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[E-devel] Evas and alpha premul color data



  
	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.