Convert CvImagePtr* (CvImage) channel to IplImage

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Hi,

I have extracted an Image message using cv_bridge as this:

cv_bridge::CvImagePtr cv_ptr;
cv_ptr = cv_bridge::toCvCopy(msg, enc::MONO8);

and i could convert it to an IplImage using:

IplImage *image2 = NULL;
allocateit( &image2, frame_size, IPL_DEPTH_8U, 1 );
image2->imageData = (char *) cv_ptr->image.data;

where allocateit is a function to allocate space. My problem comes when I want to read a color image as this:

cv_bridge::CvImagePtr cv_ptr;
cv_ptr = cv_bridge::toCvCopy(msg, enc::BGR8);
IplImage *image2 = NULL;
allocateit( &image2, frame_size, IPL_DEPTH_8U, 1 );

and want to copy one channel of *cv_ptr to the image to the IplImage *image2. I have tried:

image2->imageData = (char *) cv_ptr->image.data;

but it crashes,

cv::cvtColor(cv_ptr->image.data,image2->imageData,CV_RGB2GRAY);

but does not compile,

int from_to[] = {0,0};
cv::mixChannels(&(cv_ptr->image.data),1,image2,1, from_to,1);

does not compile either.

What would you recommend me to do? Thanks!