Now let's paint the ocean!
Load "OceansLayer.png" into GIMP. You'll notice that it looks strange - that's because the entire image with the exception of the lines you have drawn is transparent. To allow for better viewing, create an all-white new layer and place it below the existing layer.
Now create two additional new layers: "Deep Ocean" and "Shallow Ocean". Both must initially be white. Place them on top of the existing layers.
Go to the Background layer (that's what the original layer with the lines should be named like). Select the parts of the map beyond the continental shelves with the Magic Wand. Then go to the Deep Ocean Layer. Change the Foreground Color to a deep blue (I used 0032ff in HTML notation). Use the "Fill" tool with "FG color fill" and "fill whole selection" to paint the selection blue.
Now remove the selection with Select-None. Apply a Gaussian Blur with a radius of 10 px by going to "Filters->Blur->Gaussian Blur" and setting "Horizontal" and "Vertical" to 10 px.
Now add transparency to this layer. Do this via Layer->Transparency->Color to Alpha and choosing white as the color.
Duplicate this layer. Now merge the copied layer with the original one by right-clicking on it in the layer dialog and choosing "Merge Down".
This ensures that you have a gradual blending between the deep ocean and the shallower areas. We will use this technique a lot later on.
Now let's apply some effects:
- Filters->Noise->Scatter HSV with the values 2/3/160/10
- Filters->Generic->Erode
- Filters->Distort->Wind with Wind/Left/Leading/10/10
That's it for the Deep Ocean Layer. Now go to the Background Layer, select the shallow ocean parts, and go to the Shallow Ocean layer. Fill it with a light blue (I used 00afff), deselect, apply a Gaussian Blur as above, and again add transparency, dublicate it and then merge it with the original.
Now apply the following:
- Filters->Noise->Scatter HSV with the values 2/3/160/10
- Filters->Generic->Erode
Now merge the Shallow Ocean and Deep Ocean layers. Duplicate and merge them again to make sure that no transparency remans in the boundary between them.
Now go to the Background layer and select the land areas with the Magic Wand. Press SHIFT to mark more than one land mass. Go to the Oceans layer and delete all land parts by going to Edit->Cut.
Make the white layer at the bottom invisible and save the result as "OceansColor.png". Now you have a nice-looking ocean surrounding your islands!
I will cover some further examples later on, but they follow the same methods - establish basic lines with an Inkscape drawing, and then generate color effects with a variety of GIMP filters. Since I don't actually draw anything with GIMP, I can easily generate a new map with the same effects even if the Inkscape map changes.
Of course, this doesn't mean that there is no room for improvement - far from it! For example, these ocean effects look nice in color and on the screen, but they look ugly and random when you print them out with a b&w printer. And I still haven't come up with a good idea how to make them more printer friendly.