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From: Toby Thain <toby@smartgames.ca>
Subject: Re: [Tiff] is there alpha component present in
	GrayscaleorPalettecolorimage
Date: Sat, 16 Sep 2006 18:20:48 -0400
To: "Joris" <joris.at.lebbeke@skynet.be>
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Status:   


On 16-Sep-06, at 6:18 PM, Joris wrote:

> Toby,
>
> Toby Thain wrote:
>> Why is that challenging? You just omit that index from colour
>> matching consideration. In most cases, just thresholding alpha to
>> that index would be sufficient for opacity handling.
>
> You're clearly talking about the case where one palette entry is  
> made to
> code full transparency, and others are made to have no transparency at
> all.

I am. I'll postpone worrying about the more general case until I meet  
it in real life. :)

--T

>
> I'm refering instead to the general case of alpha inside palette
> entries, where for example you could have RGBA values like
> (20,20,20,100) and (20,20,20,200). That's the general case when we are
> assessing transparency inside palettes. Thus, dithering involves
> calculating differences between RGBA values. Aside from the fact that
> RGB is not a colorspace proportional to vision but a space  
> proportional
> to CRT voltages and thus the normal euclidean distance between RGB
> values doesn't make much sense in a dithering algorithm, the real
> challenge here is to somehow involve alpha in that scheme. Is RGBA
> (20,20,20,20) closest to RGBA (20,20,100,20), or is it closest to RGBA
> (20,20,20,100)? Put more generally, you need to weigh color difference
> against alpha difference, and it's not quite clear how to avoid making
> rather arbitrary weight decisions.
>
>
> Best regards,
>
> Joris Van Damme
> info@awaresystems.be
> http://www.awaresystems.be/
> Download your free TIFF tag viewer for windows here:
> http://www.awaresystems.be/imaging/tiff/astifftagviewer.html
>

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