Re: [Boost-users] [GIL] Some questions about 2D iteration

Subject: Re: [Boost-users] [GIL] Some questions about 2D iteration
From: Brett Gmoser (bgmoser_at_[hidden])
Date: 2010-04-09 03:26:33

Nicolas,

> glad I could help !

I'm glad you saw my message! Sorry it took me a day or two to get back 
into the swing of this project, I was worried you wouldn't see my reply 
just because I wanted to thank you. The previous one and this one was a 
huge help.

> Actually, you're approach is the good one. I should not have said '1d 
> iterators', its confusing. I only wanted to warn you that according to 
> http://www.boost.org/doc/libs/1_42_0/libs/gil/doc/html/giltutorial.html#OneDIteratorsSec 
> :

Actually, '1d iterators' makes perfect sense now that I understand what 
you mean (my fault for not understanding earlier). I can definitely see 
why that would be less efficient. I guess there is a padding to the 
right and bottom side of the image (I think you graphics guys call it 
the "stride"?), since by the actual data layout of the image (just an 
array) you would think the 1d iteration would actually be faster as long 
as you didn't need to know your exact X/Y coordinate.

> Just one comment about your loop, as you mentioned a need for 
> performance, be careful about the order of the loops, as the 
> performance of your traversal may vary with the memory layout of the 
> pixel data of your view. Most of the time, its recommended to iterate 
> over 'y' in the outer loop  :
>
> for(int y = 0; y < height, ++x)
>   for(x_iterator x = src.row_begin(x), end = src.row_end(); x != end; ++x)
>     /* ... */
>
> This allows, considering a typical image memory layout, to traverse 
> raw images in a cache coherent way. Note also that calling row_end() 
> at every pixel may not be nicely optimized away by your compiler.

Again, a most excellent tip! I had a bunch with x as the outer loop, 
I'll switch them over tomorrow, as well as the calling row_end() on 
every pixel. Thanks again for your help. Developing this small project 
(a CAPTCHA, if I hadn't mentioned) has been fun with GIL - it's rare I 
get to do something outside of my normal day-to-day stuff, and this is 
what makes programming fun.

That said, if anybody on this list reading this happens to be in need of 
a CAPTCHA generator using GIL (I didn't find anything that was already 
made using C++ and/or GIL), I'd be happy to release my code into the 
public domain. Some samples of the same word generated a bunch of times 
is here: http://www.codexterous.com/captchas/ - and it's actually much 
improved now (more features added, such as the anti-segmentation line is 
waved).

Brett