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Re: [E-devel] Evas and alpha premul color data



I'm forwarding the messages that Jose sent me because he seems to be
detected as a spammer by the ML service.

----- Forwarded Message -----

Date: Sat, 24 Jun 2006 07:10:42 GMT
From: "jose_ogp@juno.com" <jose_ogp@juno.com>
To_: simon.treny@free.fr
Subject: Re: [E-devel] Evas and alpha premul color data


 
> > And I forgot to mention another problem I have in mind. 
> > 
> > For now, in edje, if you want to do a color transition from 
> > color1="255 0 0 255" to color2="0 255 0 0", you have to  
> > create 2 states, the first associated to color1, and the second 
> > to color2. 
> >  
> > With "alpha premul" colors, it will become (if I understand  
> > correctly): color1'="255 0 0 255" and color2'="0 0 0 0". 
> >  
> > So how Edje will guess it has to make a transition from red 
> > to green? Or we'll have to keep "normal" colors in edje, but 
> > it won't be coherent with the API of evas :/ 
> >  
> > Simon 
> >  
>  
> 	Ummm, that's odd... Edje should not consider the color 
> 0 255 0 0 as "green". It should be transparent - that's what 
> evas will do with such an input color.. if an obj's color has 
> alpha = 0, the obj is transparent. 
>	To transition to green one should set color2 to be, 
> green! ie. 0 255 0 255. 
>  
> 	If edje is doing otherwise then it has a bug, or 
> something funky :( 
 
	No, it's ok :)  I see what you mean..  How would it 
know that it should transition thru something half-way that 
is partly-red partly-green partly-transparent, ie. that at the 
half-way point of the transition the color should be set to 
128 128 0 128 
 
	It wouldn't, that's correct. That's part of the problem 
of dealing with non-premul colors, it gives a bad idea of what 
is actually going to be displayed. 
 
	In order to get a transition that varies from opaque-red, 
to half-opaque-brownish, to transparent, you'd need to transition 
between these *three* premul colors, ie. you'd need three premul 
colors to transition from, not just two. 
 
	The thing about using premul colors is that it gives 
you a very exact, direct idea of what you'll actually get 
displayed, whereas non-premul colors give an indirect idea 
of such. 
 
	This is why I think it's such a bad idea to mix the 
two -- it creates confusion as to what should be happenning. 
If you stick to using premul colors, you always get a good 
idea of what is going to be displayed. 
 
   jose. 
 
 
PS. 
   Please feel free to forward/copy these emails to the elist.