Re: Re: associated alpha

I am still thinking about what to do about associated alpha and TIFF. 
For various reasons, GraphicsMagick does not handle alpha the same way 
that Photoshop does.  TIFF's associated alpha works the way Photoshop 
does, and Photoshop doesn't seem to want to have things any other way.

At this time, it seems that the only truely viable solution is to 
pre-multiply the pixel data on the way out, using the associated 
alpha.  The process would need to be reversed (as best possible) when 
an image is read which uses associated alpha.

Since GraphicsMagick does not use pre-multiplied alpha internally, it 
is able to store full color in pixels which are otherwise completely 
transparent.  With pre-multiplied alpha, entirely transparent pixels 
are converted to absolute black so there is no way to recover the 
original color value.

With pre-multiplied alpha, colormapped images become a problem since 
the existing colormap color values need to be pre-multiplied.  This 
essentially means that the image would need to be converted to 
DirectClass, pre-multiplied, and then color-reduced (if desired) 
again.

Bob
======================================
Bob Friesenhahn
[email protected]
http://www.simplesystems.org/users/bfriesen


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