View Full Version : Help with Channels in GIMP

11-27-2009, 09:01 PM
Greetings to all, as this is my first post!

That said, I am working on my first map using Jeremy Elford's tutorial for an antique style map (which is designed for photoshop). I have my base map in black and white and am just starting to go through his second section on antiquing. Where I'm running into trouble is creating a new alpha channel (I did manage to get a duplicate of the original alpha), and copying the map layer and pasting it into the new alpha channel. When I try to paste it into the alpha channel, I get a floating selection (pasted layer) as a new layer and can't select any other layers.

Any help I can get is much appreciated!

11-27-2009, 11:35 PM
After pasting it in the channel, it'll float. You just need to go to the layers window and "Anchor" the selection (either through the anchor icon or from the context menu IIRC).

When you anchor it, it stops floating and "falls" on the place you pasted it in the first place. Whenever you paste things in Gimp, you have the option of "anchoring" it to the layer you pasted it or, if you create a new layer after pasting (i.e., Ctrl+Shift+N), to have it as a whole new layer.

I hope that helps.

11-28-2009, 01:42 AM
That sure does clear up alot of my question. Thanks!

Was I correct to just duplicate the original alpha channel? The tutorial says to "create a new alpha layer".

11-28-2009, 10:11 AM
Unfortunately I haven't seen the tutorial yet. Can you post a link to it here so I can take a look?

I know a little bit about Photoshop and, maybe, I can help you find out what you need to do with GIMP to follow it.

11-28-2009, 11:24 AM
Jeremy Elford's Tutorial's are here (http://www.jezelf.co.uk/tutorials.htm). I have completed the basic map and just started the paper/aged tutorial. Really where I'm having problems is where the commands for PS don't line up with the commands in GIMP.

11-28-2009, 12:43 PM
From what I could gather skimming through the tutorial, you seem to be in doubt about step 1:0 "Create a new alpha channel". I don't know how much you know of these things, so I'll try to be very basic here. I'm sorry if I end up being too basic.

Please, also keep in mind that I don't know that much about Photoshop to start with.

It seems to me, from reading the tutorial, that the author means an "Alpha Channel" when he simply means a Channel in any way.

A channel is simply a black & white (well, shades of grey also work) mask that defines a range of values (usually between 0 to 255). Some sort of "stencil" to colours and other properties, if you like to think of it like that.

Most image editing programs for display on screen (not printing) usually deal with "RGBA" colours. That is: Red, Green, Blue, Alpha. And they have channels masking each of those colours.

What this means is that, when you look at GIMP's Channel tab without having any channel created by yourself, only the default ones, you see a channel for each of those properties. Red, Green, Blue are self explanatory: They mask the values (again, between 0 ~ 255) of each colour in a given image (blending those three colours you can get pretty much any other colour in the spectrum).

Alpha is the "opacity" channel. Whenever you add a bit of alpha value to the channel (0 is black, 255 is white, any shade of grey is between), you make that piece of the image more opaque. So, Black in Alpha channel is fully transparent (invisible) and white is fully opaque.

Another thing Channels are commonly used for (and that is the case in the tutorial) is to define "saved selections". Again, it might look weird at first, but selection in a program like GIMP or photoshop isn't boolean: it's not either selected or not selected. It's fuzzier than that: You can have a pixel somewhat selected, almost completely selected, almost not selected and so forth.

The effect of these fuzzy selections is that, whenever you perform an action on a selection like that (filter, fill, or something like that), it works "full force" on the completely selected areas, and then fade in the less selected ones.

My take is that the author meant just to create a new channel that he would *then* use to represent the filled parts of the map (as opposed to the sea or something like that), and that's why he called it "alpha". But it's not a special channel at all: just creating a new channel and pasting the things should have worked nice.

Now, the interesting bit is that, if you pasted a mask that takes the entire area of the image in that channel, it doesn't really matter if it was a regular channel or copy of another one: It'll overwrite the new channel completely, and the end result will be just the same. If you were pasting something with transparency, though, parts of the channel you copied would show through, and most likely not behave like you wanted.

Again, I hope it helps, though I think I wasn't as clear this time as before :(

11-28-2009, 08:52 PM
In Photoshop, every time you create a new channel, it's labeled as an alpha, even if it isn't used as one.

Have you considered starting with RobA's Gimp tutorial instead of trying to convert a PS tut the first time out? It's been among the most popular tutorials on this site. http://www.cartographersguild.com/showthread.php?t=1142

11-29-2009, 07:57 PM
@ alexandream: Thanks! That helped so much and I get the rationalization behind it now. It's been 8 years since my intro to digital media class where we briefly covered PS and Illustrator.

@Midgardsormr: I looked through RobA's tutorial and did a few basic steps out of there as well, but I really wanted an antiqued look and I just so happen to be a glutton for punishment :)