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Thread: Problems creating an Alpha Channel in GIMP

  1. #1

    Default Problems creating an Alpha Channel in GIMP

    So I have hit a bit of a wall. I have been following some of the GIMP tutorials on the site, but I am constantly running into a blockage when I am asked to create an Alpha Channel for my layer.

    I have been following Click image for larger version. 

Name:	Photoshop Map Making Tutorial 3 (Authentic Maps).pdf 
Views:	17 
Size:	1.03 MB 
ID:	57099, a tutorial written by Jeremy Elford.

    Here is the quoted section I am trying to follow. I feel like I am missing something though.

    "1:0 first thing is we are going to create an alpha channel
    select layer with map > Ctrl+A (select all) > Ctrl+C > Channels tab > create new alpha layer > Ctrl+V to paste
    in the new channel you just created that's called 'Alpha 1' by default.

    1.1: back to your layers tab
    Create new layer > Get a tan brown as foreground colour > Edit > Fill > foreground Colour - this will end up
    as the light brown area in the image below...
    (You could go with any other colour - or keep it as a line drawing on white paper - as you would see in any
    novel. To do this you'll want to ignore the bits to fill in colour, or at the end desaturate the image and adjust

    1.2: Back to Channels tab
    Hold Ctrl+Left Click (with your mouse - I might also refer to this as 'Ctrl+L click' ) on your map alpha. We do
    this to create what is known in Photoshop as a 'Selection set' . By doing this step, we create one which we'll
    use to create the outline version of our map (see below compared to above)

    1.3: Back to layers. Create new layer,get a dark brown as your foreground colour - for the line work colour.
    Again could be any colour you like if you have a different colour scheme.
    Edit>Stroke>1 pixel You should see the line work now. It used the selection set from your Alpha channel as
    mentioned above. Selection sets and Alpha channels are handy things in PS and used often. In this case,
    though it's important to use the alpha channel because it will use the varying shades of grey which you may
    note is effective for the rivers on the image below, that they are lighter as they get further inland."

    I am working from this image, which I rendered following some of RobA's tutorials.

    Click image for larger version. 

Name:	Project1b.png 
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Size:	226.2 KB 
ID:	57102

    After creating my Alpha Channel, my image changes coloration a bit, with the white turning a dark grey. Is this the appropriate response?

    Here is a screenshot of my layers tab and UI setup.

    Click image for larger version. 

Name:	Layers Example.png 
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Size:	407.9 KB 
ID:	57100

    Here is what my channels tab looks like after I have created the "Alpha Channel."

    Click image for larger version. 

Name:	Channel Example.png 
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Size:	408.2 KB 
ID:	57101

    Once I am at this point, it seems I am unable to manipulate the color scheme like the PDF suggests Upon switching back to the layers file I am unable to use the fill tool. Any help or thoughts are much appreciated.

    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	Project1a.png 
Views:	56 
Size:	315.4 KB 
ID:	57098  
    Last edited by CmorEffect; 08-22-2013 at 06:00 PM.

  2. #2
    Community Leader RobA's Avatar
    Join Date
    Apr 2007
    Toronto, Canada


    Things are different in Gimp.

    I think the intent is to create a channel (saved selection) based on the B&W outline. The simplest way to do this in Gimp is to go Ctrl+a (select all), hit the quick mask toggle (Shift-q or click):
    Click image for larger version. 

Name:	gimp-quickmask-toggle.png 
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Size:	166.3 KB 
ID:	57121
    Then hit Ctrl+v (paste) and hit the quick mask toggle again. You now have a selection based on that B&W image.
    To save that to a channel (for use later) use the Menu Select->Save to Channel. This will save the selection to a channel you can re-load as the selection later, named "Selection Mask copy".
    You will have to go back to the layers palette after this as creating a new channel that way makes the channel the active drawable.

    Then continue along.

    When it says to ctrl-left click on the channel to create a "Selection Set" in Gimp just use the right click "Channel to Selection" or the little red button in the channel palette (second from the right) to load the selection from the channel.

    Good luck!

    -Rob A>

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