Re: Color components and fixed point numbers

[patkwee@web.de (Pat)]

Thank you very much for your answer.

> First fix your type declarations ... Besides, you also omitted the semicolons.

Sorry, was just too lazy...

> Second, you got the numbers wrong, or you want something that many 
> compilers would not support as a value for 'Small, even with a rep 
> clause.  You should use:

I'm very new to Ada, so what is rep clause?

> Since that is how the hardware works, that should be what you expect.  So 
> Small_RGB'(1.0,1.0,1.0) and Wide_RGB'(1.0,1.0,1.0) are different colors. 
>  Or better stated: Small_RGB(255.0/256, 255.0/256. 255.0/256) is not 
> equal to Wide_RGB(65535.0/65536, 65535.0/65536, 65535.0/65536), and you 
> should not expect them to be equal.

Yes, this is the reason why I used 255 and 65535 as denominator. I am
not a
expert on color conversion or representation, but I think 0.0 should
represent
the darkest and 1.0 the brigthest color independent from the
representation as
integers. So 255 and 65535 should be the same brightness.

If I would use integer arithmetic I would do it like this:

  type Small_Comp is range 0 .. 255;
  type Wide_Comp is range 0 .. 65535;

  --  Small_Comp'Last and Wide_Comp'Last should represent the same
brightest color
  --  Small_COmp'First and Wide_Comp'Last should represent the same
darkest color

  w : Wide_Comp;
  s : Small_Comp:=?;

  w := s*65535 / 255; -- equals w:=s*257
  s := w / 257;

I though the reason why 1.0/255 is not working may be that there is no
exact
floating point representation of 1.0/255 and so due to rounding 255 *
(1.0/255) < 1.0 .


> Can you force an implementation to make the two brightest whites equal? 
>   It is not too difficult.  Most compilers will give the expected result 
> for:
>
>   type Small_Component is delta 1.0/256 range 1.0/256 .. 1.0;
>   type Wide_Component is delta 1.0/65536 range 1.0/65536 .. 1.0;

I think this will only move my problem, so that now the darkest colors
are
not identical.

Thank you very much, Pat