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



Hi Jose,

Mm.. It seems to be a very important change indeed. If I understand
correctly, it means that all the apps that calls
evas_object_color_set() and every edje theme will have to be updated.
This is a problem, but it can be quickly solved. The most important
problem imho is that "alpha premul" colors are really less intuitive
than "normal" colors, at least for me :)

For example, when you are designing an edje theme, you often change
the alpha of a part to try different values, and to see which alpha
value looks the best. So now, every alpha change will need a change of
the r, g, b components too, right?

Another problem, but maybe I'm wrong on that point, is for transition.
A simple example, you want an object to fade out. For now, you only
have to use a timer that makes the alpha of the object decrease
progressively. It's rather intuive for now. But with this change,
you'll also have to update the rgb values, no?!

For me, if the only advantage is to avoid some confusion inside the
evas's code (so for now, the evas-user is not confused), I think it
doesn't worth it. We'll avoid a confusion in the code of evas, but
we'll probably confuse the user (i.e. the coder that uses evas).
But maybe i'm wrong on the 2 examples I've given, maybe the change is
not so important.

Regards :)
Simon TRENY <MoOm>


On Fri, 23 Jun 2006 06:20:47 GMT,
"jose_ogp@juno.com" <jose_ogp@juno.com> wrote :

>   
> 	Just a note to all here..  
>   
> 	Sometime within the next week or three, I may send a very  
> large evas patch (probably in 3 or 4 parts) which entails a very  
> *drastic* semantic change for evas --- namely, it will support  
> only "alpha premultiplied" image data and colors which are given  
> in argb format.  
>   
> 	This would cause some pain, and break some things.. but in  
> the long run, I believe it is better.  
>  
> 	However, one has to learn to think in/with "alpha premul" 
> colors.  
>   
> 	What does this mean????  I'll explain it a bit here.  
>   
> 	In argb color space where colors are specified by 4-tuples  
> a, r, g, b, representing the alpha, red, green, and blue components  
> of a color, that this is an 'alpha premul' color means that the  
> alpha component 'a' has value >= the r,g,b components.  
>   
> 	That's all it means, nothing more.  
>   
> 	One way to obtain an alpha premul color from any color,  
> is to replace the old r,g,b values by new r',g',b' values given by  
> r' = (a*r)/255, g' = (a*g)/255, b' = (a*b)/255, hence the term  
> "alpha premultiplied".  
>   
> 	Where will evas be affected by this? Anywhere you see image  
> or gradient objects take argb32 data... this data will now be  
> *assumed* to be alpha premul by default. If you pass in data, or  
> specify colors, that are *not* premul, then you will get mostly  
> junk results.  
> 	Similarly, any api function that refers to setting a color  
> on an object somehow, it will now be assumed that the color is  
> alpha premul.. If they aren't, you will get mostly junk results.  
>   
> 	"Why not simply keep the interfaces to accept non premul  
> colors, as is now the case, and just do things internally with  
> premul if that's such a great thing...?"   
>   
> 	Because mixing premul and non-premul color spaces leads to  
> immense amounts of confusion, and is error prone..  
> 	I have seen very knowledgable and experienced people get  
> wrapped around in the confusion caused by this.  
>   
> 	Learn to think with alpha premul colors, this may seem  
> drastic.. and it is indeed.. but it's far more consistent for a  
> wide variety of things, and even more 'natural' once one gets used  
> to the change.  
>   
>   
> 	If this change seems unacceptable to most here - and I can  
> certainly understand that - or would prefer some mixture of premul  
> and non premul... that's fine, I will leave evas alone then.  
>   
> 	But I personally want nothing to do with mixing these two  
> color spaces. It's a bad idea.  
>   
>   
>    jose.  
>   
> 
> 
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