Thank you so very much for all the wonderful, fantastic help you provided!
My Summary of how I implemented your information is included in this
post, below your very helpful post. Perhaps it will serve to help someone
else who needs to do something similar? I hope so.

On Sat, Oct 18, 2008 at 8:39 PM,
> I would recommend first reading the reference documentation that is included
> with the source code.
> Online at:
> Next, your scanned files should be named sequentially in the form
> "frame_0001.pnm", "frame_0002.pnm", "frame_0003.pnm", etc. The "frame"
> part can be different, and the extension should match the filetype.
> The numbering should be consecutive with no gaps.
> Once your scanned files are named properly, you should open up the
> first one in GIMP, then perform a "Video->Frames convert"; specifying
> an extension of ".xcf" (you can change the basename and/or the
> directory path should you wish). You should then close your PNM image
> window and work with the copy.
> Yes, you MUST use XCF format for onion-skinning to function properly.
> Using XCF is also necessary for other useful GAP functionality
> (masking, selections, etc).
>> As I've mentioned above, I'd like to be able to do two things:
>> 1. Place one drawing over another and see the bottom drawing through the
>>    top drawing so I can lasso stuff in the top drawing and 'register' it
>>    with the bottom drawing.
> Use the following Onionskin settings...
> Reference Mode: Normal
> Onionskin Layers: 1
> Frame Reference: -1
> Stack Position: 0 [From Top]
> Opacity: 50%  100%
> Select Mode: All visible
> "Auto create after load" & "Auto delete before save" should both be checked.
> All other settings should use their defaults.
>> 2. Place two 'key frame' drawings beneath the top drawing, and be able
>>    to do an 'in-between' drawing on the page on top, seeing both the
>>    bottom drawings through the top page.
> Alter the above settings as follows...
> Reference Mode: Bidirectional (double)
> Onionskin Layers: 2
> This will result in the onion layers appearing ABOVE your frame layer
> (not underneath it as you specified). This is, in my opinion,
> preferable so that you don't have to adjust the opacity of your frame
> layer to see the previous and next frame layers.
> _______________________________________________
> Gimp-user mailing list
> Gimp-user@lists.XCF.Berkeley.EDU
> https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-user

Recently, I asked a question about how to use the Onionskin feature
of the Gimp Animation Package. The following is a Summary of what I
did to align a series of drawings that had registration crosshairs
with the crosshairs on the first drawing, so all the drawings would
be registered with each other. The drawings were all scanned into
the computer using a flatbed scanner. All the drawings were saved
as PNM image files from the scanner. The directions I was given were
good for onionskinning a drawing with the one right behind it in
numerical order. I slightly modified those instructions to align all
62 drawings with the very first drawing.

 1. Create new directory and save all PNM files to the new directory.
 2. Change to the new directory, and create a directory: PNM
 3. Copy all the PNM files to the PNM directory. This is a backup.
 4. Open The Gimp.
 5. File > Open > f_01.pnm
 6. Video > Frames Convert...
        Extension: .xcf
    GAP saves all the PNM files as XCF files, in sequential order.
 7. Close f_01.pnm.
 8. In f_000001.xcf window:
        Video > Onionskin > Configuration...
            Reference Mode: Normal
            Onionskin Layers: 1
            Frame Reference: -1
            Stack Position: [X] From Top
            Opacity: 50.0 / 100.0
            Select Mode: All Visible (ignore pattern)
            [X] Auto create after load
            [X] Auto delete before save
         Click [Okay]
 9. File > Open > f_000002.xcf
10. In the Layers window, make sure the onionskin_000001 layer is above
    the Background layer otherwise, the onionskin effect can't be seen.
    I used the down arrow in the Layers window to move the Background
    down one. That will make the onionskin appear.
11. Lasso both drawings.
12. View > Zoom to whatever magnification you need to align the cross-
    hairs. I used 200%.
13. With the mouse cursor inside the lassoed area, press the mouse
    button and drag until the crosshairs are aligned.
14. Click outside of the lassed area to anchor the drawing.
15. Right click the onionskin_000001 layer in the Layers windows and
    Delete Layer. Clean up drawing with eraser, or whatever.
16. Save the file.
17. File > Open > f_000002.xcf
18. Move the Background under the onionskin layer in the Layers window.
19. Video > Onionskin > Configuration...
        Frame Reference: -2
    Click [Okay]
    What this does is makes the first drawing the reference drawing.
    On each new drawing I open, I will move the Background below the
    onionskin layer, and change the Frame Reference in the Onionskin
    Configuration. The next change will be to -3, then -4, and so on.
    Each time this is done, new onionskin layers are built for the
    remaining files, so this may not be good if you have 1000's of
    drawings. It wasn't so bad with only 62 drawings.
20. Follow steps 11 to 16.

I make videos for YouTube with PPM files, so I did a Save As... after I
saved each XCF file, and saved a PPM copy of the registered image of
the drawing in a PPM directory as I went along. Likewise, I organize
the other files in the same way: XCF files go in a XCF directory, etc.

I found out that if a drawing was cocked, I could first align the
crosshairs, then use the rotate tool with the center placed on the
aligned crosshairs, to uncock it. My experience doing this has
convinced me that attaching a pegbar to the scanner is the best
way to solve the problems I experienced. This is my first flatbed
scanner, so I'm learning things the hard way..

Many thanks to "saulgoode" for the expert instructions that allowed me
to use The Gimp GAP Onionskin tool to do exactly what I wanted to do!

I believe that what I am posting is correct to the best of my knowledge.
Your mileage may vary. I am not responsible for how you use this info,
nor for any damage which may directly or indirectly occur as a result
of you using this info. Best of luck in all your Gimp-GAP adventures!
b h a a l u u at g m a i l dot c o m
Kid on Bus: What are you gonna do today, Napoleon?
Napoleon Dynamite: Whatever I feel like I wanna do. Gosh!
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