Thorsten wrote in Mon Dec 17, 2012 12:49 pm:
As for resolution, the default textures are I think usually 512x512 spread to 2000 x 2000 m, which gives a pixel size of 4x4 m. I'm not sure how large the dds base size is - probably similar. They're mapped to a variety of sizes, but 1000 x 1000 m seems the usual. Which gives a pixel size of 2 x 2 m. The pricetag is that cropgrass1.png is 617K , cropgrass.dds is 2.7 M, so the dds textures take ~4.5 times the diskspace (and download bandwidth) of their png counterparts (imagine how a widespread switch to dds would blow up the size of the GIT repository...)
The better pixel size comes at increased tiling artefacts, which the dds set counters by having very low contrast inside the texture so as to create no visual cues to tiling. However, contrast inside the texture is an important device to hide the hard landclass boundaries (see the long version of this argument here, so from higher altitude, the dds set to first oder looks like the mapserver image of the landclass distribution painted with almost monochromatic colors. If you fly low dds is better, but if you're high up dds becomes more dull
Just to debunk some myths:
Most .dds textures are 2048^2. The size they are mapped to varies according to features visible in them, -it ranges roughly from 500 to 1600- so you don't end up with a house the size of a stadium. The size increase is offset by the fact that they remain compressed in VideoRAM... as opposed to the .png ones. Also they don't need any processing at load time (no mipmap generation is necessary since that is already done).
So you get 4x the disk size for at least 16x the detail, and roughly the same memory used at runtime. It's for you to decide which is more efficient...
P.S.: @btw, they are one and the same