YUV是视频裸数据流，没有被压缩的，占用内存非常大，不适合在网络上传输，因此咱们平时网络上传输的都是经过压缩的视频流，如咱们平时看的直播都是经过编码压缩的视频流，下载到本地之后进行解码再进行渲染。今天我们就来看一下YUV数据是如何编码成H264数据流的。YUV是裸数据流，所以咱们要实现编码，就必须要知道YUV数据的格式pix_fmt、视频宽高。

一 使用命令行进行编码

ffmpeg -f yuv420p -s 540x960 -i bb1_yuv420p_540x960.yuv output.h264

-f 指定yuv数据格式为yuv420p
-s 指定视频大小为540x960

二 使用代码进行编码

1、通过avformat_alloc_output_context2函数初始化输出文件上下文

avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, h264Path.UTF8String);

2、通过avcodec_find_encoder函数找到编码器

dec = avcodec_find_encoder(AV_CODEC_ID_H264)

3、通过avcodec_alloc_context3初始化编码器上下文

dec_ctx = avcodec_alloc_context3(dec);

4、设置编码器上下文的参数，包括码率、时间基、视频宽高、像素等参数

dec_ctx->width = yuvW;
dec_ctx->height = yuvH;
dec_ctx->framerate = av_make_q(25, 1);
dec_ctx->time_base = av_make_q(1, 25);
dec_ctx->pix_fmt = AV_PIX_FMT_YUV420P;
dec_ctx->gop_size = 10;
dec_ctx->bit_rate = 800000;
dec_ctx->max_b_frames = 1;
if (dec_ctx->codec_id == AV_CODEC_ID_H264) {
    av_opt_set(dec_ctx->priv_data, "preset", "slow", 0);
}

5、通过avformat_new_stream函数新建一个视频流，并通过avcodec_parameters_from_context函数把编码器的参数拷贝给视频流

AVStream *st = avformat_new_stream(ofmt_ctx, dec);
ret = avcodec_parameters_from_context(st->codecpar, dec_ctx);

6、通过avcodec_open2函数打开编码器

avcodec_open2(dec_ctx, dec, NULL);

7、通过avio_open函数打开输出文件

avio_open(&ofmt_ctx->pb, h264Path.UTF8String, AVIO_FLAG_WRITE);

8、通过avformat_write_header写文件头

avformat_write_header(ofmt_ctx, NULL);

9、循环从yuv文件中获取视频帧，经过编码后，通过av_interleaved_write_frame函数写入文件

    while (feof(yuv_f)==0) {
        size_t size = fread(yuv_buffer, 1, yuvW*yuvH*3/2, yuv_f);
        if (size<yuvW*yuvH*3/2) {
            printf("fread fail \n");
            break;
        }
        for (int i=0; i<yuvH; i++) {
            memcpy(frame->data[0] + i*frame->linesize[0], yuv_buffer + yuvW*i, yuvW);
        }
        for (int i=0; i<yuvH/2; i++) {
            memcpy(frame->data[1] + (i*frame->linesize[1]), yuv_buffer+yuvH*yuvW + yuvW/2*i, yuvW/2);
        }
        for (int i=0; i<yuvH/2; i++) {
            memcpy(frame->data[2] + (i*frame->linesize[2]), yuv_buffer+yuvH*yuvW*5/4 + yuvW/2*i, yuvW/2);
        }
        frame->pts = pts_i++;
        ret = avcodec_send_frame(dec_ctx, frame);
        if (ret<0) {
            printf("avcodec_send_frame fail \n");
            break;
        }
        while (1) {
            ret = avcodec_receive_packet(dec_ctx, pkt);
            if (ret==AVERROR(EAGAIN) || ret == AVERROR_EOF) {
                break;
            } else if (ret<0) {
                printf("avcodec_receive_packet fail \n");
                break;
            }
            ret = av_interleaved_write_frame(ofmt_ctx, pkt);
            if (ret<0) {
                printf("av_interleaved_write_frame fail \n");
                break;
            }
            av_packet_unref(pkt);
        }
    }
    ret = avcodec_send_frame(dec_ctx, NULL);
    if (ret<0) {
        printf("avcodec_send_frame fail \n");
        goto __FAIL;
    }
    while (1) {
        ret = avcodec_receive_packet(dec_ctx, pkt);
        if (ret==AVERROR(EINVAL) || ret == AVERROR_EOF) {
            break;
        } else if (ret<0) {
            printf("avcodec_receive_packet fail \n");
            break;
        }
        ret = av_interleaved_write_frame(ofmt_ctx, pkt);
        if (ret<0) {
            printf("av_interleaved_write_frame fail \n");
            break;
        }
        av_packet_unref(pkt);
    }

这里需要注意的是后面还有一个while循环，这是因为视频帧有I、B、P帧的区别，这就导致可能还有数据在缓存中没有编码完，因此使用一个while循环去读出来。
10、通过av_write_trailer写入文件尾

av_write_trailer(ofmt_ctx);

完整代码如下：

+ (void)convert
{
    NSString *yuvPath = [[NSBundle mainBundle] pathForResource:@"bb1_yuv420p_540x960.yuv" ofType:nil];
    NSString *h264Path = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).firstObject stringByAppendingPathComponent:@"bb1.h264"];
    NSLog(@"%@", h264Path);
    int yuvW = 540;
    int yuvH = 960;
    int ret;
    AVFormatContext *ofmt_ctx = NULL;
    AVCodecContext *dec_ctx = NULL;
    AVCodec *dec = NULL;
    AVPacket *pkt = NULL;
    AVFrame *frame = NULL;
    FILE *yuv_f = fopen(yuvPath.UTF8String, "rb+");
    ret = avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, h264Path.UTF8String);
    if (ret<0) {
        printf("avformat_alloc_output_context2 fail \n");
        goto __FAIL;
    }
    dec = avcodec_find_encoder(AV_CODEC_ID_H264);
    if (!dec) {
        printf("avcodec_find_encoder fail \n");
        goto __FAIL;
    }
    dec_ctx = avcodec_alloc_context3(dec);
    dec_ctx->width = yuvW;
    dec_ctx->height = yuvH;
    dec_ctx->framerate = av_make_q(25, 1);
    dec_ctx->time_base = av_make_q(1, 25);
    dec_ctx->pix_fmt = AV_PIX_FMT_YUV420P;
    dec_ctx->gop_size = 10;
    dec_ctx->bit_rate = 800000;
    dec_ctx->max_b_frames = 1;
    if (dec_ctx->codec_id == AV_CODEC_ID_H264) {
        av_opt_set(dec_ctx->priv_data, "preset", "slow", 0);
    }

    ret = avio_open(&ofmt_ctx->pb, h264Path.UTF8String, AVIO_FLAG_WRITE);
    if (ret<0) {
        printf("avio_open fail \n");
        goto __FAIL;
    }
    ret = avcodec_open2(dec_ctx, dec, NULL);
    if (ret<0) {
        printf("avcodec_open2 fail \n");
        goto __FAIL;
    }
    AVStream *st = avformat_new_stream(ofmt_ctx, dec);
    ret = avcodec_parameters_from_context(st->codecpar, dec_ctx);
    if (ret<0) {
        printf("avcodec_parameters_from_context fail \n");
        goto __FAIL;
    }
    
    ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret<0) {
        printf("avformat_write_header fail \n");
        goto __FAIL;
    }
    
    uint8_t *yuv_buffer = av_malloc(yuvW*yuvH*3/2);
    frame = av_frame_alloc();
    if (!frame) {
        printf("av_frame_alloc fail \n");
        goto __FAIL;
    }
    frame->width = dec_ctx->width;
    frame->height = dec_ctx->height;
    frame->format = dec_ctx->pix_fmt;
    ret = av_frame_get_buffer(frame, 0);
    if (ret<0) {
        printf("av_frame_get_buffer fail \n");
        goto __FAIL;
    }
    pkt = av_packet_alloc();
    if (!pkt) {
        printf("av_packet_alloc fail \n");
        goto __FAIL;
    }
    int pts_i = 0;
    while (feof(yuv_f)==0) {
        size_t size = fread(yuv_buffer, 1, yuvW*yuvH*3/2, yuv_f);
        if (size<yuvW*yuvH*3/2) {
            printf("fread fail \n");
            break;
        }
        for (int i=0; i<yuvH; i++) {
            memcpy(frame->data[0] + i*frame->linesize[0], yuv_buffer + yuvW*i, yuvW);
        }
        for (int i=0; i<yuvH/2; i++) {
            memcpy(frame->data[1] + (i*frame->linesize[1]), yuv_buffer+yuvH*yuvW + yuvW/2*i, yuvW/2);
        }
        for (int i=0; i<yuvH/2; i++) {
            memcpy(frame->data[2] + (i*frame->linesize[2]), yuv_buffer+yuvH*yuvW*5/4 + yuvW/2*i, yuvW/2);
        }
        frame->pts = pts_i++;
        ret = avcodec_send_frame(dec_ctx, frame);
        if (ret<0) {
            printf("avcodec_send_frame fail \n");
            break;
        }
        while (1) {
            ret = avcodec_receive_packet(dec_ctx, pkt);
            if (ret==AVERROR(EAGAIN) || ret == AVERROR_EOF) {
                break;
            } else if (ret<0) {
                printf("avcodec_receive_packet fail \n");
                break;
            }
            ret = av_interleaved_write_frame(ofmt_ctx, pkt);
            if (ret<0) {
                printf("av_interleaved_write_frame fail \n");
                break;
            }
            av_packet_unref(pkt);
        }
    }
    ret = avcodec_send_frame(dec_ctx, NULL);
    if (ret<0) {
        printf("avcodec_send_frame fail \n");
        goto __FAIL;
    }
    while (1) {
        ret = avcodec_receive_packet(dec_ctx, pkt);
        if (ret==AVERROR(EINVAL) || ret == AVERROR_EOF) {
            break;
        } else if (ret<0) {
            printf("avcodec_receive_packet fail \n");
            break;
        }
        ret = av_interleaved_write_frame(ofmt_ctx, pkt);
        if (ret<0) {
            printf("av_interleaved_write_frame fail \n");
            break;
        }
        av_packet_unref(pkt);
    }
    ret = av_write_trailer(ofmt_ctx);
    if (ret<0) {
        printf("av_write_trailer fail \n");
    }
__FAIL:
    if (ofmt_ctx->pb) {
        avio_close(ofmt_ctx->pb);
    }
    if (dec_ctx) {
        avcodec_close(dec_ctx);
    }
    if (yuv_buffer) {
        av_free(yuv_buffer);
    }
    if (ofmt_ctx) {
        avformat_free_context(ofmt_ctx);
    }
    if (frame) {
        av_frame_free(&frame);
    }
    if (pkt) {
        av_packet_free(&pkt);
    }
}