction = x4->noise_reduction;
    if (avctx->me_subpel_quality >= 0)
        x4->params.analyse.i_subpel_refine   = avctx->me_subpel_quality;
#if FF_API_PRIVATE_OPT
FF_DISABLE_DEPRECATION_WARNINGS
    if (avctx->b_frame_strategy >= 0)
        x4->b_frame_strategy = avctx->b_frame_strategy;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    if (avctx->keyint_min >= 0)
        x4->params.i_keyint_min = avctx->keyint_min;
#if FF_API_CODER_TYPE
FF_DISABLE_DEPRECATION_WARNINGS
    if (avctx->coder_type >= 0)
        x4->coder = avctx->coder_type == FF_CODER_TYPE_AC;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    if (avctx->me_cmp >= 0)
        x4->params.analyse.b_chroma_me = avctx->me_cmp & FF_CMP_CHROMA;

    if (x4->aq_mode >= 0)
        x4->params.rc.i_aq_mode = x4->aq_mode;
    if (x4->aq_strength >= 0)
        x4->params.rc.f_aq_strength = x4->aq_strength;
    PARSE_X264_OPT("psy-rd", psy_rd);
    PARSE_X264_OPT("deblock", deblock);
    PARSE_X264_OPT("partitions", partitions);
    PARSE_X264_OPT("stats", stats);
    if (x4->psy >= 0)
        x4->params.analyse.b_psy  = x4->psy;
    if (x4->rc_lookahead >= 0)
        x4->params.rc.i_lookahead = x4->rc_lookahead;
    if (x4->weightp >= 0)
        x4->params.analyse.i_weighted_pred = x4->weightp;
    if (x4->weightb >= 0)
        x4->params.analyse.b_weighted_bipred = x4->weightb;
    if (x4->cplxblur >= 0)
        x4->params.rc.f_complexity_blur = x4->cplxblur;

    if (x4->ssim >= 0)
        x4->params.analyse.b_ssim = x4->ssim;
    if (x4->intra_refresh >= 0)
        x4->params.b_intra_refresh = x4->intra_refresh;
    if (x4->bluray_compat >= 0) {
        x4->params.b_bluray_compat = x4->bluray_compat;
        x4->params.b_vfr_input = 0;
    }
    if (x4->avcintra_class >= 0)
#if X264_BUILD >= 142
        x4->params.i_avcintra_class = x4->avcintra_class;
#else
        av_log(avctx, AV_LOG_ERROR,
               "x264 too old for AVC Intra, at least version 142 needed\n");
#endif
    if (x4->b_bias != INT_MIN)
        x4->params.i_bframe_bias              = x4->b_bias;
    if (x4->b_pyramid >= 0)
        x4->params.i_bframe_pyramid = x4->b_pyramid;
    if (x4->mixed_refs >= 0)
        x4->params.analyse.b_mixed_references = x4->mixed_refs;
    if (x4->dct8x8 >= 0)
        x4->params.analyse.b_transform_8x8    = x4->dct8x8;
    if (x4->fast_pskip >= 0)
        x4->params.analyse.b_fast_pskip       = x4->fast_pskip;
    if (x4->aud >= 0)
        x4->params.b_aud                      = x4->aud;
    if (x4->mbtree >= 0)
        x4->params.rc.b_mb_tree               = x4->mbtree;
    if (x4->direct_pred >= 0)
        x4->params.analyse.i_direct_mv_pred   = x4->direct_pred;

    if (x4->slice_max_size >= 0)
        x4->params.i_slice_max_size =  x4->slice_max_size;

    if (x4->fastfirstpass)
        x264_param_apply_fastfirstpass(&x4->params);

    /* Allow specifying the x264 profile through AVCodecContext. */
    if (!x4->profile)
        switch (avctx->profile) {
        case FF_PROFILE_H264_BASELINE:
            x4->profile = av_strdup("baseline");
            break;
        case FF_PROFILE_H264_HIGH:
            x4->profile = av_strdup("high");
            break;
        case FF_PROFILE_H264_HIGH_10:
            x4->profile = av_strdup("high10");
            break;
        case FF_PROFILE_H264_HIGH_422:
            x4->profile = av_strdup("high422");
            break;
        case FF_PROFILE_H264_HIGH_444:
            x4->profile = av_strdup("high444");
            break;
        case FF_PROFILE_H264_MAIN:
            x4->profile = av_strdup("main");
            break;
        default:
            break;
        }

    if (x4->nal_hrd >= 0)
        x4->params.i_nal_hrd = x4->nal_hrd;

    if (x4->motion_est >= 0) {
        x4->params.analyse.i_me_method = x4->motion_est;
#if FF_API_MOTION_EST
FF_DISABLE_DEPRECATION_WARNINGS
    } else {
        if (avctx->me_method == ME_EPZS)
            x4->params.analyse.i_me_method = X264_ME_DIA;
        else if (avctx->me_method == ME_HEX)
            x4->params.analyse.i_me_method = X264_ME_HEX;
        else if (avctx->me_method == ME_UMH)
            x4->params.analyse.i_me_method = X264_ME_UMH;
        else if (avctx->me_method == ME_FULL)
            x4->params.analyse.i_me_method = X264_ME_ESA;
        else if (avctx->me_method == ME_TESA)
            x4->params.analyse.i_me_method = X264_ME_TESA;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    }

    if (x4->coder >= 0)
        x4->params.b_cabac = x4->coder;

    if (x4->b_frame_strategy >= 0)
        x4->params.i_bframe_adaptive = x4->b_frame_strategy;

    if (x4->profile)
        if (x264_param_apply_profile(&x4->params, x4->profile) < 0) {
            int i;
            av_log(avctx, AV_LOG_ERROR, "Error setting profile %s.\n", x4->profile);
            av_log(avctx, AV_LOG_INFO, "Possible profiles:");
            for (i = 0; x264_profile_names[i]; i++)
                av_log(avctx, AV_LOG_INFO, " %s", x264_profile_names[i]);
            av_log(avctx, AV_LOG_INFO, "\n");
            return AVERROR(EINVAL);
        }

    x4->params.i_width          = avctx->width;
    x4->params.i_height         = avctx->height;
    av_reduce(&sw, &sh, avctx->sample_aspect_ratio.num, avctx->sample_aspect_ratio.den, 4096);
    x4->params.vui.i_sar_width  = sw;
    x4->params.vui.i_sar_height = sh;
    x4->params.i_timebase_den = avctx->time_base.den;
    x4->params.i_timebase_num = avctx->time_base.num;
    x4->params.i_fps_num = avctx->time_base.den;
    x4->params.i_fps_den = avctx->time_base.num * avctx->ticks_per_frame;

    x4->params.analyse.b_psnr = avctx->flags & AV_CODEC_FLAG_PSNR;

    x4->params.i_threads      = avctx->thread_count;
    if (avctx->thread_type)
        x4->params.b_sliced_threads = avctx->thread_type == FF_THREAD_SLICE;

    x4->params.b_interlaced   = avctx->flags & AV_CODEC_FLAG_INTERLACED_DCT;

    x4->params.b_open_gop     = !(avctx->flags & AV_CODEC_FLAG_CLOSED_GOP);

    x4->params.i_slice_count  = avctx->slices;

    x4->params.vui.b_fullrange = avctx->pix_fmt == AV_PIX_FMT_YUVJ420P ||
                                 avctx->pix_fmt == AV_PIX_FMT_YUVJ422P ||
                                 avctx->pix_fmt == AV_PIX_FMT_YUVJ444P ||
                                 avctx->color_range == AVCOL_RANGE_JPEG;

    if (avctx->colorspace != AVCOL_SPC_UNSPECIFIED)
        x4->params.vui.i_colmatrix = avctx->colorspace;
    if (avctx->color_primaries != AVCOL_PRI_UNSPECIFIED)
        x4->params.vui.i_colorprim = avctx->color_primaries;
    if (avctx->color_trc != AVCOL_TRC_UNSPECIFIED)
        x4->params.vui.i_transfer  = avctx->color_trc;

    if (avctx->flags & AV_CODEC_FLAG_GLOBAL_HEADER)
        x4->params.b_repeat_headers = 0;

    if(x4->x264opts){
        const char *p= x4->x264opts;
        while(p){
            char param[4096]={0}, val[4096]={0};
            if(sscanf(p, "%4095[^:=]=%4095[^:]", param, val) == 1){
                OPT_STR(param, "1");
            }else
                OPT_STR(param, val);
            p= strchr(p, ':');
            p+=!!p;
        }
    }

    if (x4->x264_params) {
        AVDictionary *dict    = NULL;
        AVDictionaryEntry *en = NULL;

        if (!av_dict_parse_string(&dict, x4->x264_params, "=", ":", 0)) {
            while ((en = av_dict_get(dict, "", en, AV_DICT_IGNORE_SUFFIX))) {
                if (x264_param_parse(&x4->params, en->key, en->value) < 0)
                    av_log(avctx, AV_LOG_WARNING,
                           "Error parsing option '%s = %s'.\n",
                            en->key, en->value);
            }

            av_dict_free(&dict);
        }
    }

    // update AVCodecContext with x264 parameters
    avctx->has_b_frames = x4->params.i_bframe ?
        x4->params.i_bframe_pyramid ? 2 : 1 : 0;
    if (avctx->max_b_frames < 0)
        avctx->max_b_frames = 0;

    avctx->bit_rate = x4->params.rc.i_bitrate*1000;

    x4->enc = x264_encoder_open(&x4->params);
    if (!x4->enc)
        return AVERROR_EXTERNAL;

    if (avctx->flags & AV_CODEC_FLAG_GLOBAL_HEADER) {
        x264_nal_t *nal;
        uint8_t *p;
        int nnal, s, i;

        s = x264_encoder_headers(x4->enc, &nal, &nnal);
        avctx->extradata = p = av_mallocz(s + AV_INPUT_BUFFER_PADDING_SIZE);
        if (!p)
            return AVERROR(ENOMEM);

        for (i = 0; i < nnal; i++) {
            /* Don't put the SEI in extradata. */
            if (nal[i].i_type == NAL_SEI) {
                av_log(avctx, AV_LOG_INFO, "%s\n", nal[i].p_payload+25);
                x4->sei_size = nal[i].i_payload;
                x4->sei      = av_malloc(x4->sei_size);
                if (!x4->sei)
                    return AVERROR(ENOMEM);
                memcpy(x4->sei, nal[i].p_payload, nal[i].i_payload);
                continue;
            }
            memcpy(p, nal[i].p_payload, nal[i].i_payload);
            p += nal[i].i_payload;
        }
        avctx->extradata_size = p - avctx->extradata;
    }

    cpb_props = ff_add_cpb_side_data(avctx);
    if (!cpb_props)
        return AVERROR(ENOMEM);
    cpb_props->buffer_size = x4->params.rc.i_vbv_buffer_size * 1000;
    cpb_props->max_bitrate = x4->params.rc.i_vbv_max_bitrate * 1000;
    cpb_props->avg_bitrate = x4->params.rc.i_bitrate         * 1000;

    return 0;
}

大概的工作

• 设置 X264Context 的参数。X264Context 主要完成了 libx264 和 FFmpeg 对接的功能。可以看出代码主要在设置一个 params 结构体变量，该变量的类型即是 x264 中存储参数的结构体 x264_param_t。
• 调用 libx264 的 API 进行编码器的初始化工作。例如调用 x264_param_default() 设置默认参数，调用 x264_param_apply_profile() 设置 profile，调用 x264_encoder_open() 打开编码器等等。

参考

• 雷霄骅 - FFMPEG