Re: [boost] Bitmap pixel iterators?

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2001-03-24 14:02:32

On Fri, 23 Mar 2001, Michael D. Crawford wrote:
crawfo>     // Alternatively, if iterator support is built into ZDCPixmap then we can
crawfo>     // initialize from a member function that returns an iterator:
crawfo>     //
crawfo>     // PixelIterator iter( myPix.TopLeft() );

An alternative that avoids touching the existing XDCPixmap is to
write TopLeft as a free function instead of a member function.

PixelIterator iter = top_left(myPix);

crawfo> But there's a problem - how can they be used two-dimensionally in a loop?  One
crawfo> can only loop over one index at a time.  One possible solution although a
crawfo> little ugly is to put inner loop iterators inside to limit loops

There's a couple more alternatives here.

1. Use two iterators, the "outer iterator" iterates over rows
of the image, and the "inner iterator" iterates  down each row.

PixelRowIterator row_iter = rows_begin(myPix);
for (; row_iter != rows_end(myPix); ++row_iter) {
  PixelIterator iter = begin(*row_iter);
  for (; iter != end(*row_iter); ++iter)
    ...
}

MTL provides this kind of interface for matrices.

2. Use a single iterator that knows how to wrap around. The
 problem with this approach is that it introduces and if statement
 into the inner loop, which hurts performance.

Cheers,
Jeremy

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 Jeremy Siek                        www: http://www.lsc.nd.edu/~jsiek/
 Ph.D. Candidate                  email: jsiek_at_[hidden]
 Univ. of Notre Dame         work phone: [phone]
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