Realistic Folds in a Map
I had a lot of trouble trying to make a realistic fold in a map. I think I got it reasonably right in the end after a lot of mistakes. So here is a tut on what I learned. Most of you will already know how to do this but for some others it may be of help. The same procedure works in Photoshop. I present this tut for Gimpers as I am more familiar with Gimp.

A fold in a map has a depression or ridge in the paper and casts a shadow on the map. It also distorts the print as the paper follows the bends of the fold.. Lastly, it lifts the paper unequally from the underlying surface.This unequal lift causes a varying shadow.

Basically the process is to use a displacement filter to create the distortion. This is acoomplished with a displacement map. (note that the word map here means a layer) . The displacement map must be the same size as the layer that is to be distorted . The displacement map must be a grayscale map.
(Gimp Help 12.3 Displace has an excellent explanation on how the filter works) Note that a b/w rgb map works and also a colored map works but the effects become unreliable.So it is a good idea to use a grayscale map

To help give realism I use the same grayscale displacement map as a layer in Multiply mode to show the bends in the paper of the Atlas map.

Using the paths tool I trace a small section on the edge of the map to be culled. Lastly, a small dropshadow is applied to show a varying lift from the underlying surface.

Here is the map, I want to fold , spread out and lay flat on a table. (1080x830 -1000x750 atlas content and 40 px margins all round) I have deliberately made the map margin not white and have accentuated the colours to show the effect of the displacement filter.
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The displacement filter does just that, it displaces the pixels so your nice 40pixel margin no longer exists after displacement. Even a 3 pixel displacement in the margin is quite noticeable. To overcome this I extend the 40pixel margins by 10 pixels if I want a displacement of 5 pixels. The new atlas map is now 1100x850 -1000x750 atlas content and 50 px margins all round.
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This is the fold map I scanned from an A4 piece of paper.The lower left fold was almost invisible so I duplicated the layer twice in multiply mode. To give definition to the lower left fold. The other folds were then too dark , so I toned these down with the dodge and burn tool to make it all roughly the same darkness. Also I simply resized the image to 1100x850

I applied the displacement map with a factor of 4 to displace the atlas image and applied the same displacement map overlay in mutiply mode. You will see that the displacement agrees with the shading of the multiply mode overlay. Also note that the 50 px margin is now 46px . I can now cut a nice map with 40px margin again to represent my original map.(1080x830)

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The last step is to cut a very narrow edge to show lift of paper at the folds. I do this by placing my map on the marble table giving a 100 px marble edge. And selecting a rectangle on the map , selecting the inverse and erasing the edge to show a small variation in the edge of the atlas map. About 3to 4 px is plenty. The edge of the map is now “jagged” . Alternatively use the paths tool, create a selection from the path and Ctl X to cut away the jaggies. A small blur Gaussian 2-3 to take the edge of the hard pixels. The blur is only applied to the narrow band where the jagged edge was created
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Create a new layer underneath the jagged map layer (about 3-4 pixels smaller than the jagged edge perimeter and apply a drop shadow (no offset) to just show the edge of the atlas map . I made this layer black to show where it goes , but it can be any colour or picture !The effect will be that it appears that the paper is raised around folds and is slightly above the bottom surface in some places. I had to duplicate the drop shadow to show a good effect
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I tried to show a finished map, but after trying for some time I saw that I can only use 5 attachments per post. You can see the effects in the last picture where the drop shadow shows the different height of paper to tabletop at folds and crinkles.

Hope this of help to someone.