Very Computer

1. PS7: Layer Transparency vs Alpha Channel(s)?

In photoshop, besides the color channels (usually R,G and B or C,Y,M
and K) a layer can have transparency and multiple alpha channels.
Obviously, the transparency can be regarded as an "extra channel"
which you just don't get to see or edit explicitly.

I have 3 questions about transparency and alpha channels:

1. How do I "convert" one to another? To use an alpha channel as
transparency, I ctrl-click on the alpha channel I want, invert
selection and delete that selection from the particular layer (i.e.
from its RGB or CMYK channels). The data from the alpha channel is now
exactly the transparency. However this does not work correctly if the
layer is already transparent at some spots: areas in the layer which
were transparent, will stay transparent after this trick - no matter
what the alpha channel looks like in those areas.

Is there a correct way to achieve this, I mean something that
'overwrites' the original transparency with the data from the alpha
channel? And how about the reverse: getting the transparency data into
an alpha channel?

2. When saving images, some file formats (such as BMP) can only save
alpha channels and discard transparency, other formats only store
transparency and throw away alpha (like PNG). There's also formats
which are able to store both (only TIF afaik, besides PSD of course).
Is there a way to know which format stores what?

3. I noticed that Photoshop tends to blend (partially) transparent
pixels to white. For example I make an RGB image with a single layer,
which has transparency (i.e. it's not a background layer). When I
flatten it, transparent pixels become white (and partially transparent
pixels partially blend to white). I'm totally clueless about why it
does this, especially where it gets the color white from. I understand
it has to become _some_ color, but why white, even if my currently
selected background color is not white? Wouldn't it make more sense if
the pixels just keep their original RGB values when flattening?

Even more strange is when I save the image (before flattening it, so
it's still a single layer with transparency) as RAW: photoshop does
not save alpha channels in .raw images, so it just saves 24 bit RGB
data, and then the pixels which where semi-transparent also blends to
white! Why doesn't it just leave the RGB data as it is, and simply
throws away the alpha or transparency if it can't save that?

Thanks a lot!

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