meanwhile, I used my F-TexTools for making VTs in DXT1 format. I first converted the above, original 30662x15332 .CUB data file to a true 32k x 16k size, using the ISIS3 tool 'enlarge' with Cubic Convolution Interpolation
. Next I converted the 32k .CUB to a binary grayscale file with 'isis2raw' and quickly generated 512 VT tiles in high-quality DXT1 format from it with my 'txtilesDXT' tool. The running was all completed within just a few minutes...
From the resulting 32k VT set, let me show one close-up example for the high definition of the Mercury surface
that is available now, due to the Messenger probe:
Don't miss clicking on the image!!
It was gently colorated with GIMP from the true-color photo template that I also used for my 8k Mercury texture in the 1.6.0 Celestia distribution.