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I've never done any graphic art or design. So I have a super basic question. I'm learning to use Krita. I keep seeing the word "Alpha", what does this mean/refer to?

When I choose to create a new document in Krita, a dialog box pops up so that you can select different options for the document. The options for color model all have "/alpha" next to them. So the options look like this:

  • RGB/alpha
  • CMYK/alpha
  • Grayscale/alpha
  • XYZ/alpha
  • etc.

Each option has that after it.

Also I just learned what an alpha channel is. So... I guess I've really only seen it twice, but still, I want to know that means. Are there other "alpha" things too that I haven't learned about yet?

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    Jul 5, 2017 at 9:43

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'Alpha' is the opacity channel. Each layer can have such a channel, governing how transparent or opaque every pixel on the layer is.

Such a channel is essentially a greyscale image of exactly the same dimensions as the layer. Every pixel on the alpha channel corresponds to the same pixel on the layer. The colour of a pixel on the alpha channels affects the opacity of the corresponding pixel on the layer.

Usually, 100% black means 100% transparent (and thus invisible) and 100% white means completely opaque (or 0% transparent). Any percentage of grey is an intermediate opacity.

This way, an alpha channel allows you to control each pixel's opacity separately.

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Alpha channel means the opacity of an individual pixel. This allows you to layer pictures on top of each other neatly as each pixel knows how much it should contribute to the compound image.

The name alpha to my understanding derives form the formulas used for blending which predate alpha channels, these used alpha character in them to parameterise the amount of blending (as to why that is I dont know, probably arbitrary choice). Thus if you wanted to drive each pixel with a separate blend value you needed a source for this alpha value. Thus calling the driving source an alpha channel is pretty logical, and straightfoward extension.

Side note: From the fact that you have 4 channel follows that there is no such thing as a transparent pixel. As even a fully invisible pixel has a color, and this can come and haunt you when you do image scaling or other operations on them.

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The "alpha channel" is the opacity of the pixel (0% transparent, 100% opaque, intermediate values are possible). So, instead of being characterized by three channels (RGB, for instance), pixels are denoted by 4 (RGBA). So (100%,100%,100%,100%) is an opaque white pixel while (100%,100%,100%,0%) is a transparent white pixel (that looks exactly the same as a (0%,0%,0%,0%) transparent back pixel, until you start playing with the alpha channel.

You can often edit the alpha channel directly using a layer mask.

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  • Also worth adding that rgba is also sometimes (at least in CSS) written as color values and the alpha channel as a decimal rather than percentage. Example: rgba(174, 38, 76, 0.5), which would be 50% (0.5) transparent.
    – Scribblemacher
    Jul 5, 2017 at 13:44
  • @Scribblemacher Thats the same thing % meaning hudredths or 0.01. In either case alpha is still most likely calculated in image channels as values ranging from 0 to 255 0 being 0 and 255 being eqivalent of 1.
    – joojaa
    Jul 5, 2017 at 15:04
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    @joojaa I'm aware. I just figured it would be worth pointing out that these are sometimes notated different but are equivalent (if someone isn't sure what alpha is they may also not realize that 100%=255 in this instance)
    – Scribblemacher
    Jul 5, 2017 at 15:19
  • @Scribblemacher my point being that there is always a bit of indepth information be added to any answers scope. Explaining everything is going to make reading things just tedious. Some level of competency just has to be assumed, unless you want to make all answers always 2 pages long. Because that is what happens when you are thorough, unfortunately it also takes a very loooong time to write those kinds of answers (as you can see here, and i am not sure if im properly finished yet).
    – joojaa
    Jul 6, 2017 at 6:52

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