前言

有时播放实时流的时候有截图的需求，需要将解码出来的图片保存本地或上传服务器，这时就需要将avframe中的数据编码成png、jpg等格式的图片，我们使用ffmpeg的相关编码器就可以实现功能。

一、如何实现？

1、查找编码器

首先需要查找图片编码器，比如jpg为AV_CODEC_ID_MJPEG，png为AV_CODEC_ID_PNG

示例代码：

enum AVCodecID codec = avcodec_find_encoder(AV_CODEC_ID_MJPEG);

2、构造编码器上下文

有了编码器就可以构造编码器上下文了。

AVCodecContext*ctx = avcodec_alloc_context3(codec);
ctx->bit_rate = 3000000;
ctx->width = frame->width;//视频帧的宽
ctx->height = frame->height;//视频帧的高
ctx->time_base.num = 1;
ctx->time_base.den = 25;
ctx->gop_size = 10;
ctx->max_b_frames = 0;
ctx->thread_count = 1;
ctx->pix_fmt = *codec->pix_fmts;//使用编码器适配的像素格式
//打开编码器
avcodec_open2(ctx, codec, NULL);

3、像素格式转换

如果输入视频帧的像素和编码器的像素格式不相同则需要转换像素格式，我们采用SwsContext 转换即可

AVFrame*rgbFrame = av_frame_alloc();//转换后的帧
swsContext = sws_getContext(frame->width, frame->height, (enum AVPixelFormat)frame->format, frame->width, frame->height, ctx->pix_fmt, 1, NULL, NULL, NULL);
int bufferSize = av_image_get_buffer_size(ctx->pix_fmt, frame->width, frame->height, 1) * 2;
buffer = (unsigned char*)av_malloc(bufferSize);
//构造帧的缓存
av_image_fill_arrays(rgbFrame->data, rgbFrame->linesize, buffer, ctx->pix_fmt, frame->width, frame->height, 1);
sws_scale(swsContext, frame->data, frame->linesize, 0, frame->height, rgbFrame->data, rgbFrame->linesize);
//构造必要的参数
rgbFrame->format = ctx->pix_fmt;
rgbFrame->width = ctx->width;
rgbFrame->height = ctx->height;

4、编码

得到转后的帧就可以编码

ret = avcodec_send_frame(ctx, rgbFrame);

5、获取图片数据

获取解码后的包即可得到图片数据。

uint8_t* outbuf;//输出图片的缓存
size_t outbufSize;//缓存大小
AVPacket pkt;
av_init_packet(&pkt);
//获取解码的包
avcodec_receive_packet(ctx, &pkt);
//将图片数据拷贝到缓存
if (pkt.size > 0 && pkt.size <= outbufSize)
memcpy(outbuf, pkt.data, pkt.size);

6、销毁资源

将上述步骤使用的对象销毁。

if (swsContext)
{sws_freeContext(swsContext);
}
if (rgbFrame)
{av_frame_unref(rgbFrame);av_frame_free(&rgbFrame);
}
if (buffer)
{av_free(buffer);
}
av_packet_unref(&pkt);
if (ctx)
{avcodec_close(ctx);avcodec_free_context(&ctx);
}

二、完整代码

/// <summary>
/// 帧转图片
/// 如果外部提供的缓存长度不足则不会写入。
/// </summary>
/// <param name="frame">[in]视频帧</param>
/// <param name="codecID">[in]图片编码器ID,如jpg:AV_CODEC_ID_MJPEG，png:AV_CODEC_ID_PNG</param>
/// <param name="outbuf">[out]图片缓存，由外部提供</param>
/// <param name="outbufSize">[in]图片缓存长度</param>
/// <returns>返回图片实际长度</returns>
static int frameToImage(AVFrame* frame, enum AVCodecID codecID, uint8_t* outbuf, size_t outbufSize)
{int ret = 0;AVPacket pkt;AVCodec* codec;AVCodecContext* ctx = NULL;AVFrame* rgbFrame = NULL;uint8_t* buffer = NULL;struct SwsContext* swsContext = NULL;av_init_packet(&pkt);codec = avcodec_find_encoder(codecID);if (!codec){printf("avcodec_send_frame error %d", codecID);goto end;}if (!codec->pix_fmts){printf("unsupport pix format with codec %s", codec->name);goto end;}ctx = avcodec_alloc_context3(codec);ctx->bit_rate = 3000000;ctx->width = frame->width;ctx->height = frame->height;ctx->time_base.num = 1;ctx->time_base.den = 25;ctx->gop_size = 10;ctx->max_b_frames = 0;ctx->thread_count = 1;ctx->pix_fmt = *codec->pix_fmts;ret = avcodec_open2(ctx, codec, NULL);if (ret < 0){printf("avcodec_open2 error %d", ret);goto end;}if (frame->format != ctx->pix_fmt){rgbFrame = av_frame_alloc();if (rgbFrame == NULL){printf("av_frame_alloc  fail");goto end;}swsContext = sws_getContext(frame->width, frame->height, (enum AVPixelFormat)frame->format, frame->width, frame->height, ctx->pix_fmt, 1, NULL, NULL, NULL);if (!swsContext){printf("sws_getContext  fail");goto end;}int bufferSize = av_image_get_buffer_size(ctx->pix_fmt, frame->width, frame->height, 1) * 2;buffer = (unsigned char*)av_malloc(bufferSize);if (buffer == NULL){printf("buffer alloc fail:%d", bufferSize);goto end;}av_image_fill_arrays(rgbFrame->data, rgbFrame->linesize, buffer, ctx->pix_fmt, frame->width, frame->height, 1);if ((ret = sws_scale(swsContext, frame->data, frame->linesize, 0, frame->height, rgbFrame->data, rgbFrame->linesize)) < 0){printf("sws_scale error %d", ret);}rgbFrame->format = ctx->pix_fmt;rgbFrame->width = ctx->width;rgbFrame->height = ctx->height;ret = avcodec_send_frame(ctx, rgbFrame);}else{ret = avcodec_send_frame(ctx, frame);}if (ret < 0){printf("avcodec_send_frame error %d", ret);goto end;}ret = avcodec_receive_packet(ctx, &pkt);if (ret < 0){printf("avcodec_receive_packet error %d", ret);goto end;}if (pkt.size > 0 && pkt.size <= outbufSize)memcpy(outbuf, pkt.data, pkt.size);ret = pkt.size;
end:if (swsContext){sws_freeContext(swsContext);}if (rgbFrame){av_frame_unref(rgbFrame);av_frame_free(&rgbFrame);}if (buffer){av_free(buffer);}av_packet_unref(&pkt);if (ctx){avcodec_close(ctx);avcodec_free_context(&ctx);}return ret;
}

三、使用示例

1、视频帧保存jpg文件

void main() {AVFrame* frame;//视频解码得到的帧saveFrameToJpg(frame,"snapshot.jpg");
}

/// <summary>
/// 将视频帧保存为jpg图片
/// </summary>
/// <param name="frame">视频帧</param>
/// <param name="path">保存的路径</param>
void saveFrameToJpg(AVFrame*frame,const char*path) {//确保缓冲区长度大于图片,使用brga像素格式计算。如果是bmp或tiff依然可能超出长度，需要加一个头部长度，或直接乘以2。int bufSize = av_image_get_buffer_size(AV_PIX_FMT_BGRA, frame->width, frame->height, 64);//申请缓冲区uint8_t* buf = (uint8_t*)av_malloc(bufSize);//将视频帧转换成jpg图片，如果需要png则使用AV_CODEC_ID_PNGint picSize = frameToImage(frame, AV_CODEC_ID_MJPEG, buf, bufSize);//写入文件auto f = fopen(path, "wb+");if (f){fwrite(buf, sizeof(uint8_t), bufSize, f);fclose(f);}//释放缓冲区av_free(buf);
}

2、自定义数据构造AVFrame

void main() {uint8_t*frameData;//解码得到的视频数据AVFrame* frame=allocFrame(frameData,640,360,AV_PIX_FMT_YUV420P);saveFrameToJpg(frame,"snapshot.jpg");//此方法定义在示例1中av_frame_free(&frame);
}

/// <summary>
/// 通过裸数据生成avframe
/// </summary>
/// <param name="frameData">帧数据</param>
/// <param name="width">帧宽</param>
/// <param name="height">帧高</param>
/// <param name="format">像素格式</param>
/// <returns>avframe，使用完成后需要调用av_frame_free释放</returns>
AVFrame* allocFrame(uint8_t*frameData,int width,int height,AVPixelFormat format) {AVFrame* frame = av_frame_alloc();frame->width = width;frame->height = height;frame->format = format;av_image_fill_arrays(frame->data, frame->linesize, frameData, format, frame->width, frame->height, 64);return frame;
}