在ffmpeg中写代码实现一个bitstream filter，对hevc码流数据处理，产生各种类型的错误码流

好的，下面是一个简单的示例代码，它实现了一个bitstream filter，用于对HEVC码流进行处理，生成各种类型的错误码流：

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <libavcodec/avcodec.h>
#include <libavutil/opt.h>

typedef struct {
    AVBSFContext *bsf;
    int error_type;
} MyBitstreamFilterContext;

static int my_avc_bsf_filter(AVBSFContext *bsf, AVPacket *pkt)
{
    MyBitstreamFilterContext *ctx = bsf->priv_data;
    uint8_t *data = pkt->data;
    int size = pkt->size;

    switch (ctx->error_type) {
        case 1: // 丢弃前面的2个NALU
            if (size >= 4 && data[0] == 0 && data[1] == 0 && data[2] == 0 && data[3] == 1) {
                data += 8;
                size -= 8;
            }
            break;
        case 2: // 将第一个NALU的类型改为I帧
            if (size >= 4 && data[0] == 0 && data[1] == 0 && data[2] == 0 && data[3] == 1) {
                data[4] &= 0x7E;
                data[4] |= 0x18;
            }
            break;
        case 3: // 将第二个NALU的类型改为P帧
            if (size >= 8 && data[4] == 0 && data[5] == 0 && data[6] == 0 && data[7] == 1) {
                data[8] &= 0x7E;
                data[8] |= 0x1B;
            }
            break;
        case 4: // 将第三个NALU的类型改为B帧
            if (size >= 12 && data[8] == 0 && data[9] == 0 && data[10] == 0 && data[11] == 1) {
                data[12] &= 0x7E;
                data[12] |= 0x1C;
            }
            break;
        default:
            break;
    }

    return av_packet_copy_props(pkt, bsf->par_in);
}

static int my_avc_bsf_init(AVBSFContext *bsf)
{
    MyBitstreamFilterContext *ctx;
    const char *error_type_str;
    int ret;

    ctx = av_mallocz(sizeof(*ctx));
    if (!ctx) {
        return AVERROR(ENOMEM);
    }

    bsf->priv_data = ctx;
    ctx->bsf = bsf;

    if ((ret = av_opt_get(bsf->priv_data, "error_type", AV_OPT_SEARCH_CHILDREN, (uint8_t **)&error_type_str)) < 0) {
        av_log(bsf, AV_LOG_ERROR, "Failed to get error_type option value: %s\n", av_err2str(ret));
        return ret;
    }

    if (!strcmp(error_type_str, "discard_first_two_nalus")) {
        ctx->error_type = 1;
    } else if (!strcmp(error_type_str, "change_first_nalu_to_iframe")) {
        ctx->error_type = 2;
    } else if (!strcmp(error_type_str, "change_second_nalu_to_pframe")) {
        ctx->error_type = 3;
    } else if (!strcmp(error_type_str, "change_third_nalu_to_bframe")) {
        ctx->error_type = 4;
    } else {
        av_log(bsf, AV_LOG_ERROR, "Invalid error_type option value: %s\n", error_type_str);
        return AVERROR(EINVAL);
    }

    return 0;
}

static void my_avc_bsf_close(AVBSFContext *bsf)
{
    MyBitstreamFilterContext *ctx = bsf->priv_data;
    av_free(ctx);
}

AVBitStreamFilter my_avc_bsf = {
    .name = "my_avc_bsf",
    .filter = my_avc_bsf_filter,
    .priv_data_size = sizeof(MyBitstreamFilterContext),
    .init = my_avc_bsf_init,
    .close = my_avc_bsf_close,
};

int main(int argc, char **argv)
{
    const AVBitStreamFilter *bsf = &my_avc_bsf;
    AVBSFContext *ctx = NULL;
    AVPacket pkt = { 0 };
    AVCodecParameters *par;
    AVCodec *codec;
    AVFormatContext *fmt_ctx = NULL, *ofmt_ctx = NULL;
    AVStream *in_stream, *out_stream;
    int ret;

    if (argc < 3) {
        fprintf(stderr, "Usage: %s input output\n", argv[0]);
        exit(1);
    }

    av_register_all();

    if ((ret = avformat_open_input(&fmt_ctx, argv[1], NULL, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Failed to open input file '%s': %s\n", argv[1], av_err2str(ret));
        goto end;
    }

    if ((ret = avformat_find_stream_info(fmt_ctx, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Failed to retrieve input stream information: %s\n", av_err2str(ret));
        goto end;
    }

    if ((ret = avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, argv[2])) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Failed to create output context: %s\n", av_err2str(ret));
        goto end;
    }

    for (int i = 0; i < fmt_ctx->nb_streams; i++) {
        par = fmt_ctx->streams[i]->codecpar;

        if (par->codec_type != AVMEDIA_TYPE_VIDEO) {
            continue;
        }

        codec = avcodec_find_decoder(par->codec_id);
        if (!codec) {
            av_log(NULL, AV_LOG_ERROR, "Failed to find decoder for codec ID %d\n", par->codec_id);
            goto end;
        }

        in_stream = fmt_ctx->streams[i];
        out_stream = avformat_new_stream(ofmt_ctx, codec);
        if (!out_stream) {
            av_log(NULL, AV_LOG_ERROR, "Failed to create new output stream\n");
            goto end;
        }

        if ((ret = avcodec_parameters_copy(out_stream->codecpar, par)) < 0) {
            av_log(NULL, AV_LOG_ERROR, "Failed to copy codec parameters: %s\n", av_err2str(ret));
            goto end;
        }
    }

    if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
        if ((ret = avio_open(&ofmt_ctx->pb, argv[2], AVIO_FLAG_WRITE)) < 0) {
            av_log(NULL, AV_LOG_ERROR, "Failed to open output file '%s': %s\n", argv[2], av_err2str(ret));
            goto end;
        }
    }

    if ((ret = avformat_write_header(ofmt_ctx, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Failed to write output file header: %s\n", av_err2str(ret));
        goto end;
    }

    for (;;) {
        ret = av_read_frame(fmt_ctx, &pkt);
        if (ret < 0) {
            if (ret == AVERROR_EOF) {
                break;
            }
            av_log(NULL, AV_LOG_ERROR, "Error occurred while reading frame: %s\n", av_err2str(ret));
            goto end;
        }

        in_stream = fmt_ctx->streams[pkt.stream_index];
        out_stream = ofmt_ctx->streams[pkt.stream_index];

        if (pkt.stream_index == 0) {
            ctx = NULL;
            if ((ret = av_bsf_alloc(bsf, &ctx)) < 0) {
                av_log(NULL, AV_LOG_ERROR, "Failed to allocate bitstream filter context: %s\n", av_err2str(ret));
                goto end;
            }

            if ((ret = avcodec_parameters_copy(ctx->par_in, in_stream->codecpar)) < 0) {
                av_log(NULL, AV_LOG_ERROR, "Failed to copy codec parameters: %s\n", av_err2str(ret));
                goto end;
            }

            if ((ret = av_opt_set(ctx->priv_data, "error_type", "discard_first_two_nalus", 0)) < 0) {
                av_log(NULL, AV_LOG_ERROR, "Failed to set error_type option value: %s\n", av_err2str(ret));
                goto end;
            }

            if ((ret = av_bsf_init(ctx)) < 0) {
                av_log(NULL, AV_LOG_ERROR, "Failed to initialize bitstream filter: %s\n", av_err2str(ret));
                goto end;
            }

            if ((ret = av_bsf_send_packet(ctx, &pkt)) < 0) {
                av_log(NULL, AV_LOG_ERROR, "Failed to send packet to bitstream filter: %s\n", av_err2str(ret));
                goto end;
            }

            while ((ret = av_bsf_receive_packet(ctx, &pkt)) == 0) {
                if ((ret = av_write_frame(ofmt_ctx, &pkt)) < 0) {
                    av_log(NULL, AV_LOG_ERROR, "Failed to write packet to output file: %s\n", av_err2str(ret));
                    goto end;
                }
            }

            if (ret == AVERROR(EAGAIN)) {
                continue;
            }

            if (ret == AVERROR_EOF) {
                av_bsf_free(&ctx);
                av_packet_unref(&pkt);
                continue;
            }

            av_log(NULL, AV_LOG_ERROR, "Failed to receive packet from bitstream filter: %s\n", av_err2str(ret));
            goto end;
        }

        if ((ret = av_write_frame(ofmt_ctx, &pkt)) < 0) {
            av_log(NULL, AV_LOG_ERROR, "Failed to write packet to output file: %s\n", av_err2str(ret));
            goto end;
        }

        av_packet_unref(&pkt);
    }

    if ((ret = av_write_trailer(ofmt_ctx)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Failed to write output file trailer: %s\n", av_err2str(ret));
        goto end;
    }

end:
    avformat_close_input(&fmt_ctx);
    if (ofmt_ctx && !(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
        avio_closep(&ofmt_ctx->pb);
    }
    avformat_free_context(ofmt_ctx);

    return ret;
}

这个bitstream filter实现了四种不同的错误类型：

1. 丢弃前两个NALU。
2. 将第一个NALU的类型更改为I帧。
3. 将第二个NALU的类型更改为P帧。
4. 将第三个NALU的类型更改为B帧。

你可以通过修改代码来实现其他类型的错误码流。此外，还可以通过在命令行中使用`-bsf:v my_avc_bsf=error_type=<type>`参数来指定错误类型。例如，如果要生成第一种类型的错误码流，可以使用以下命令：

ffmpeg -i input.hevc -bsf:v my_avc_bsf=error_type=discard_first_two_nalus output.hevc