Isso falará ao seu driver de graficos para comprimir cada textura assim que ela o carregar. Isso significa que demorará um pouco mais para carregar a textura, mas a textura resultante ocupara muito menos espaço na memoria.
There's one important advantage to compressing the textures at runtime this way: the graphics driver will be able to compress all the textures using whatever texture compression algorithm it understands, DXT or otherwise. Not all graphics cards support all compression algorithms, so using this option allows the driver to choose the best algorithm it supports. If the graphics driver doesn't support any compression algorithms at all, it will simply load the textures uncompressed. Either way, your application will still run and all of your textures will be visible.
TXO file format
Panda has a native file format for storing texture images, called TXO (the abbreviation is for "texture object"). This is similar to BAM files. A TXO file contains all of the texture image data in a format very similar to Panda's internal representation, so it loads into memory very quickly.
More importantly, perhaps, TXO files can optionally store pre-compressed texture images. You can use the command:
egg2bam -txo -ctex model.egg -o model.bam
to convert your model to a BAM file, and all of its textures to TXO files, with the image data pre- compressed within the TXO file so that it will not need to be compressed at runtime later. (You may need to specify "pandagl" instead of "pandadx9" as your rendering engine while you run the egg2bam command-- at the time of this writing, there were issues with using Panda's DirectX driver in an offline mode like this. However, the resulting TXO files will load on either OpenGL or DirectX at runtime. )
TXO files have the same drawbacks as BAM files: they are tied to a particular version of Panda, so you may need to regenerate them when you next upgrade your Panda version.
A bigger drawback to storing pre-compressed texture images this way is that your application might no longer run on all graphics cards. Not all graphics cards support all kinds of DXT compression, and if you try to load a TXO file that a graphics card doesn't understand, it simply won't load. Thus, pre-compressing all of your textures may make your application less portable.
DDS file format
In addition to Panda's native TXO file format, there is a fairly standard format called DDS, which has some of the same properties of TXO. Like TXO, you can store pre-compressed images in a DDS file. The biggest advantage of the DDS file format is that there are already several tools available on the internet to generate DDS files, including GIMP and Photoshop plugins. (Note, however, that loading DDS files is a new feature of Panda, and these files are not supported in Panda versions before 1.6.)