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I am porting a game from PSP to PS2. But in the game, there are some image files whose format is DXT1.PSP supports DXT1,but PS2 can't. Now I converted DXT1 to INDEX8 with tools, however INDEX8 will spend more memory than DXT1 in PS2. Can I decompress DXT1 by PS2? And how?

Decoding DXT is a bit too much for the GS. It can do it, but you will need to derive it yourself (it's a lot of passes since there is no hardware functionality), and in the end the cost of decoding is not worth it.

There are alternatives though:

* Use 4-bit CLUT whenever you can, then you get the cost down to half of 8-bit CLUT.

* VQ texture compression is a good alternative if you need a better ratio. It gives you 2 bits per pixel and the result can be very good, but is heavily dependant on the quality of the encoder. My recommendation is: Get a proper palette quantizer (don't use the VQ algorithm for this) and rasterize the image to 256 colors with floyd-steinberg before running the vector quantizer over the image.

Decompression is done by setting the VQ codebook as palette, and rendering the compressed image to a 4x larger buffer on the GS, which is very cheap.

Do note that because of the way the PS2 works you need to swizzle the data before compressing it (since you'll be rendering 8-bit into 32-bit but then read the resulting data as 8-bit).

thanks for your advice