Question

I have following function. I want to copy back some data from Mat to
IplImage * type and return it to the main control.
This is silly, but I couldn’t find a proper way to do this! The cheat
sheet havent say anything about Mat->IplImage * conversion WITH data
copy (since I need it outside the function).

Any idea or pointer is appreciated.
Best
–Ali

int test(IplImage **srcImage, int num_images)
{
  vector<Mat> images(num_images); 
  for (int i = 0; i < num_images; ++i) 
  { 
    images[i] = Mat(srcImage[i]); // I guess should be correct! 
      .... 
     // some manipulations on images[i] 
  } 

  // this should be wrong!
  for (int i = 0; i < num_images; ++i) 
  { 
     cvReleaseImage(&srcImage[i]); 
     srcImage[i] = new IplImage(images[i]); 
     images[i].clear();
   } 
  return 0;
 }

Answer 1

Short version: Convert to a temporary IplImage, then use cvCopy.

However, there are several issues with your code:

int test(IplImage **srcImage, int num_images)
{
  vector<Mat> images(num_images); 
  for (int i = 0; i < num_images; ++i) 
  { 
    images[i] = Mat(srcImage[i]); // I guess should be correct!

so far, yes.

      .... 
     // some manipulations on images[i] 

If the manipulations are in-place (don’t reallocate the Mats), you don’t need to copy the data back, since the Mat constructor didn’t copy the data in the first place. If you do reallocate, then …

  } 

  // this should be wrong!
  for (int i = 0; i < num_images; ++i) 
  { 
     cvReleaseImage(&srcImage[i]);

This can be problematic. images[i] might still be using the same memory.

     srcImage[i] = new IplImage(images[i]);

new IplImage isn’t going to do you any good. It doesn’t have meaningful constructors, use cvCreateImage.

     images[i].clear();

This is not neccessary, as the vector goes out of scope anyway.

   } 
  return 0;
 }

The last loop should look something like this:

for (int i = 0; i < num_images; ++i) { 
     IplImage* old = srcImage[i]; // store pointer for later deallocation in case of shared data
     IplImage src = images[i];
     srcImage[i] = cvCreateImage(...); // fill appropriate parameters here. If you didn't change size/type/etc, you might not need to create/deallocate(*)
     cvCopy(&src, srcImage[i]);
     cvReleaseImage(&old); // now it is safe to deallocate(*)
}