C to delphi method conversion of openCV sample code - delphi

Question

I have a code snippet from openCV example as follows:
CvScalar sum_line_pixels( IplImage* image, CvPoint pt1, CvPoint pt2 )
{
CvLineIterator iterator;
int blue_sum = 0, green_sum = 0, red_sum = 0;
int count = cvInitLineIterator( image, pt1, pt2, &iterator, 8, 0 );
for( int i = 0; i < count; i++ ){
blue_sum += iterator.ptr[0];
green_sum += iterator.ptr[1];
red_sum += iterator.ptr[2];
CV_NEXT_LINE_POINT(iterator);
/* print the pixel coordinates: demonstrates how to calculate the
coordinates */
{
int offset, x, y;
/* assume that ROI is not set, otherwise need to take it
into account. */
offset = iterator.ptr - (uchar*)(image->imageData);
y = offset/image->widthStep;
x = (offset - y*image->widthStep)/(3*sizeof(uchar)
/* size of pixel */);
printf("(%d,%d)\n", x, y );
}
}
return cvScalar( blue_sum, green_sum, red_sum );
}
I got stuck on the line:
offset = iterator.ptr - (uchar*)(image->imageData);
Iterator structure is:
PCvLineIterator = ^TCvLineIterator;
TCvLineIterator = packed record
ptr: ^UCHAR;
err: Integer;
plus_delta: Integer;
minus_delta: Integer;
plus_step: Integer;
minus_step: Integer;
end;
image->imageData is
imageData: PByte;
Could someone help me convert the offset line to delphi?
Thanks!

Answer 1

The line that calculates offset is simply calculating the number of bytes between the pointers iterator.ptr and image->imageData. Assuming you are using the same variable names a Delphi version of that code would be like this:
offset := PByte(iterator.ptr) - image.ImageData;
However, since you are using an older version of Delphi, the above code will not compile. Older Delphi versions (pre Delphi 2009) don't permit pointer arithmetic on types other than PAnsiChar. So you will need to write it like this:
offset := PAnsiChar(iterator.ptr) - PAnsiChar(image.ImageData);
I suspect that what is confusing you in the C code is (uchar*). That is the C syntax for a type cast.
As an aside, it is a mistake to use packed records for OpenCV structs. If you take a look at the C header files you will see that these structs are not packed. This is benign in the case of CvLineIterator since it has no padding, but you will get caught out somewhere down the line if you get into the bad habit of packing structs that should not be packed.