IOS实时视频聊天问题

因为项目需要，在视频聊天时，要对视频流进行处理，看到CallViewController.m里面的captureOutput方法，似乎对视频输出进行了压缩和旋转，能否大概解释一下这块的逻辑？还用为什么要调用YUV420spRotate90方法，进行旋转？
- (void)captureOutput:(AVCaptureOutput *)captureOutput

didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer

       fromConnection:(AVCaptureConnection *)connection

{

    if (_callSession.status != eCallSessionStatusAccepted) {

        return;

    }

    

#warning 捕捉数据输出，根据自己需求可随意更改

    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);

    if(CVPixelBufferLockBaseAddress(imageBuffer, 0) == kCVReturnSuccess)

    {

        

//        UInt8 *bufferbasePtr = (UInt8 *)CVPixelBufferGetBaseAddress(imageBuffer);

        UInt8 *bufferPtr = (UInt8 *)CVPixelBufferGetBaseAddressOfPlane(imageBuffer, 0);

        UInt8 *bufferPtr1 = (UInt8 *)CVPixelBufferGetBaseAddressOfPlane(imageBuffer, 1);

//       printf("addr diff1:%d,diff2:%d\n",bufferPtr-bufferbasePtr,bufferPtr1-bufferPtr);

        

//        size_t buffeSize = CVPixelBufferGetDataSize(imageBuffer);

        size_t width = CVPixelBufferGetWidth(imageBuffer);

        size_t height = CVPixelBufferGetHeight(imageBuffer);

//        size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);

        size_t bytesrow0 = CVPixelBufferGetBytesPerRowOfPlane(imageBuffer, 0);

        size_t bytesrow1  = CVPixelBufferGetBytesPerRowOfPlane(imageBuffer, 1);

//        size_t bytesrow2 = CVPixelBufferGetBytesPerRowOfPlane(imageBuffer, 2);

//        printf("buffeSize:%d,width:%d,height:%d,bytesPerRow:%d,bytesrow0 :%d,bytesrow1 :%d,bytesrow2 :%d\n",buffeSize,width,height,bytesPerRow,bytesrow0,bytesrow1,bytesrow2);




        if (_imageDataBuffer == nil) {

            _imageDataBuffer = (UInt8 *)malloc(width * height * 3 / 2);

        }

        UInt8 *pY = bufferPtr;

        UInt8 *pUV = bufferPtr1;

        UInt8 *pU = _imageDataBuffer + width * height;

        UInt8 *pV = pU + width * height / 4;

        for(int i =0; i < height; i++)

        {

            memcpy(_imageDataBuffer + i * width, pY + i * bytesrow0, width);

        }

        

        for(int j = 0; j < height / 2; j++)

        {

            for(int i = 0; i < width / 2; i++)

            {

                *(pU++) = pUV[i<<1];

                *(pV++) = pUV[(i<<1) + 1];

            }

            pUV += bytesrow1;

        }

        

        YUV420spRotate90(bufferPtr, _imageDataBuffer, width, height);

        [[EaseMob sharedInstance].callManager processPreviewData:(char *)bufferPtr width:width height:height];

        

        /*We unlock the buffer*/

        CVPixelBufferUnlockBaseAddress(imageBuffer, 0);

    }

}