to compress dds files with nvdxt the format options -dxt1c, dxt3 or dxt5 is what is compressing them. dxt5 has the best compression i think. -u888 is no compression.
dds will have larger file sizes than png but png files are rendered slower. If your trying to save diskspace the format to use would be jpg.
2.785.408 tx_0_0.dds
3.146.108 tx_0_0.png
2.785.408 tx_0_1.dds
568.770 tx_0_1.png
2.785.408 tx_1_0.dds
2.781.312 tx_1_0.png
2.785.408 tx_1_1.dds
2.512.357 tx_1_1.png
2.785.408 tx_2_0.dds
5.166.365 tx_2_0.png
2.785.408 tx_2_1.dds
1.858.949 tx_2_1.png
2.785.408 tx_3_0.dds
3.618.339 tx_3_0.png
2.785.408 tx_3_1.dds
2.322.031 tx_3_1.png
These different file sizes have a lot to do with the information contained in the files. Png and dds compress differently. The file tx_0_1 is mostly ocean i think. That is easily compressed with png because a lot of the pixels are the same value. DDS seems to compress by file size and not the pixel values. I'm sure someone will correct me if I'm wrong.
Correction: It seems that dxt5 uses 4:1 compression and dxt1 uses 6:1 compression
The DXT1 format compresses RGB images by a factor of 6:1, for an average of 4 bits per texel
The DXT5 format compresses an RGBA image by a factor of 4:1, using a byte per texel on average
quotes taken from NVSDK Bump_Map_Compression.pdf
DXT1 would probally get the best compression. Not sure about dxt3
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