Hi,

Cathy Irwin writes:

I am a relatively new user of the GIMP and am experimenting with writing my own plug-ins. I am still having trouble working out when it is most appropriate to use layers and when to use channels. I understand that channels are to do with RGBA values.

You have misunderstood the concept of channels then. Channels are saved selection masks. They consist of what could be seen as grayscale data and are used to store the selection. The top three or four "channels" that you see in the Channels tab in the user interface aren't really channels. They just appear there for historical reasons. Perhaps because Photoshop puts them there as well...

Sven

Sven Neumann wrote:
> Channels are

saved selection masks. They consist of what could be seen as grayscale data and are used to store the selection. The top three or four "channels" that you see in the Channels tab in the user interface aren't really channels. They just appear there for historical reasons. Perhaps because Photoshop puts them there as well...

I bet to differ -- the top three or four 'channels' ARE channels, and the rest AREN'T. The rest are, as you describe, 'saved selection masks'. I guess they're historically in the same dialog because Photoshop liked to pretend that saved selection masks were 'auxilliary channels' (barf), and AFAIK practically no-one found a real use for genuine 'auxilliary channels' (probably good for spot colours, but I'm guessing that ironically, Photoshop then proceeded to handle spot colours differently anyway).

--Adam

Hi,

"Adam D. Moss" writes:

I bet to differ -- the top three or four 'channels' ARE channels, and the rest AREN'T. The rest are, as you describe, 'saved selection masks'. I guess they're historically in the same dialog because Photoshop liked to pretend that saved selection masks were 'auxilliary channels' (barf), and AFAIK practically no-one found a real use for genuine 'auxilliary channels' (probably good for spot colours, but I'm guessing that ironically, Photoshop then proceeded to handle spot colours differently anyway).

Well, of course you are right. But in order to avoid further confusion I used the term "channel" to describe the GimpChannel object as well as the corresponding wrapper struct in libgimp. This is what the plug-in API exposes as channels even though perhaps the name choice is rather unfortunate.

Sven

Cathy Irwin writes:

Can anybody point me to some documentation that explain the differences between the various drawables comprehensively?

At a conceptual level, the difference is very simple: A layer is a visible part of an image. A channel is not: it is a grayscale drawable that acts to modify the behavior of some part of the image.

As Sven wrote, part of the confusion arises because the top three or four "channels" that you see in the Channels tab in the user interface aren't really channels, in the GIMP sense of the word "channel".

There does not really exist a good developer-oriented explanation of the differences between types of drawables, but if you combine the material in the Help docs at

http://docs.gimp.org/en/

(particularly sections 2.4, 4.2, and 4.3) with the API docs for gimplayer and gimpdrawable at

http://developer.gimp.org/api/2.0/libgimp/index.html

it might be enough to be helpful.

Best, -- Bill

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