Re: [Boost-users] Boost-users Digest, Vol 2650, Issue 4

22 Apr 2011

      On Mar 07, 2011, at 07:44 AM, Robert Lupton the Good  wrote:
...
...
Sorry for the slow response. I simply defined the following:
typedef boost::gil::pixel gray_float_pixel_t;
typedef boost::gil::image gray_float_image_t;
typedef gray_float_image_t::view_t gray_float_view_t;
typedef gray_float_image_t::const_view_t gray_float_const_view_t;
As Dr. Bourdev advised, I do not use with these typedefs any of the color conversion operations offered by GIL. Apparently those rely on the scaling properties of the float wrapper type.
So far, all I have done with these typedefs is ...
1. construct views using interleaved_view and pixel data pointed to by [const] float*, as illustrated near the beginning of the GIL tutorial
2. create x- and y- iterators for the views to apply row- and column-oriented algorithms
Sorry for _my_ slow response (I was in Japan). I don't care about colour conversions, but I think that these types will convert to integral types with scaling unless you add more boilerplate. I added the following -- but it may not all have been needed; this was quite a while ago and I was new to gil. I also have some remaining conversion problems where I have to rewrite expressions to make them compile, but this may be something else. Anyway, I'd really like to see native gil support for a complete set of NON-scaling types.
namespace boost { namespace gil {
/*
* Define a type that's a pure float, without scaling into [0, 1]
*/
typedef float bits32f_noscale;
GIL_DEFINE_BASE_TYPEDEFS(32f_noscale, gray)
GIL_DEFINE_ALL_TYPEDEFS_INTERNAL(32f_noscale, dev2n, devicen_t<2>, devicen_layout_t<2>)
template<> struct channel_multiplier : public std::binary_function {
bits32f_noscale operator()(bits32f_noscale a, bits32f_noscale b) const { return a*b; }
};
template <typename DstChannelV>
struct channel_converter :
public std::unary_function {
DstChannelV operator()(bits32f_noscale x) const { return DstChannelV(x + 0.5f); }
};
template <typename SrcChannelV>
struct channel_converter :
public std::unary_function {
bits32f_noscale operator()(SrcChannelV x) const { return bits32f_noscale(x); }
};
//
// Totally specialised templates to resolve ambiguities
//
#define CONVERT_NOOP(T1, T2) \
template <> \
struct channel_converter : public std::unary_function { \
T2 operator()(T1 x) const { return static_cast<T2>(x); } \
}; \
\
template <> \
struct channel_converter : public std::unary_function { \
T1 operator()(T2 x) const { return static_cast<T1>(x); } \
}
CONVERT_NOOP(unsigned char, short);
CONVERT_NOOP(unsigned char, unsigned short);
CONVERT_NOOP(unsigned char, int);
CONVERT_NOOP(unsigned short, short);
CONVERT_NOOP(unsigned short, int);
CONVERT_NOOP(short, int);
#undef CONVERT_NOOP
Using only the four typedefs that I provided earlier, I don't see any sign that the GIL classes that I'm using are scaling the floats behind the curtain.  I haven't been able to follow the details of the metaprogramming inside GIL, but I have observed two key facts that suggest that no scaling is occuring:

   1. The end result has the scale I expect (without scaling).
   2. std::less works with InIter = gray_float_const_view_t::x_iterator (or y_iterator).

Hopefully, Christian or Lubomir will provide a more authoritative response  <nudge/>

Re: [Boost-users] Boost-users Digest, Vol 2650, Issue 4

Curtis Gehman