[Haskell-cafe] Colour tutorial (Was: AC-Vector, AC-Colour and AC-EasyRaster-GTK)

[Derek Elkins]

On Sat, Jul 11, 2009 at 12:54 PM, Derek Elkins<derek.a.elkins at gmail.com> wrote:
> On Fri, Jul 10, 2009 at 12:42 AM, <roconnor at theorem.ca> wrote:
>> On Thu, 9 Jul 2009, roconnor at theorem.ca wrote:
>>
>>> You can use by lib without worrying about the CIE.  You can use my library
>>> without ever importing or using the word CIE.  However, the CIE stuff is
>>> there for those who need it.
>>>
>>> Perhaps I (maybe with some help) need to make a tutorial on the haskell
>>> wiki to try to make it less intimidating.
>>
>> Okay, I threw together a quick introduction at
>> <http://www.haskell.org/haskellwiki/Colour>.  Any changes, comments,
>> corrections, and addtions are welcome.  It's a wiki!
>>
>> The word CIE does occur at all in the document.
>
> I read this and it irks me that opaque is not a monoid homomorphism
> despite being the natural injection of non-transparent colours into
> semi-transparent colours with pureColour being the projection back.
> [Incidentally, you have a typo in pureColour, ac `over` mempty should
> be ac `over` black or opaque black presumably, or even opaque mempty,
> which I think was what you were going for, illustrating my point.]
> It's like defining mappend on Integers as (+) and on Reals as (*);
> actually, I think this is very close to what is actually happening.

I'm mistaken about the typo in pureColour, but luckily the mistake
just further illustrates my point.