Hi Carol,

Carol Spears wrote:
> this makes no sense to me.  it might be because i do not understand
> alpha.  the way i understand it is that each pixel is assigned a number
> that represents a color ....

That's right. The number is between 0 and 255. And that number
gets mapped to the full color (that is, an RGB triplet in the
GIMP). But there is no reason why this index could not refer to a
different colour, say an RGBA color. So that a 50% opaque red
could be part of your palette in index 23 (for example). 

> so you might be talking about (a light just turned on) a number that
> represents a color that has four parts.

Yup :) That's the one.

> doh.  except when you are still
> limited to 256 this will really effect the actual colors and make the
> image less eh, colorful after the indexing is done.  meaning if all
> those shades of alpha need a different color from the 256 that can be
> used then less real colors can be used.
> is this how that works?

Well, kind of... since there are 256*256*256 RGB colors, and
256*256*256*256 RGBA colors, you end up with certain types of
images that index less well, because you're picking 256 colors
from a bigger set. But most images would do quite well. And you
always have the option of throwing away the alpha channel, or
thresholding it, if you want.


        David Neary,
        Lyon, France
CV: http://dneary.free.fr/CV/
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