Paint fields in GIMP using satellite texture as base. Paint cultivated or plowed fields. GE is extremely bad software to use so painting fields using it is a torture, it is far better to paint them in gimp.
cultivator_density uses cultivated land RGB 1,0,0 and ploughed land RGB 2,0,0 colors.
This image assumes you have high resolution satellite image for real world data terrain available, use 8192 x 8192 resolution for 8km and 4096 x 4096 resolution for 4km terrains.
- open satellite image in gimp, create fields layer and paint fields as some bright color like red
- resize to cultivator_density resolution (usually smaller), background RGB 0,0,0 and red color to RGB 1,0,0 export to cultivator_density.png
- replace RGB 0,0,0 with RGB 130,0,0 and RGB 1,0,0 with RGB 0,0,0 export to fruit_density.png
- edit terrainname.i3d GDM to PNG on these two density images
- GE load terrainname.i3d and verify that fields are painted without grass in them, save
Idea: why not screenshot GE top-down view with field rectangles visible, then just gimp resize it to like 4096 res, color replace the blue with RGB 1,0,0 to create cultivated fields almost automatically? Hmm :)