Re: [Boost-users] [GIL] gray pixel iterator in generic algorithm

Subject: Re: [Boost-users] [GIL] gray pixel iterator in generic algorithm
From: Curtis Gehman (curtis2006_at_[hidden])
Date: 2011-02-16 17:13:37

Lubomir,

I'm finally getting back to this.  In addition to what you suggested, I boldly (or naively) tried to declare const versions.

typedef boost::gil::pixel<const float, boost::gil::gray_layout_t> gray_float_const_pixel_t;
typedef boost::gil::image<gray_float_const_pixel_t,false> gray_float_const_image_t;
typedef gray_float_const_image_t::view_t gray_float_const_view_t;

When VC++2010 gets into the last line, it stops after reporting the following error:

1>c:\perforce\software\thirdparty\boost\boost_1_44_0\boost\gil\color_base.hpp(82): error C2758: 'boost::gil::detail::homogeneous_color_base<Element,Layout,K>::_v0' : must be initialized in constructor base/member initializer list
1>          with
1>          [
1>              Element=const float,
1>              Layout=boost::gil::gray_layout_t,
1>              K=1
1>          ]
1>          c:\perforce\software\thirdparty\boost\boost_1_44_0\boost\gil\color_base.hpp(76) : see declaration of 'boost::gil::detail::homogeneous_color_base<Element,Layout,K>::_v0'
1>          with
1>          [
1>              Element=const float,
1>              Layout=boost::gil::gray_layout_t,
1>              K=1
1>          ]
..

I'm no metaprogramming expert.  So figuring this out is rather intimidating.  Is there anything obvious that you can suggest?

Note that I'm using Boost 1.44.

Thanks.

On Dec 22, 2010, at 11:23 AM, Lubomir Bourdev <lbourdev_at_[hidden]> wrote:

> Hi Curtis,
>
> GIL's default float type is not "float" but a special float wrapper to ensure that the range is 0..1
> The built-in float has a range from -infinity to infinity and that does not work well when converting a value from another channel type.
> If you want an image with a standard float you can create it like this (untested)
>
> typedef pixel<float,gray_layout_t> gray_float_pixel_t;
> typedef image<gray_float_pixel_t,false> gray_float_image_t;
> typedef gray_float_image_t::view_t gray_float_view_t;
> ...
>
>
> However, don't use such images with algorithms that require channel ranges, such as color conversion.
>
> Lubomir
>
>
> On Dec 20, 2010, at 11:53 PM, Curtis Gehman wrote:
>
>> I'm trying to use GIL pixel iterators in a generic STL-style algorithm.  I'm primarily using GIL for the generic 2D iterators.  And in my context, I'm only interested in grayscale images.
>>
>> The value_type associated with GIL iterators is a pixel type, which typically is an object representing multiple channels (e.g., colors).  It makes sense that comparison operators (e.g., <) are not defined for general color pixel types.
>>
>> For grayscale images, however, it makes sense that common arithmetic and comparison operators would be defined for the pixel type.  This seems to be the case for the 8-bit grayscale GIL type (gray8_view_t).  The iterator's value_type appears to be seamlessly evaluated to an unsigned char, exactly as I would expect.
>>
>> When I try to use a float image view type, however, the same magic doesn't seem to happen.  Below is some toy code that demonstrates the issue.  When the last line is knocked out with "//", the code compiles.  But with that second template instantiation (for floats) there, the compiler (gcc 42 and VC++2010) exits when it does not find an operator< for the iterator's value_type when instantiating for gray32fc_view_t.
>>
>> I suppose that I can roll my own less<iterator_traits<SrcView::x_iterator>::value_type> functors.  But I just started using GIL, and I'm hoping that there is a simpler solution.
>>
>> Any ideas?
>>
>>
>> #include <algorithm>
>> #include <iterator>
>> #include <vector>
>>
>> #include <boost/gil/gil_all.hpp>
>>
>> namespace g = boost::gil;
>>
>> using std::vector;
>> using std::iterator_traits;
>>
>> template <typename SrcView> inline
>> vector<typename iterator_traits<typename SrcView::x_iterator>::value_type>
>> row_min(const SrcView& src)
>> {
>> typedef typename iterator_traits<typename SrcView::x_iterator>::value_type value_t;
>> vector<value_t> result;
>> for (int y = 0; y < src.height(); ++y)
>> {
>> result.push_back(*std::min_element(src.row_begin(y), src.row_end(y)));
>> }
>> return result;
>> }
>>
>>
>> // Explicit template instantiation
>>
>> // Works for gray8c_view_t.
>> template vector<iterator_traits<g::gray8c_view_t::x_iterator>::value_type> row_min(const g::gray8c_view_t& src);
>>
>> // Compile error for gray32fc_view_t.
>> // No operator< for iterator_traits<g::gray32fc_view_t::x_iterator>::value_type.
>> template vector<iterator_traits<g::gray32fc_view_t::x_iterator>::value_type> row_min(const g::gray32fc_view_t& src);
>>
>> <ATT00001..txt>
>
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