Wasn't it SharkD who wrote:
>On 10/28/2009 11:01 PM, Mike Williams wrote:
>> It will not be possible to find graphics applications that
>> would be able to manipulate the image as if it were greyscale.
>
>What can you do in grayscale that you can't do in RGB?

One thing that would go wrong is that bits wouldn't be carried between
channels.

Consider the border of regions with these 32-bit greyscale values
        00000001-00000000-00000000-00000000
        00000000-00000000-00000000-00000000
What we'd like is for a three-pixel-radius blur to output values like
        00000001-00000000-00000000-00000000
        00000000-11000000-00000000-00000000
        00000000-10000000-00000000-00000000
        00000000-01000000-00000000-00000000
        00000000-00000000-00000000-00000000
as we approach and cross the boundary, but if the channels are blurred
independently that can't happen.

An application that treats the image as RGBA won't produce bright blue
by blurring together what it sees as dark red and black.

-- 
Mike Williams
Gentleman of Leisure

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