Overlay Transparency With Shading

If I have a simple 16-colour bitmap I can use it to overlap another
bitmap with transparency by somehow defining a "transparent
colour" (usually the bottom-left pixel colour). Whenever a pixel in
the transparent colour is found in the overlay, then the source colour
pixel is used, otherwise the overlay colour is used.

But how can I achieve this effect when I have 16.7M colours and have
shading at the edges of an overlay, and so need a resultant pixel
which is similarly shaded, but with the source colour, not the
"transparent colour".

Is there built-in windows or Delphi functionality which will do this.

Information, knowledge, how-to, please.

Alan Lloyd

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