Re: [boost] Promote Traits -- Review of Generic Image Library (GIL)

From: Ullrich Koethe (koethe_at_[hidden])
Date: 2006-10-17 15:18:28

Hi Lubomir,

Lubomir Bourdev wrote:

> We don't believe the type of the result could automatically be inferred
> simply from the types of arguments. The optimal type depends on your
> intent and the context.
> 

True. But the result type is usually not automatically inferred in VIGRA 
(it is inferred from the destination image you passed in). Promote traits 
are mainly used for the definition of intermediate types (e.g. temporary 
images).

Essentially, promotion traits make explicit what the compiler is doing 
anyway. For example, the type of an expression adding two signed chars is 
always int, regardless of the type of the variable the result is 
ultimately assigned to. We wanted to achieve the same for the addition of 
images. (We did not always get it right, but this can be corrected.)

> For example, VIGRA's promotion traits choose double as the promotion for
> int when dealing with floating point operations. Why not float? On many
> architectures float is faster, and precision may not be an issue in the
> context in which you use it.
> 

Right, some customization would be nice. But the defaults have worked very 
well for us most of the time, so we didn't feel an urgent need to change 
the behavior.

> Why should int always be the result type of adding two ints? 

Because that's what the C++ standard says (AFAIR).

> Int for summing two shorts
> will not overflow, but what if you are building, say, the integral image
> and you want to sum the values of all pixels in your image? 
> 

This can be handled by means of an appropriate Inspector with value_type 
double or BigInt. The  VIGRA Inspector concept is like a Functor, except 
that it doesn't return a value, but collects some statistics about the 
values it has seen. (Note that VIGRA is also defined in terms of concepts, 
although not quite as nice as GIL.)

> This is why in GIL we allow the type of the result to be specified
> explicitly upon invoking the operation. Promotion traits can at best be
> useful for a reasonable default, 

Resonable defaults are everything in generic programming, otherwise it's 
unusable.

> We feel that promotion traits belong
> in boost::type_traits and not in an image library, as the problem
> generalizes beyond images.
> 

I agree.

Regards
Ulli

-- 
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| Ullrich Koethe  Universitaet Hamburg / University of Hamburg   |
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