I just did a very quick test on the same way of runways decals:
- I create a 4096x4096 file in photoshop or gimp.
- I drawn some dirty trails
- I remove this dirty trails of alpha channel
- save it in .tif with alpha channel
- in my .tsc file i added a new decal layer called "tfff_dirty"
- In AC3D i created a new group called "tfff_dirty"
- I applied the dirty .tif image where i want
- saved it and export all .tgi files
- converted it into .tmb
And result:
I think I'm doing something wrong with the alpha layer because it looks white in afs2 but it still works.
Didn't we include this "dirt" layer in our king-man airport example?
Anyway it is possible to assign another texture layer for the tyre markings and oil stains, etc and as far as I know we do this for pretty much all default airports.
You speak about this file probabily:
It's a good base but it does not seem transparent and it's limited in use (corner for example)
Edit: White are caused by opacity of gradient. Dont use gradient.