"Kevin Myers" <[EMAIL PROTECTED]> writes:

> Does the value of the alpha channel provide the transparency level
> for each pixel, and if so, then what does that have to do with
> "alpha"?

Your guess is right, or rather, almost right. The alpha channel
actually provides the opacity levels for each pixel since usually a
fully transparent pixel has an alpha value of 0 while a fully opaque
pixel has an alpha value of 1.0 (or 100% or 255 or 65535 or ...,
depending on how the data is actually stored).

> Also, how is level of transparency actually applied in order compute
> the final display values for a pixel when a semi-transparent pixel
> is overlaid onto an underlying non-transparent pixel?

The formula is basically

  dest = (1 - alpha) * dest  +  alpha * src

We assume that the alpha value runs from 0.0 to 1.0 here.

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