对rgb图像转yuv，可以直接对像素进行处理实现，但可能写出来的代码执行效率不高，发现ffmpeg的执行速度很快，后续再研究其转换的函数sws_scale的源码，下面是转换实现的代码，

SwsContext* swsCtx = NULL;

swsCtx = sws_getContext(WIDTH, HEIGHT, AV_PIX_FMT_RGB24, WIDTH, HEIGHT, AV_PIX_FMT_YUV420P, SWS_BICUBIC, NULL, NULL, NULL);

uint8_t* rgbBuf = (uint8_t*)av_malloc(frameSize * sizeof(uint8_t));

AVFrame* rgbFrame = av_frame_alloc();

rgbFrame->format = AV_PIX_FMT_BGR24;

rgbFrame->width = WIDTH;

rgbFrame->height = HEIGHT;

rgbFrame->linesize[0] = WIDTH * 3;

av_image_fill_arrays(rgbFrame->data, rgbFrame->linesize, rgbBuf, AV_PIX_FMT_RGB24, WIDTH, HEIGHT, 1);

int outSliceH = sws_scale(swsCtx, rgbFrame->data, rgbFrame->linesize, 0, HEIGHT, frame->data, frame->linesize);