On 18 February 2013 16:52, Phil Endecott <spam_from_boost_dev@chezphil.org> wrote:
Christian Henning wrote:
3) scanline_read_iterator is an input iterator, do you also plan output one too?
Not before the release. But logically I don't think there is much interest. What do you think?
The example that I originally presented for needing line-by-line access was to read in a very large image and cut it into tiles, without needing RAM proportional to the image size.
Phil, Thanks for bringing this subject. It would be an important use case for myself too. I have assumed, at some point we may achieve this by the following: * add tile_read_iterator which would dereference to tiles as views * use scanline_read_iterator to access lines of current view (a memory of tile data) I'm thinking along the lines of TIFF spec, perhaps similar approach could be based applied to strips-based access combined with scanline_read_iteartor.
c) Any particular reason you don't use result of the iterator deference directly?
I agree that the way the iterator is dereferenced in that code ("*it;") is non-obvious. I can't think of any other case where dereferencing an iterator has such significant side-effects i.e. it writes into an implied buffer and advances itself. More conventional code might look like:
buffer_t buffer = ......; iterator begin(.....); iterator end(); for( iterator it = begin; it != end; ++it ) { const buffer_t& b = *it; copy_pixels( .... ); }
Here I am explicitly (a) incrementing the iterator, and (b) using the result of the dereference. I think you can do that and still get the same performance.
Thanks for completing the picture, I agree with that. Best regards, -- Mateusz Loskot, http://mateusz.loskot.net