Re: [Boost-users] [GIL] Why does 'get_color' take a tag object instead of its type only?

Subject: Re: [Boost-users] [GIL] Why does 'get_color' take a tag object instead of its type only?
From: Christian Henning (chhenning_at_[hidden])
Date: 2013-01-29 10:26:23

>> > I'm curious of why gil::get_color(pixel, tag) is defined that way.
>> > Any need to take the tag value?
>> >
>> > If not, I'd prefer gil::get_color<Tag>(pixel) or just
>> > gil::get<Tag>(pixel)
>> > or even Tag::get(pixel).
>>
>> You can use the "at_c" function.
>>
>> rgb8_pixel_t p( 10, 20, 30);
>> unsigned char c = at_c<0>( p );
>> assert( c == 10 );
>
>
> That doesn't really answer my question...
> I just wonder the rationale for taking the tag value.
>
> I know there are at_c/semantic_at_c things, but they don't fit my need.
> Those functions are useful if I'm an end-user who knows the exact format,
> but I'm writing a 2D graphics lib building on GIL that intends to support
> all types the user gave,
> so index means nothing here, I must get the component by tag, thus
> get<red_t>(c) won't compile if the user gives cmyk.

By tag is useful when dealing with swapped channels, like rgb and bgr.
For instance when reading a bmp file I read it as bgr but the end user
most likely wants the rgb format.

I'm not sure I follow your arguments. Why would you care for the tag?
Just take the number of channels and enumerate over them. Is that not
general enough for you?

But maybe you wanna specialize your algorithm by the pixel layout
type, e.g. rgb_layout_t.

>
>>
>> >
>> > And if possible, I hope GIL would just make pixel a Fusion sequence.
>> >
>>
>> I'm very interested in ideas to improve gil! Since I'm not familiar
>> with a fusion sequence could you elaborate a little further.
>
>
> I imagine it to be an Associative Sequence in Fusion (naturally or adapted).
> So I can just use the algorithms provided by Fusion, which seem to overwhelm
> the static_xxxs in GIL.

Can it handle swapped channels? Like:

rgb8_pixel_t a( 10, 20, 30 );
bgr8_pixel_t b( 30, 20, 10 );

assert( get_color( a, red_t() ) == get_color( b, red_t() ) );

Regards,
Christian