Hello,
I've been in the process of creating a mask, the type typically found in the world/texture folder.
The best I've been able to do after using BMP2000, DXTbmp and MS imagetool is an image 32 X 2048 that is in Mask format but after conversion has black and white vertical lines with a hint of the desired pattern and the same result in the sim. Any hints or help would be appreciated.

John
Flight Ontario

Discovered how to do it;
Using other Mask as example.
Select 8 various combinations of corners of texture you want Mask of in black with anti-aliasing each combo creating 256 X 256 selections.
stack the 8 textures resulting in 256 X 2048 image then convert to grey scale
re-size stack from 256 X 2048 to 32 X 2048 (should look like sample.
Next with Microsoft image tool load image and set format to Mask then create mips. Save file with file naming convention as in "world" "texture" folder ( ones with "M" in their file name.
John