Adobe’s definition of "alpha channel"

BS
Posted By
boarder_s_paradise
Oct 23, 2008
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852
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5
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Closed
Since there were so many people who absolutely wanted to discuss the exact definition of "alpha-channel" to the point where the discussion about my other thread’s original topic became impossible, I hereby create an entire thread exclusively dedicated to you.

Don’t expect me to comment here.

I only post the starting point and then you guys can continue your flaming and fingerpointing.
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BS
boarder_s_paradise
Oct 23, 2008
The story so far: I used the term “alpha channel” as follows:

Add "alpha-channel" to the channels palette. Much like the R channel displays the red values of a pixel, the alpha channel would display the transparency values of the pixels. […] like in a layer mask, i.e. paint with darker color and the pixels get more transparent, paint with lighter color and the pixels get more opaque.

to make the idea clearer I should have said: a permanent and auto-updating "alpha channel". I added these adjectival complements later on in the thread.

I got called out for misusing the term.

But my use of the term is coherent with Adobe’s own help files. See:

< http://livedocs.adobe.com/en_US/Photoshop/10.0/help.html?con tent=WSfd1234e1c4b69f30ea53e41001031ab64-76c2.html>

quote:

Masks are stored in alpha channels. Masks and channels are grayscale images, so you can edit them like any other image with painting tools, editing tools and filters. Areas painted black on a mask are protected, and areas painted white are editable.

image caption: Selection saved as an alpha channel in Channels palette
MV
Mathias_Vejerslev
Oct 23, 2008
Do you know how Quick Mask works? You just press Q and youve got your visible mask.
BS
boarder_s_paradise
Oct 23, 2008
unrelated
CC
Chris_Cox
Oct 23, 2008
Yes, selections/masks can be stored in alpha channels – in any of the 50+ that you can create. But selections/masks and transparency/opacity are not the same thing. Saving a selection as an alpha channel is just that — saving it to another channel. That has nothing to do with transparency/opacity.

Any channel beyond the color channels is an alpha channel. It could be a stored selection, a bump map, a reflection map, transparency, notes to your art directory, or any other information stored in the form of an image channel.

Transparency/opacity is a special subset of alpha channels, something very tightly connected to the color data. In Photoshop, the transparency channel is not directly visible or editable without operating on the color data – while other alpha channels are visible and editable.

When the term "alpha channel" was first coined it meant a single transparency/opacity channel. That definition lasted maybe 5 years before the term was used to mean any additional data channels. Many image formats support alpha channels, but do not support transparency. Other file formats support multiple alpha channels, allowing one to be used for transparency.

Some industries rely on the older definition (alpha = transparency), while others use the more common definition (any extra channel). This causes confusion in communication. If someone says "alpha channel" it is unclear if they mean transparency or just any additional channel. Communication improves when people are more specific "transparency channel" vs. "saved mask" vs. "spot color channel".
BS
boarder_s_paradise
Oct 23, 2008
Thanks for the long definition and history lesson. I promise to call it "transparency channel" henceforth (at least, to me it seems that’s what the preferred terminology is for you).

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