将视频从彩色转换为灰度

我想读入彩色视频，并将其输出为灰度视频。下面是我的代码，运行得很好，但输出的是我的同色视频。有人能发现问题吗？

//Convert a video to grayscale
//argv[1]: input video file
//argv[2]: name of new output file
#include "cv.h"
#include "highgui.h"
int main( int argc, char* argv ) {
CvCapture* capture = cvCaptureFromAVI("C:\walking\lady walking.avi");
if(!capture){
    return -1;
}
IplImage *bgr_frame=cvQueryFrame(capture);//Init the video read
double fps = cvGetCaptureProperty (
    capture,
    CV_CAP_PROP_FPS
    );
CvSize size = cvSize(
    (int)cvGetCaptureProperty( capture, CV_CAP_PROP_FRAME_WIDTH),
    (int)cvGetCaptureProperty( capture, CV_CAP_PROP_FRAME_HEIGHT)
    );
CvVideoWriter *writer = cvCreateVideoWriter(
    "Output video",//filename for new file
    CV_FOURCC('M','J','P','G'),//video codec to compress video stream
    fps,
    size
    );
IplImage* logpolar_frame = cvCreateImage(
    size,
    IPL_DEPTH_8U,
    3
    );
while( (bgr_frame=cvQueryFrame(capture)) != NULL ) {
    cvLogPolar( bgr_frame, logpolar_frame,
        cvPoint2D32f(bgr_frame->width/2,
        bgr_frame->height/2),
        40,
        CV_INTER_LINEAR+CV_WARP_FILL_OUTLIERS );
    cvWriteFrame( writer, logpolar_frame );
}
cvReleaseVideoWriter( &writer );
cvReleaseImage( &logpolar_frame );
cvReleaseCapture( &capture );
IplImage*image;
CvCapture*recorder = cvCaptureFromAVI("C:\walking\lady walking.avi");
cvNamedWindow("Output video",1);
while(1)    {
    image=cvQueryFrame(recorder);   
    if(image==NULL)
    {
        break;
    }
    cvShowImage("Output video",image);
    char c= cvWaitKey(30);
    if(c==27)
    {
        break;
    }
}
cvReleaseCapture( &capture );
cvDestroyWindow( "Output video" );
return 0;
}